• TABLE OF CONTENTS
HIDE
 Front Cover
 Front Matter
 Table of Contents
 The disease
 Causal organism
 Seasonal development of the...
 Control
 Summary
 Literature cited






Group Title: Bulletin - University of Florida Agricultural Experiment Station ; 249
Title: Gray leafspot
CITATION PAGE IMAGE ZOOMABLE PAGE TEXT
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00026529/00001
 Material Information
Title: Gray leafspot a new disease of tomatoes
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 35 p. : ill. ; 23 cm.
Language: English
Creator: Weber, George F ( George Frederick ), b. 1894
Hawkins, Stacy O
Kelbert, D. G. A ( David G. A )
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1932
 Subjects
Subject: Tomatoes -- Diseases and pests -- Florida   ( lcsh )
Stemphylium solani -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Bibliography: p. 34-35.
Statement of Responsibility: by George F. Weber, Stacy Hawkins, and David G. A. Kelbert.
General Note: Cover title.
Funding: This collection includes items related to Florida’s environments, ecosystems, and species. It includes the subcollections of Florida Cooperative Fish and Wildlife Research Unit project documents, the Florida Sea Grant technical series, the Florida Geological Survey series, the Howard T. Odum Center for Wetland technical reports, and other entities devoted to the study and preservation of Florida's natural resources.
 Record Information
Bibliographic ID: UF00026529
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000924108
oclc - 18204687
notis - AEN4712

Table of Contents
    Front Cover
        Page 1
    Front Matter
        Page 2
    Table of Contents
        Page 3
        Page 4
    The disease
        Page 5
        Page 6
        Host range
            Page 7
            Page 8
            Page 9
        Symptoms
            Page 10
            Page 11
            Page 12
    Causal organism
        Page 13
        Taxonomy
            Page 13
            Page 14
            Page 15
        Morphology
            Page 16
            Page 17
            Page 18
        Physiology
            Page 19
            Page 20
            Page 21
            Page 22
            Page 23
        Pathogenicity
            Page 24
            Page 25
            Page 26
    Seasonal development of the disease
        Page 27
        Page 28
        Page 29
    Control
        Page 30
        Page 31
        Page 32
    Summary
        Page 33
    Literature cited
        Page 34
        Page 35
Full Text




UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
GAINESVILLE, FLORIDA
Wilmon Newell, Director


GRAY LEAFSPOT, A NEW DISEASE

OF TOMATOES
By
GEORGE F. WEBER, STACY HAWKINS
AND DAVID G. A. KELBERT


Fig. 1.-Views of top and bottom sides of tomato leaflets showing lesions
resulting from natural infection with Stemphylium solani.

TECHNICAL BULLETIN

Bulletins will be sent free upon application to the
AGRICULTURAL EXPERIMENT STATION
GAINESVILLE, FLORIDA
LIPAk .
FLORIDA EXPERIMEV. Y -"
OAINESVILLE, PLUtKIUA


Bulletin 249


June, 1932










EXECUTIVE STAFF
John J. Tigert, M.A., LL.D., President of the
University
Wilmon Newell, D.Sc., Director
H. Harold Hume, M.S., Asst. Dir., Research
Sam T. Fleming, A.B., Asst.Dir., Administration
J. Francis Cooper, M.S.A., Editor
R. M. Fulghum, B.S.A., Assistant Editor
Ida Keeling Cresap, Librarian
Ruby Newhall, Secretary
K. H. Graham, Business Manager
Rachel McQuarrie, Accountant


MAIN STATION, GAINESVILLE

AGRONOMY
W. E. Stokes, M.S., Agronomist
W. A. Leukel, Ph.D., Associate
G. E. Ritchey, M.S.A., Assistant*
Fred H. Hull, M.S., Assistant
J. D. Warner, M.S., Assistant
John P.. Camp, M.S., Assistant

ANIMAL HUSBANDRY
A. L. Shealy, D.V.M., Veterinarian in Charge
E. F. Thomas, D.V.M., Assistant Veterinarian
W. W. Henley, B.S.A., Assistant Veterinarian
R. B. Becker, Ph.D., Associate in Dairy Inves-
tigations
W. M. Neal, Ph.D., Asst. in Animal Nutrition
P. T. Dix Arnold, B.S.A., Assistant in Dairy In-
vestigations

CHEMISTRY
R. W. Ruprecht, Ph.D., Chemist
R. M. Barnette, Ph.D., Associate
C. E. Bell, M.S., Assistant
J. M. Coleman, B.S., Assistant
H. W. Winsor, B.S.A., Assistant
H. W. Jones, M.S., Assistant

ECONOMICS, AGRICULTURAL
C. V. Noble, Ph.D., Agricultural Economist
Bruce McKinley, A.B., B.S.A., Associate
M. A. Brooker, Ph.D., Associate
Zach Savage, M.S.A., Assistant

ECONOMICS, HOME
Ouida Davis Abbott, Ph.D., Head
L. W. Gaddum, Ph.D., Biochemist
C. F. Ahmann, Ph.D., Physiologist

ENTOMOLOGY
J. R. Watson, A.M., Entomologist
A. N. Tissot, Ph.D., Assistant
H. E. Bratley, M.S.A., Assistant
E. F. Grossman, M.A., Asso., Cotton Insects
P. W. Calhoun, Assistant, Cotton Insects

HORTICULTURE
A. F. Camp, Ph.D., Horticulturist
Harold Mowry, B.S.A., Associate
M. R. Ensign, M.S., Associate
A. L. Stahl, Ph.D., Assistant
G. H. Blackmon, M.S.A., Pecan Culturist
C. B. Van Cleef, M.S.A., Greenhouse Foreman

PLANT PATHOLOGY
W. B. Tisdale, Ph.D., Plant Pathologist
George F. Weber, Ph.D., Associate
R. K. Voorhees, M.S., Assistant
Erdman West, M.S., Mycologist
*In cooperation with U.S.D.A.


BOARD OF CONTROL
P. K. Yonge, Chairman, Pensacola
A. H. Blanding, Bartow
Raymer F. Maguire, Orlando
Frank J. Wideman, West Palm Beach
Geo. H. Baldwin, Jacksonville
J. T. Diamond, Secretary. Tallahassee



BRANCH STATIONS

NORTH FLORIDA STATION, QUINCY
L. O. Gratz, Ph.D., Associate Plant Pathologist
in Charge.
R. R. Kincaid, M.S., Asst. Plant Pathologist
W. A. Carver, Ph.D., Asso. Cotton Specialist
R. M. Crown, B.S.A., Asst. Agronomist. Cotton
Jesse Reeves, Farm Superintendent

CITRUS STATION, LAKE ALFRED
John H. Jefferies, Superintendent
Geo. D. Ruehle, Ph.D., Asst. Plant Pathologist
W. A. Kuntz, A.M., Asst. Plant Pathologist
B. R. Fudge, Ph.D., Assistant Chemist
W. L. Thompson, B.S., Assistant Entomologist

EVERGLADES STATION, BELLE GLADE
R. V. Allison, Ph.D., Soils Specialist in Charge
R. W. Kidder, B.S., Farm Foreman
R. N. Lobdell, M.S., Associate Entomologist
F. D. Stevens, B.S., Sugarcane Agronomist
H. H. Wcdgeworth, M.S., Asso. Plant Path.
B. A. Bourne, M.S., Associate Sugarcane Physi-
ologist
J. R. Neller, Ph.D., Associate Biochemist
A. Daane, Ph.D., Associate Agronomist
M. R. Bedsole, M.S.A., Assistant Chemist

SUB-TROPICAL STATION, HOMESTEAD
H. S. Wolfe, Ph.D., Asso. Horticulturist in Chg.
btacy 0. Hawkins, M.A., Assistant Plant
Pathologist




FIELD STATIONS

Leesburg
M. N. Walker, Ph.D., Asso. Plant Pathologist
W. B. Shippy, Ph.D., Asst. Plant Pathologist
K. W. Loucks, M.S., Asst. Plant Pathologist
C. C. Goff, M.S., Assistant Entomologist
J. W. Wilson, Ph.D., Assistant Entomologist
Plant City
A. N. Brooks, Ph.D., Asso. Plant Pathologist
R. E. Nolen, M.S.A., Lab. Asst. in Plant Path.
Cocoa
A. S. Rhoads, Ph.D., Asso. Plant Pathologist
Hastings
A. H. Eddins, Ph.D., Asso. Plant Pathologist
West Palm Beach
D. A. Sanders, D.V.M., Associate Veterinarian

Monticello
Fred W. Walker, Assistant Entomologist
Bradenton
David G. Kelbert. Asst. Plant Pathologist



















CONTENTS

PAGE
THE DISEASE ................... ................................. 5
Geographical Distribution .................................... 5
Economic Importance ........................................ 5
Host Range ................................................. 7
Symptoms ............................................... 10

CAUSAL ORGANISM ................................................. 13
Taxonomy ............................................... 13
Morphology ................................................ 16
Physiology ........................................ ......... 19
Pathogenicity .............. ........................... 24

SEASONAL DEVELOPMENT .......................................... 27

CONTROL ........................ ................... .............. 30

SUMMARY ........................................................... 33

LITERATURE CITED .................................................... 34









GRAY LEAFSPOT, A NEW DISEASE OF TOMATOES
By GEORGE F. WEBER, STACY HAWKINS AND
DAVID G. A. KELBERT

Gray leafspot of tomatoes was found in Florida (29) (30)
during the summer of 1924, although certain old growers claim to
have known of the disease for years and referred to it as "red
rust." During that year no attempt was made to classify the
parasite. In 1925-1926, the disease was again conspicuous in the
vicinity of Gainesville, and in Manatee County, Florida. Since
1926, it has been found annually in the tomato sections of the
State. Collections were made and laboratory examination re-
vealed fungous spores on the lesions on both surfaces of the leaf.
The fungus was obtained in pure culture, its identity and patho-
genicity were established and a preliminary report of these
studies has been published (30), in which the name Stemphylium
solani sp. nov. was assigned the parasite. Since this report was
published further information has been obtained in both field and
laboratory relative to the parasite, the disease and control meth-
ods.
THE DISEASE
Geographical Distribution:-Gray leafspot was first discovered
in the vicinity of Gainesville, Florida, on Globe tomato plants
growing in an abandoned field in the summer. A survey showed
that its distribution was more or less general in Alachua County
at that time. The lateness of the season prevented further sur-
vey work in the more southern tomato sections. Early in 1925,
the disease was found in several seedbeds in Manatee County,
near Palmetto. During the spring of 1926 it was found near
Terra Ceia, causing considerable damage in a field of bearing to-
matoes and, by the end of the season, the disease was scattered
over most of the county. In 1928 the disease showed up in the
extensive fields of Dade County on the lower east coast of Flor-
ida and was more or less common in most of the commercial plant-
ings examined. Since it has beer collected in these three widely
separated sections, it would be only natural to presume that it has
a more or less general distribution in Florida. The writer has not
examined tomato fields outside the boundaries of Florida for this
disease and no reports have been found in the literature concern-
ing its occurrence elsewhere.
Economic Importance:-The disease has not been found on the






Florida Agricultural Experiment Station


fruit in any stage of development. Consequently, its actual eco-
nomic importance cannot be easily calculated. In 1926, in a 30-
acre field near Terra Ceia, the disease was found generally dis-
tributed on the lower leaves (Fig. 1) of staked tomato plants,
which were in the stage of setting the first cluster of fruit. Dur-
ing the following three weeks, the leaves on the lower half of the
plants were killed and the infection had spread to the tops. All
of the leaves were finally shed because of the disease, except
small tufts at the growing tips. The fruit did not develop and
the planting was a total loss. Since that time, numerous produc-
ing fields in both the West and East Coast tomato sections have
been observed that suffered partial or total losses.
The disease is more or less common and often destructive in
seedbeds in both the West and East Coast tomato sections of the
State. Young seedlings are not killed previous to the transplant-
ing stage, except under conditions most favorable for the devel-
opment of the disease. When the disease is severe, however,
100 percent of the plants may be infected. At transplanting time,
infected plants carry the disease to the fields where it spreads and
causes disastrous results.
During the past season, certain fields were planted in the pre-
viously uncultivated lower Everglade section of Florida, 10 miles
distant from any place where tomatoes had been grown previous-
ly. One grower lost his first seedlings because of heavy rains and,
in order to produce a crop as early as possible, secured seedlings
from seedbeds in the old tomato-growing sections. The disease
was carried to the new location on the seedlings and finally re-
sulted in the abandonment of several hundred acres of planted
fields at the time the first picking should have been made. The


Fig. 2.-Check plant A, and the dis-
ease produced by artificial inoculation on
B, tomato; C, pepper; D, ground cherry;
E, eggplant; and F, Solanum aculeatissi-
mum.





Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 7

losses due to this disease amounted to about 5 percent of the total
crop during the seasons of 1925, 1926, and 1927. The losses to
Florida growers for 1928 have been estimated to be 15 percent,
for 1929, 10 percent, for 1930, 2 percent, and for 1931, 2 percent.


Fig. 3.-Spots produced by natural infection of Stemphylium solani on A,
tomato; B, Solanum verbascifolium.

Host Range:-This disease has been important only on toma-
toes, although it has been found occurring naturally on ground
cherries (Physalis pubescens L.), eggplants (Solanum melongena
L.), and peppers (Capsicum annuum L.) and caused considerable
defoliation of these plants (Fig. 2). Natural infection has also
been found on the wild solanaceous plants, Solanum aculeatissi-
mum Jacq., S. blodgettii Chapm. and S. verbascifolium L. (Figs.
3 and 4), all of which grow wild in the tomato-growing sections
of South Florida. A number of inoculations have been made dur-





Florida Agricultural Experiment Station


MIB

















Fig. 4.-Natural infection of leaves of A, Solanum bahamense; B, S. pseu-
do-capsicum; C, S. aculeatissimum; D, S. blodgettii; and E, pepper, caused
by Stemphylium solani.







Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 9

ing the progress of these investigations which show that the fol-
lowing plants are susceptible to the disease:


Solanum aculeatissimum Jacq.
Solanum aggregatum
Solanum bahamense L.
Solanum blodgettii Chapm.
Solanum carolinense L.
Solanum citrullifolium A. Br.
Solanum floridanum Shuttlw.
Solanum gilo
Solanum macrocarpum
Solanum melongena L.
Solanum melongena var. esculentum
Solanum nigrum L.
Solanum nigrum var. douglasii


Solanum nigrum var. guineense L.
Solanum pseudo-capsicum L.
Solanum rostratum Dunal.
Solanum texanum
Solanum torreyi Gray
Solanum tuberosum L.
Solanum verbascifolium L.
Physalis pubescens L.
Capsicum annuum L.
Capsicum frutescens L.
Lycopersicon esculentum Mill.
Lycopersicon lycopersicon (L)
Karst.


The disease on each of these hosts is very similar in that it
produces a distinct leaf spot. There is considerable variation in
the size, color and rate of development of the spots on the leaves
and their effect on the time and extent of defoliation of the dif-
ferent species of susceptible plants. In a number of hosts, when
infection is not severe, a flecking of the leaf blades appears, which
usually does not cause defoliation. There appears to be two pos-
sible reasons for the flecking, namely: unfavorable environmental


Fig. 5.-Tomato seedlings in seedbed showing defoliation caused by the
disease.







Florida Agricultural Experiment Station


conditions for infection and variations in susceptibility and re-
sistance among individual plants within a species.
Except for a single species found on tomato by Samuel (26) in
South Australia, Stemphylium spp. have not been reported un
tomatoes or other solanaceous plants. Certain species have been
reported on grapes (Vitis sp.) (1), heather (Calluna vulgaris
Salis.) (4), cranberry (Vaccinium macrocarpon Ait.) (6), oats
(Avena sativa L.) (11), apple (Pyrus malus L.) (12) (18)
(21) (24) (27), wheat (Triticum aestivumL.) (13) (14) (22),
holly (Ilex aquifolium L.) (28), cucumber (Cucumis sativus L.)
(20), orange (Citrus sinensis Osb.) (23), and certain other
plants (31). Other species of Stemphylium have been found in
milk, cheese, and butter (7). (17).
Symptoms:-Both in the seedbed and in the field, gray spot has
been found to be limited almost entirely to the leaf blades. Under
very favorable conditions, occasional lesions have been observed
on the petioles and the more tender parts of the growing stems. In
these instances, the spots are linear and parallel with the stem.
No infection has been found on the fruit.
Serious infections in the seedbed result in marked defoliation
of the plants without conspicuous yellowing (Fig. 5). In the field,
however, the yellowing of the lower leaves after the spots develop
is a conspicuous symptom. The disease advances rapidly from
the lower leaves to the growing tips of the branches. The af-
fected leaves die rapidly, become brown and are shed. In se-
vere cases all of the leaves are shed, except small infected leaves
at the growing tips of the branches, where the development of
new leaves and the destruction of them by the fungus have ap-
parently come to a balance, leaving small tufts of new leaflets
at the branch tips.
Gray leafspot first appears as minute brownish-black specks
(Figs. 1 and 6). In light infections, there may be one to several
spots on a leaflet, or, in severe infections, they may be so thick
that half the entire surface of the leaf blade is occupied by these
tiny spots, less than one millimeter in diameter. These spots
are occasionally marginal and, in such places, are somewhat
elongated or irregular in outline. On other portions of the leaf,
the spots are more or less circular, and variously scattered over
the blade, without any apparent restriction by the veins. The
individual spots show simultaneously on both surfaces of the
leaf and are surrounded by a narrow halo band when viewed with
transmitted light.







Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 11


Fig. 6.--Naturally infected tomato leaf showing condition shortly before
shedding occurs.


N






Florida Agricultural. Experiment Station


With reflected light, there is a sharp contrast between the
brownish spot and the normal green of the leaf blade. The small-
est spots, barely visible, measuring 1/10 to 1/5 millimeter in diam-
eter, do not usually show this sharp line of demarcation between
the diseased and healthy green tissue.
At this time there is no apparent yellowing of the leaf. As the
spots enlarge the central killed areas change from a brownish-
black to a grayish-brown and the color contrast between the
healthy and diseased tissue becomes pronounced. The whole spot
becomes somewhat shiny and glazed. These changes continue
until the spots attain a size of about two millimeters in diameter;
by this time there is, in most cases, a definite yellow area appar-
ent around them. These are seldom found with greater diameter
than two millimeters, except on the very oldest leaves near the
base of old plants. Under such conditions, individual spots may
attain a diameter of four millimeters or more.
As the centers of the spots dry out, they often crack from side
to side in various patterns. The central area may break away
entirely from the leaf, leaving jagged-edged holes of various diam-
eters in the center of the individual spots, giving a shothole ap-
pearance to the leaf when viewed in its entirety. It is often in
this stage that the yellowing of the entire leaflet becomes most
conspicuous, specially if the infection is severe. This condition
is followed in quick succession by drooping and wilting and, even-
tually, dying and shedding of the leaves.
The spores have not been found on the diseased areas until the
upper surface of the spot shows the grayish-brown color and the
glazed or shiny surface. They are readily found after that, ap-
pearing first on the erect conidiophores in the central portion of
the spot on both surfaces, but more plentifully on the lower sur-
face of the leaf blade. As the spot ages the fruiting area in-
creases and the conidiophores are found almost to the blackish
margin on both surfaces of the leaf. Occasionally, on the older,
somewhat yellowish leaves that are more or less shaded at the
base of the plant and appear to be subnormal in their functions,
the spots coalesce, involve and kill large areas of the leaf blades,
which become brown and dried. In contrast with other leaf spots
of tomatoes, caused by Alternaria sp., Macrosporium sp., and
Phoma sp., the Stemphylium spots are small, more regular and
evenly distributed, and do not enlarge rapidly. They are almost
circular, of a uniform grayish-brown over the killed area, and
show no concentric zonation, as is characteristic of spots caused







Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 13

by fungi mentioned above. Spots caused by Cladosporium sp.
differ from gray spot in that they appear as large yellow blotches
on the upper surface of the leaf, immediately above the infection
on the lower surfaces. The spots caused by Septoria sp. differ in
that they usually show a light colored central area or frogeye
spot that is more or less speckled with black pycnidia.

CAUSAL ORGANISM

Taxonomy:-The genus Stemphylium Wallr., including the
imperfect forms of certain fungi included under the Moniliales-
Dematiaceae-Dictyosporae classification, is at the present time
not clearly defined in its relationship to the genera Alternaria
Nees and Macrosporium Fries. The early descriptions of these
genera have been amended and the type material appertaining to
them has been reassigned by numerous writers, the more recent
being Elliott (8), Bolle (3) and Mason (16). The three genera
are similar in the following characteristics: more or less globose
or oblong to obclavate, muriform spores, hyphae and conidio-
phores different, and fuligineus hyphae, conidiophores and co-
nidia. They differ further, according to descriptions, in manner
of hyphal growth, whether decumbent, ascending or erect, in pro-
duction and shape of spores, whether single, catenulate, capitate,
subglobose or beaked. There are other minor details pertaining
to them that are used in various keys for differentiation that need
not be considered here. According to Saccardo (25) Alternaria
is separated from Macrosporium and Stemphylium by producing
conidia in chains. This differentiation in most cases is entirely
sufficient, but the difficulty arises when one attempts to separate
Macrosporium and Stemphylium on the basis of whether the
hyphae are erect, ascending, or decumbent. The terms can read-
ily be assigned to either genus or interchanged. Species within
each genus differ from each other to the limits of these descriptive
terms. Engler and Prantl (9), except in a few minor details,
follow the general scheme of Saccardo as mentioned above. El-
liott (8) made a study of Saccardo's key and descriptions given in
certain volumes of the Sylloge and concluded that "all obclavate,
ovate, cuneate or elongate-pointed spores of the Macrosporium-
Alternaria type form chains and belong to Alternaria."
"Of these spores, all globular, sarcinaeform, cubed or oblong
spores without apex or beak belong to Stemphylium." These ge-
nera are evidently separated from Macrosporium because of their







Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 13

by fungi mentioned above. Spots caused by Cladosporium sp.
differ from gray spot in that they appear as large yellow blotches
on the upper surface of the leaf, immediately above the infection
on the lower surfaces. The spots caused by Septoria sp. differ in
that they usually show a light colored central area or frogeye
spot that is more or less speckled with black pycnidia.

CAUSAL ORGANISM

Taxonomy:-The genus Stemphylium Wallr., including the
imperfect forms of certain fungi included under the Moniliales-
Dematiaceae-Dictyosporae classification, is at the present time
not clearly defined in its relationship to the genera Alternaria
Nees and Macrosporium Fries. The early descriptions of these
genera have been amended and the type material appertaining to
them has been reassigned by numerous writers, the more recent
being Elliott (8), Bolle (3) and Mason (16). The three genera
are similar in the following characteristics: more or less globose
or oblong to obclavate, muriform spores, hyphae and conidio-
phores different, and fuligineus hyphae, conidiophores and co-
nidia. They differ further, according to descriptions, in manner
of hyphal growth, whether decumbent, ascending or erect, in pro-
duction and shape of spores, whether single, catenulate, capitate,
subglobose or beaked. There are other minor details pertaining
to them that are used in various keys for differentiation that need
not be considered here. According to Saccardo (25) Alternaria
is separated from Macrosporium and Stemphylium by producing
conidia in chains. This differentiation in most cases is entirely
sufficient, but the difficulty arises when one attempts to separate
Macrosporium and Stemphylium on the basis of whether the
hyphae are erect, ascending, or decumbent. The terms can read-
ily be assigned to either genus or interchanged. Species within
each genus differ from each other to the limits of these descriptive
terms. Engler and Prantl (9), except in a few minor details,
follow the general scheme of Saccardo as mentioned above. El-
liott (8) made a study of Saccardo's key and descriptions given in
certain volumes of the Sylloge and concluded that "all obclavate,
ovate, cuneate or elongate-pointed spores of the Macrosporium-
Alternaria type form chains and belong to Alternaria."
"Of these spores, all globular, sarcinaeform, cubed or oblong
spores without apex or beak belong to Stemphylium." These ge-
nera are evidently separated from Macrosporium because of their







Florida Agricultural Experiment Station


Fig. 7.-Camera lucida drawings of Stemphylium solani in artificial cul-
ture showing: A, continued development of conidiophore and formation of
additional conidia; B, production of secondary and tertiary conidia from
original conidium still attached to original conidiophore.







Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 15

catenulate spores. The first important difference in the two clas-
sifications is, according to Saccardo, that Stemphylium sp. does
not produce spores in chains. The second difference is that El-
liott places all "elongate-pointed spore" types in the genus Alter-
naria. Bolle (3) states that Stemphylium differs from Alternaria
in the presence of round, generally four-celled conidia. Wiltshire
(32) apparently separates Stemphylium from Alternaria by the
beakless spores of the former, stating that both produce spores
in chains.
The fungus found parasitizing tomatoes in Florida has not been
observed to produce conidia in chains during the past six years,
either on hosts or on various artificial media. The conidia-are
acrogenous on the host and in young cultures. Later, in culture,
the conidiophore may elongate or branch, terminating in second-
ary spores but causing the original spore to appear pleurogenous.
This type of development, illustrated in Fig. 7, A, would appar-
ently make unfounded the proposal of Souza da Camara (27) to
set up a new genus known as Soreymatosporium for Stemphylium
species producing conidia pleurogenous!y and retaining those in
Stemphylium that produce conidia acrogenously. The majority
of the conidia are obtusely pointed at one end but not beaked and
more or less rounded at the opposite end. The conidia, when ex-
tremely young, are single-celled and with age and enlargement
become divided into as many as 50 or more cells, averaging prob-
ably half that many. The hyphae may be considered either de-
cumbent or ascending. The foregoing discussion is summarized
in the following key:
(a) Conidia produced in chains..................... ... Alternaria
(b) Conidia not produced in chains................ (1) or (2)
(1) Conidia beakless ......................... Stemphylium
(2) Conidia long beaked....................... Macrosporium
The senior author has placed this fungus in the genus Stemphy-
lium because the conidia are beakless and are not produced in
chains.
Comparisons have been made of the descriptions of other spe-
cies of Stemphylium that appear in literature. The species on
tomato differs from:
Stemphylium cucurbitacearum Osner....on cucumber (20)
Stemphylium ericocotonum ........... .on heather (4)
Stemphylium botryosum Oude .........on milk (7) (17)
Stemphylium graminis (Corda) Bon.... on oats (11)
Stemphylium congestum Newt..........onapple (4) (24) (12) (18) (21)
Stemphylium congestum var. minor Rueh.on apple (24)
Stemphylium maculans ...............on apple (21)
Stemphylium parasiticum (Thum.) Ell..on wheat (13)







Florida Agricultural Experiment Station


Stemphylium tritici Pat. .............on wheat (22)
Stemphylium citri Pat. & Chas.......... on oranges (23)
Stemphylium dendriticum da C......... on apple (27)
principally by the larger, more uniform type of spores.
Atkinson (2) described a leafspot of cotton caused by Macro-
sporium sp. which differs entirely from Stemphylium leafspots
on tomato, but pictures of the fungus appear very similar to the
fungus on tomato. Samuel (26) described a Stemphylium on
tomato from South Australia but his drawings show it to be dif-
ferent from the Florida fungus in its rigid, branching conidio-
phores and that it produced spots on the fruit which the Florida
parasite has not done. After careful consideration of descriptions
and illustrations of known species of Stemphylium, the tomato
fungus in Florida was found to be distinct and consequently was
described (30) as a new species.

STEMPHYLIUM SOLANI WEBER
Hyphae dark, variously branched, septate and intercullar. A dense
growth is made on various media with a conspicuous and abundant produc-
tion of conidia (Fig. 8). Conidiophores dark, septate, slightly larger than
the infertile hyphae, rigid, 130-200 x 4-7u, swollen tips and with irregular
shaped bases. Conidia produced acrogenously on simple septate conidio-
phores on the host and on potato dextrose agar after 6-10 days. In older
cultures, conidiophores become branched and conidia often germinate in situ,
producing 1-8 secondary conidia and occasionally tertiary conidia slightly
smaller than the secondary conidia. Conidia muriform, fuscous to very dark,
oblong-rectangular, more or less rounded at ends or one end often somewhat
obtusely pointed, several longitudinal septae, constricted near medial sep-
tum; other septations more or less irregular, transverse septae several to
many, depending on age of conidia, wall smooth when young, slightly reticu-
late after maturity, 50 percent or more of conidia measure 45-50 x 20-23u,
average 48.08 x 22.43u, ascigerous stage of fungus not known. Isolated from
leaves of Globe, Marglobe and Earliana tomatoes. Type material in Florida
Agricultural Experiment Station Herbarium.
Morphology:-On potato-dextrose agar Stemphylium solani de-
velops a scanty submerged, hyaline, undulating, septate mycelium
readily distinguished from the fuscous to black surface or aerial
hyphae. The submerged hyphae are slightly narrower than the
surface hyphae. The growing tips of the surface mycelium are
often somewhat swollen and more or less hyaline. They become
colored a short distance back from the growing tip. This growth
later becomes dense and quite opaque. The fertile and infertile
hyphae are not distinguishable and all aerial growing tips may
eventually develop spores. The septations are at right angles to
the axis of the hyphae, irregularly spaced and do not cause a
restriction of the hyphae. When grown in an undesirable en-
vironment which inhibits rapid growth, the hyphae become vacuo-







Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 17


Fig. 8.-Camera lucida drawings of Stemphylium solani showing: A, Co-
nidia from host, (1) young, (2) mature (typical), (3) old; B, Conidiophores
from host; C, Conidiophores and attached conidia from culture; D, Mycelium
from culture.






Florida Agricultural Experiment Station


late, irregularly septate and the cells develop various shapes. The
contents of the colored hyphae are more or less homogeneous,
while a fine granulation is often visible in the less densely colored
strands.
Wiltshire (31) found that certain Alternaria saltants developed
Stemphylium-like spores and Brett (5) showed that these salta-
tion phenomena appeared at intervals and that certain cultures
usually developed both Alternaria-like spores in chains and were
intermixed with Stemphylium-like spores and that this was in-
fluenced by the richness of the medium. In relation to Stemphyl-
ium solani, the writer has not found any indication of saltation in
cultures. The fungus has been grown almost continuously in
petri dish cultures on various nutrients for seven years and dur-
ing this time, the cultures were conspicuously free from any signs
of saltation.
The conidia are borne acrogenously at first. They are delicate-
ly attached to the slightly swollen rounded tip of the conidiophore,
which may be decumbent or ascending, usually the latter. At first
the conidia are small, oval to elongate, sing!e-celled structures.
The| rfi*dial crsh septum develops, dividing the young spore al-
most equally Annd causing definite restrictions on the sides where
it meets the walls. Later, a wall develops in the lower half of the
spore, \perpendicular to the cross septum. This wall may show
side branches before it is plainly- visible throughout its entire
length. After the lower half of the conidiun definitely shows
additional cell walls developing, the same process takes place in
the top half. The spore has doubled in size during this process.
The division of the spore into cells takes place rapidly and it be-
comes more or less rectangular in outline with a square or rounded
base, obtusely pointed tip and is definitely restricted near the
middle. Further ageing results in the formation of additional
cells in the spores and a more or less irregularity in the general
outline of the spore. The fungus does not fruit freely on the host,
except under favorable conditions.
Severe spotting of the leaves causes them to become yellow and
shed. They fall on the soil and rapidly become brown, although
they may occasionally cling to the plant, being pendant. The
leaves fallen on the soil are rapidly overgrown in their entirety by
the fungus which develops spores in abundance over the whole
area not in contact with the soil. These spores are so numerous
that they produce a black velvety appearance. They are normal
in every way and not distinguishable from spores developed on






Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 19

partially-killed leaves still attached to the plant. The length of
the conidiophores that develop on fallen leaves varies to a certain
extent in proportion to the available moisture. Those produced
under humid conditions may often be twice or three times as long
as those produced on the drier, more exposed portions of the leaf.
Physiology:-Cultural studies of the fungus have been in prog-
ress during the past five years and at no time has there appeared
on the different culture media fruiting structures that in any way
resemble the perfect or perithecial stage. Stemphylium solani
was isolated in pure culture on potato-dextrose agar by planting
surface sterilized leafspots collected in nature, by pouring dilu-
tion plates of the spores produced on the leafspots occurring
under natural conditions and by making dilution plates of spores
collected from leaves that had shed and become covered with
spores while resting on the soil. Certain short cuts in the method
of making single spore isolations of the fungus have been devel-
oped. Keitt (15) first described a method by which the spores
were poured in dilution agar plates in petri dishes, located with a
microscope through the bottom of the petri dish, marked by ink
dots on the glass and later, upon germination, removed by special
aseptic methods. Ezekiel (10) offered an improvement in which
he placed the spore dilution on the surface of the poured agar
plate in narrow strips by means of a flattened transfer needle.
By this method, the spores were deposited in a single plane and
consequently they were more easily found. The writer, in pour-
ing agar plates, added two or three drops of 2 percent lactic acid,
depending on the amount of the medium used, to the petri dish in
one spot close to the edge after the melted agar had been poured
into it and before it had congealed. The dish was not disturbed
until the agar congealed, but the acid penetrated the medium for
a considerable distance so that in a single dish a range of acidity
was obtained from about pH 3.00 to the original pH of the me-
dium. The point of insertion of the acid was marked on the glass
plate. The spore suspension was spread over the surface of the
plate in strips converging toward the point the acid was placed,
by means of a small sterilized cotton swab attached to a transfer
needle. The cotton swab was used in preference to a flattened
needle because of the difficulty of making continuous strips across
the surface of the agar without cutting it. A fine-pointed stip-
pling pen is preferred to others in marking the location of the
spores, because it makes smaller dots which can be placed more
accurately while being observed through the microscope.






Florida Agricultural Experiment Station


Having been located, the single spores are transferred by the
use of a spatula-tipped transfer needle. The uncovered culture
dish is raised in a slightly tilted position from the vertical so that
the ink dots can be observed by transmitted light through the
medium. The dish is held with one hand while the spore is
scoopped up with a very thin layer of agar by the transfer needle
held in the other hand. The single spores are placed on the sur-
face of a second hard agar plate. About 20 transfers are made
to a single plate and there they are examined for several days.
After becoming thoroughly acquainted with this fungus in cul-
ture the following method has been used almost entirely. Tomato
leaves containing spots observed to be producing spores were held
several inches above an uncovered poured agar plate and were
scratched and rubbed with a needle. This freed the conidia and
they fell into the petri dish. They were located under the micro-
scope, marked and transferred after germination. This method
is very rapid, efficient, and convenient, especially with some of the
large, loosely attached spores of the fungi belonging to the Monil-
iales.
The fungus grows well on all of the commonly used laboratory
media, probably better on 2 percent potato-dextrose agar than
on any of the others. Consequently, this medium has been used.
Growth takes place rapidly and usually covers the surface of the
medium in petri dishes in about 100 hours at room temperature.
Cultures placed on a laboratory shelf exposed to daylight and
darkness show concentric zones in contrast to no visible zones
in cultures grown in complete darkness. Cultures grown in al-
ternate daylight and darkness until half the plate was covered
showed the concentric zones; when then grown in darkness until
the plate was completely covered, the zoning did not continue.
Where grown first in darkness and then in daylight and darkness,
the central portion was not zoned and the outer portion was zoned.
Temperature relations of the fungus were determined by plac-
ing a series of petri dish cultures in dark incubators at different
constant temperatures. After three days, the cultures were re-
moved and the data taken. The results showed the minimum
temperature for perceptible growth for this period of time to be
10C. The optimum temperature for growth was 23-24C. and
the maximum temperature was 36C. or slightly below, as abso-
lutely no growth took place at 36C. (Fig. 9.) The growth of the
fungus at all the temperatures was of the same general texture
except at 32C. At this temperature the fungus colonies were






Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 21

much more dense than those grown at lower temperatures which
caused them to appear blacker than the others. These colonies
also showed a very sharp, more or less irregular margin, whereas
the colonies grown at lower temperatures developed very even
margins, thin at the edges and thickening toward the center.
Spores developed more abundantly at the higher temperatures
than at the low temperatures.
The media used for testing the growth of the fungus at dif-
ferent hydrogen-ion concentrations was made at one time in a
large quantity. It was then apportioned into different lots and
each of these lots was adjusted to a certain pH value, tubed,
labeled and sterilized. After sterilization, samples were taken
from each lot for further testing, while the remaining tubes were
poured into petri-dishes, inoculated with the fungus and in-
cubated. The sample tubes were tested for the hydrogen-ion con-
centration by the electrometric method.


Fig. 9.-Petri dish cultures of Stemphylium solani growing on potato
dextrose agar at different temperatures in Degrees Centigrade as indicated.





Florida Agricultural Experiment Station


The growth of the fungus on potato-dextrose agar adjusted to
different hydrogen-ion concentrations showed very little growth
at pH 3.3. At pH 4.2, the growth was quite characteristic but


3.3 4.2 5.1 .



i'O
6.9 .3


.1,10 *1



Fig. 10.-Petri dish cultures of Stemphylium solani showing colonies grow-
ing on potato dextrose agar adjusted to different pH values as indicated.

somewhat restricted (Fig. 10). The growth between pH 5.1 and
pH 7.8 was typical of the normal development of the fungus de-
scribed previously. At pH 8.3 and beyond, the growth showed
inhibiting effects of the alkali.
Germination of spores of average development ordinarily takes
place in tap water in less than two hours (Fig. 11). Preceding
the development of germination tubes, the spores swell very little.
The germination tubes may appear from a single cell of the spore
or there may be a half dozen making their appearance almost
simultaneously from as many different cells of the spore. They
are large, septate, and with age become dark brown. A clump of
mycelium is developed by the profuse branching of the hyphae.
The dark brown colored hyphae and conidiophores, which
are distinguished with difficulty, grow into a macroscopically
black fungus colony. The spores germinate as well on the surface
of potato-dextrose agar plates adjusted to pH 6.5 as in tap water.
They tolerate a high hydrogen-ion concentration, as determined
by a Youden hydrogen-ion concentration apparatus, before germ-
ination is inhibited. The minimum germination takes place at
pH 4 and below and pH 8.6 and above, the maximum at about 6.2
-6.8 or slightly on the acid side of neutrality. The spores






Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 23


1


Fig. 11.-Camera lucida drawings of Stemphylium solani showing: A,
Germinating conidia; B, Mycelium produced by conidiophores; C, Mycelium
produced by fragments of mycelium; D, (1) single conidium, (2) same as
(1) after one hour in water showing germination tubes, and (3) growth of
germination tubes during second hour.






Florida Agricultural Experiment Station


developed on the host plant have germinated more quickly and
shown more vigor in the subsequent development of the colony
than spores developed in pure culture, even though spores from
both sources usually show 100 percent germination. Spores col-
lected from diseased, fallen tomato leaves have shown germina-
tion tubes after 30 minutes when sown on agar plates. Colonies
of the fungus resulting from the growth of these single spores
have produced new spores that would germinate after 30 hours,
thus establishing an almost unbelievably short time for the cycle
from spore to spore in artificial culture.
The fungus sporulates readily in culture on all of the nutrient
media used. The formation of conidia begins a short distance
back from the advancing margins of the colony. The conidio-
phores are more or less ascending, producing the conidia on their
tips in extreme regularity and in close proximity to each other.
With age, in culture, the hyphae grow around and over the
conidia, hiding them. The older conidia usually become extremely
irregular in shape and often much larger than the average. They
appear to germinate in situ, producing from one to eight or 10
germ-tube-like projections from as many different cells (Fig. 7,
B). These projections which are more or less rigid, dark colored,
septate and from 4 to 20 times longer than the spores, function as
conidiophores and produce secondary conidia of average size,
shape and color on their tips. The length of these secondary
conidiophores varies according to the previously mentioned ex-
tremes on a single "mother-spore" and extend at right angles
from the surfaces of the spore. The conidia are easily detached
and germinate on a nutrient medium in a normal way.
The conidiophore, having produced a conidium at its tip, may
renew growth and elongate from the point of attachment to the
spore (Fig. 7, A). This growth automatically causes the conidium
to become pleurogenous. This conidiophore may produce a second
conidium which is acrogenous, as was the primary conidium.
Broken pieces of the hyphae and conidiophores in culture dishes
produce mycelium and in this way initiate cok!nies of the fungus.
In nature no mycelium of the fungus is visible on the surface of
the diseased area of the host plant, except the conidiophores. The
conidiophores arise from the stomata and are erect, more or less
rigid and short, bearing a conidium at the tip.
Pathogenicity:-The pathogene was isolated from the host
many times by methods already given. It grew and produced
conidia abundantly in culture. The conidia thus produced were






Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 25

used in a water suspension as inoculum for determining the
pathogenicity and host range of the fungus.
The conidia were obtained by loosening them in water, with a
camel's hair brush, from the mycelium growing in petri dishes.
Masses of mycelium also were broken loose by this process but
the large wefts were removed by straining through a 50-mesh

















Fig. 12.-Check plant (A) and artificially inoculated plants (B and those
to right) showing development of disease five days after inoculation.

screen. This excluded any material that would not easily pass
through a DeVilbiss atomizer. In this manner a heavy spore
suspension was readily obtained.
The plants that were inoculated were all grown in the green-
house in seedbeds and later placed in pots. The plants were
usually in the seedling stage, at least prior to blossoming, when
inoculated. The inoculum was atomized onto the foliage and
stems so that the plant parts were entirely wetted by it. The
leaves were not rubbed or wounded either prior to or after being
inoculated. After they were inoculated the plants were placed
in a moist chamber for a period of 36 hours, and then they were
removed and placed on a greenhouse bench at a temperature of
about 65-700F. The plants were examined daily and usually the
disease developed enough for recognition within three or four days
after inoculation (Fig. 12). The disease often showed up on cer-
tain tomato varieties the second day.
Inoculation experiments were conducted over a period of several
months during different years but for convenience the results have







26 Florida Agricultural Experiment Station

all been included in Table I. In some instances, the conidia were
removed from diseased tomato leaves and used directly as in-
oculum, while other experiments were conducted in which the in-
oculum was made up of conidia of different ages. However, the
results were always comparable. No variation in incubation
period, severity or time of infection was noted.

TABLE I.-RESULTS OBTAINED BY INOCTILATING DIFFERENT SPECIES OF
SOLANACEOUS PLANTS WITH Stemphylium solani.


Plants


Physalodes physalodes
(L) Britt.* ........
Physalis angulata L.* ;
Physalis francheti
Hort. .............
Physalis pubescens L..
Solanum aculeatissi-
mum Jacq.* .......
Solanum aggregatum .
Solanum bahamense L.*
Solanum balbisii Dun..
Solanum blodgettii
Chapm.* ..........
Solanum capsiastrum
Link. ............
Solanum carolinense L.*
Solanum citrullifolium
A.Br. .............
Solanum echinatum ...
Solanum flavum ......
Solanum floridanum
Shut.* ............
Solanum gilo ........
Solanum gracile Link.*
Solanum ledorodendron
Solanum macrocarpurm
Solanum melongena L..
Solanum melongena
var. esculentum Nees
Solanum mexicanum ..
Solanum ministrum ..
Solanum nigrum L..;.


Number
Inocu- Dis-
lated eamed


0
0

0
42

49
12
12
0

12

0
12

12
0
0

12
12
0
0
12
12

47
0
0
0


Plants

Solanum nigrum var.
guineense L. ......
Solanum nigrum var.
douglassii .........
Solanum pseudo-capsi-
cum .... ......... ..
Solanum pyracanthum
Jacq .. ...........
Solanium racenigrum..
Solanum rostratum
Duval* ...........
Solanum sisymbrii Sam
Solanum sisymbrifol-
ium Lan.* .........
Solanum texanum ....
Solanum torreyi Gray*
Solanum tuberosum L..
Solanum verbascifol-
ium L.* ..........
Lycopersicon esculen-
tum Mill..........
Lycopersicon lycoper-
sicon (L) Karst.* .
Capsicum annuum L.'.
Capsicum flutescensL.*
Lycium carolinianum
. W alt.* ...........
Cestrum diurnum L.*..
Cestrum nocturnum L.
Datura metel L.* ....
Datura stramonium L.*
Nicotiana tabacum L...
Petunia hybrida Vilm..


Number
Inocu- Dis-
lated eased


*Native or naturalized in Florida.

The host plants used in these experiments include representa-
tives of the genera of the Solanaceae, native or naturalized, in
Florida and the important cultivated crop plants of this family
in the State. It is of interest to note that no species of the genera
Physalodes, Lycium, Cestrum, Datura, Nicotiana and Petunia in
the Solanaceae were infected by Stemphylium solani. The culti-
vated plant, Physalis pubecens L., was the only species of that
genus inoculated that became diseased. Two species of each of






Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 27

the genera Lycopersicon and Capsicum, one native and one cul-
tivated, became infected. There was considerable variation
among the species of Solanum in this respect. All of the species
of Solanum native to or naturalized in Florida (marked by an
asterisk), with the exception of S. gracile and S. sisymbrifolium,
were readily infected. S. gracile occasionally showed flecks in
the inoculated leaf blades that indicated incipient infection, but
the fungus did not sporulate or even kill host tissue and since its
definite establishment did not take place, infection was not con-
sidered as completed. This situation also was applicable to
Physalodes physalodes, Physalis angulata, Datura stramonium
and several others. Under conditions most favorable for the
development of the fungus, infection took place on Solanum
nigrum, S. rostratum and S. tuberosum. Under field conditions,
S. gracile was not infected by artificial inoculations. The disease
was never observed on this plant, although it is the most common
weed in tomato fields on the Florida East Coast and was often
found growing in abandoned tomato seedbeds in which most of
the tomato seedlings had been killed by Stemphylium solani. The
fungus is actively pathogenic on the following cultivated plants
in their respective order, the first being the most susceptible:
Lycopersicon esculentum, Physalis pubescens, Capsicum annuum,
Solanum melongena var. esculentum and S. tuberosum.
The fungus was readily reisolated from the diseased plant parts
following inoculation. It was also identified definitely by the
conidia which developed in most cases within a few days after the
appearance of the disease.

SEASONAL DEVELOPMENT OF THE DISEASE
The tomato growing seasons in the United States are divided
according to the time of production, namely, early, second-early,
intermediate, and late. The early tomato crop originates mostly
in subtropical areas, namely, Mexico, Cuba, and the closely ad-
jacent islands, and the peninsular portion of Florida, including
both the East and West Coast sections. Central, North and West
Florida are included with certain Gulf and South Atlantic States
in the second-early group. Tomato seedbeds are planted any time
from July to October on the lower East Coast depending on loca-
tion and during November and December on the West Coast. Thus,
the peak of production on the West Coast follows that of the East
Coast sections by about two months, varying somewhat with en-
vironmental factors.






Florida Agricultural Experiment Station


The disease begins in the seedbeds often when the plants are
in the first true leaf stage. Thus, its first appearance in the ex-
treme southern part of the State is in the late summer and early
fall. The development and spread of the disease usually keeps
pace with the growth of the seedlings under average conditions.
If their growth is rapid the disease is often scarce, while on the
other hand, if the development of the seedlings is below normal,
the disease may destroy the seedlings prior to transplanting time.
When the plants are removed to the field and planted, the period
of recovery and resumption of growth may be from a few days
to a week or two.
During this time, the disease, which may be barely visible when
the plants are set, makes considerable progress and occasionally
kills extensive plantings. Under average conditions the disease
is not severe enough over the whole section to cause extensive
losses. However, in certain instances, it has been the most im-
portant factor in preventing even a single picking of large acre-
ages.
The disease develops rapidly with the advent of warm weather
and with the additional aid of spore dissemination by the pickers
usually results in a short picking period and reduced yields. The
leaves of the plants become 100 percent diseased and are killed
rapidly and shed. This process continues until only limited tufts
or undersized leaves remain at the growing tips. The older ones
are infected and killed as rapidly as the young, new leaves develop.
This development of the disease takes place from Septembr to
March in the lower East Coast sections of Florida and six weeks
to two months later in the West Coast sections. Its spread is gen-
eral over the Central and Northern part of the State from this
time on until the end of the tomato season in mid-summer.
In Florida, the fungus remains viable on tomato plants which
grow in some areas of the State throughout the entire year. The
part of the year when tomato plants are not cultivated in the fields
in South Florida is from about April to August or September.
During this time, however, infected plants have been found grow-
ing in gardens and as volunteers in field and along roadsides which
undoubtedly function as sources of inoculum for the early fall
plantings in seedbeds. It was found that the fungus spores showed
94 percent germination after 14 months storage. In this experi-
ment several badly diseased tomato plants were enclosed in a cloth
bag and stored in a shed. Additional evidence on longevity of







Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 29

viability was obtained from old cultures. Spores developed on cul-
tures in petri dishes have been found viable after 19 months.
The spores are spread over extensive areas by the wind. Dur-
ing the growing season in South Florida, there is usually a con-
tinuous south, south-
east or east wind and
it has been observed
that the fungus has
spread in a north or
northwesterly direc-
tion across fields from
certain diseased seed-
beds along the south
edge of the fields
(Fig. 13). The usual
mode of spread of the
disease, however, is in
the transplanting of
diseased seedlings
from infected seed-
beds to the fields.
This was the case in
regard to a specific
field in the Ever-
glades, that was lo-
cated more than 10
miles from the near-
est tomato field. The
source of the inocu-
lum also may be cen- Fig. 13.-A badly infected abandoned seedbed
tered around wild so- acting as a source of inoculum for field plantings.
lanaceous plants, such
as Solanum verbascifolium, S. bahamense and S. aculeatissimum.
Natural infection takes place on the seedlings as soon as they
appear above ground, although the infection of cotyledons is usu-
ally not severe. A certain percent of infection has usually been
found in most of the seedbeds. The tomato plant is subject to
infection at any time during its life. All aerial parts of the plant
except the fruit have been found showing the disease. The stems
have shown less of the disease and the leaf blades most of the
disease, when compared with the other susceptible parts of the
plant. The spores germinate quickly in the presence of water and,







Florida Agricultural Experiment Station


under favorable conditions on a tomato leaf, produce extensive
hyphal growth over the surface of the leaf during a single night.
These hyphae penetrate the epidermis directly or gain entrance to
the host between the epidermal cells, in addition to direct growth
through the stomata. Within the host the fungus develops inter-
cellularly for 48 to 72 hours before the cells most severely affected
show indications of parasitism. The fungus enters the cells fol-
lowing an apparent weakening of them caused by the reaction of
the walls to some substance supplied by the fungus. After three
days infection can be distinguished macroscopically, because of
the browning of a few killed cells. The mycelium does not grow
extensively through the parenchyma tissue of the leaf, but be-
comes thickly matted and interwined and remains more or less
limited to a small area. A yellowed area develops around the
infection, but it does not become invaded by the penetrating
hyphae until after the leaf has been shed.
The conidiophores are short and erect on the host, protruding
from the stomata or directly through the epidermis of the killed
cells. The conidia are produced at their tips within a few hours
and remain in place until mechanically removed by some outside
agency. There is no evidence to show that a second conidium will
develop in place of the original one, should it be removed. The
conidiophores in nature are shorter than they are in petri dish
cultures, although on fallen infected leaves in moist places on the
soil often they are twice as long as on the host. On fallen infected
leaves they are very numerous and with terminal black conidia
produce a velvety surface. On leaves still attached to the host,
the conidiophores are more or less scattered.
About 200 tomato varieties and strains have been grown in ex-
perimental plots and all of them have shown the disease in nature.
The Marglobe developed for resistance to Fusarium wilt and later
found to be resistant to nailhead spot has not shown noticeable
resistance to this disease.

CONTROL
The control of this disease, for the present time at least, un-
doubtedly will require the application of fungicides (Fig. 14),
beginning in the seedbed, since existing varieties of tomatoes do






Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 31

not show resistance.
Sanitation may aid
somewhat but the
source of inoculum
cannot be eliminated
cheaply. Rotation is
not possible, since the
new, more desirable
fields are not plenti-
ful, although in the
past this kind of rota-
tion has been the
practice in most gen-
eral use. Many grow-
ers have attempted to
cultivate new land
each season, while Fig. 14.-Dusting tomato seedbeds for the con-
others attempt to trol of the disease.
grow tomatoes year
after year on the same land and leave it only after a number of
seasons, because the soil "becomes saturated with disease." The
first season or two in new locations have often been very satisfac-
tory but carelessness in regard to sanitation has prevented longer
periods of freedom from disease.
Experiments* for control of the disease by the application of
fungicides were inaugurated during the 1929 season in the lower
East Coast and West Coast sections of Florida in the vicinity of
Homestead and Bradenton, respectively. The fungicides were
applied in January and February, 1929, and continued weekly until
five to 10 applications were made to each plot. Each plot consisted
of four rows, approximately 200 feet long, set and spaced as in
commercial plantings. The fertilization, cultivation and pickings
were similar in all plots. The only variable was the fungicide. The
applications of both spray and dust were made on the same day
or on successive days. The dust was put on the plants early in
the morning when they were still wet with dew and little or no
wind was blowing, while the liquid sprays were usually applied in
the afternoon. The experiments were replicated a number of
times and the locations selected included representative fields in
the better sections of the district around Homestead and Braden-
*Conducted at Homestead by Stacy Hawkins and at Bradenton by D. G. A.
Kelbert,







Florida Agricultural Experiment Station


ton. The total yield in pounds of fruit was taken as the measure
of control obtained, since the disease does not involve the fruit
directly. The fungicides, both sprays and dusts, were applied by
hand machinery. The costs were calculated by combining the
retail cost of material and the cost of time-labor at the time the
experimental work was performed.


TABLE II.-RESULTS OBTAINED THROUGH THE USE OF FUNGICIDES FOR THE
CONTROL OF GRAY LEAFSPOT OF TOMATO AT HOMESTEAD AND BRADENTON
IN 1929.
Cost Yield in pounds
Size Replications Fungicide per plot per plot
1/8 A. 11 copper-lime dust $1.25 473
11 1-1.5-50 bordeaux .27 451
S11 copper carbonate 2.15 382
S11 sulphur 1.20 380
S12 4-6-50 bordeax .36 376
S9 2-3-50 bordeaux .30 372
S30 check .... 365

The important items of the above table are the number of
replications, the variation in costs, and the yields. The plots
treated with copper-lime dust produced the largest yield at a cost
of lA cents for each pound of fruit increase over check. The plot
sprayed with 1-1.5-50 bordeaux mixture, while producing less
total pounds of fruit, averaged 3.1 pounds increase for each 1 cent
spent for spraying. The plots dusted with copper carbonate and
sulphur produced fairly good yields, but the cost per pound in-
crease in each case is relatively high and the same is true of the
two remaining bordeaux-sprayed plots. The check plots produced
365 pounds of fruit at no additional cost. Consequently, the in-
creased yields of all of the plots, except the first two, show very
little advantage when costs are considered. The copper-lime
dusted plot, however, shows an increase of 108 pounds at an added
cost of $1.25, while the 1-1.5-50 bordeaux sprayed plot shows an
increase of 86 pounds at an added cost of 27 cents. From these
viewpoints the 1-1.5-50 bordeaux sprayed plots are the best,
producing an increased yield of 3.1 pounds of fruit for each 1 cent
of cost for disease control.
The control experiments for the season of 1930 were conducted
in the same manner as those of the previous year and the data are
summarized in Table III.







Bulletin 249, Gray Leafspot, a New Disease of Tomatoes 33


TABLE III.-RESULTS OBTAINED THROUGH THE USE OF FUNGICIDES FOR THE
CONTROL OF GRAY LEAFSPOT OF TOMATO AT HOMESTEAD AND BRADENTON
IN 1930.
Cost Yield in pounds
Size Replications Fungicide per plot per plot
1/8 A. 13 4-6-50 bordeaux $ .88 502
12 2-3-50 bordeaux .75 497
11 copper-lime dust 1.15 392
28 check .... 389
10 copper carbonate 2.30 388
S11 sulphur .75 292

The data above are quite comparable to those in Table II. It is
of interest to note, however, that the plots sprayed with the
bordeaux mixtures yielded better than the ones treated with the
copper-lime dust and that the ones dusted with copper carbonate
and sulphur did not equal the check plots in yield.
The plots sprayed with the weaker bordeaux mixture produced
1%A pounds of fruit for each 1 cent of fungicide cost, while the
plots sprayed with the 4-6-50 formula produced 1.4 pounds in-
crease of fruit for each 1 cent of fungicide cost.
Experiments also were conducted in which 1-1.5-50 bordeaux
mixture was applied to the seedbeds weekly until transplanting
time, beginning a day or two after the cotyledons emerged. The
sprayed plants were planted in the field beside unsprayed seed-
lings and observed during the season. The sprayed seedlings were
larger and free from the disease, while the unsprayed plants
showed considerable infection at transplanting time. The above
noted difference prevailed until toward the end of the picking
season, when all of them showed the disease.

SUMMARY

Gray leafspot of tomato caused by Stemphylium solani Weber,
a recently described fungus, is generally distributed in the to-
mato growing sections of Florida.
Losses caused by this disease have averaged approximately 5
percent of the total crop during the past seven years.
The disease has been found on tomatoes, peppers, eggplants and
ground cherries of the cultivated crop plants and a score of wild
plants in the Solanaceae family.
Symptoms of the disease and its distinguishing characteristics
are given.







34 Florida Agricultural Experiment Station

The causal organism is described and its taxonomic position,
morphological characteristics and physiological reactions are
stated. :i
Seasonal development, spore production and pathological
anatomy are described.
Control methods consist in weekly applications of 1-1.5-50 bor-
deaux mixture in the seedbeds followed by field spraying with
2-3-50 bordeaux mixture.






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