• TABLE OF CONTENTS
HIDE
 Title Page
 Credits
 Table of Contents
 Introduction
 Downy mildew
 Bacterial diseases
 Damping-off
 Anthracnose
 Gray mold
 Bottom rot
 Tip-burn
 Mosaic
 Seeding
 Alternaria blight of escarole
 Southern blight
 Leaf spots
 Other diseases
 Seeded sterilization
 Seed disinfection














Group Title: Bulletin - University of Florida. Agricultural Experiment Station ; no. 195
Title: Diseases of lettuce, romaine, escarole, and endive
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00027364/00001
 Material Information
Title: Diseases of lettuce, romaine, escarole, and endive
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: p. 299-333 : ill. ; 23 cm.
Language: English
Creator: Weber, George F ( George Frederick ), b. 1894
Foster, A. C
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1928
 Subjects
Subject: Lettuce -- Diseases and pests -- Florida   ( lcsh )
Belgian endive -- Diseases and pests -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by G.F. Weber and A.C. Foster.
General Note: Cover title.
General Note: "In cooperation with the Office of Vegetable and Forage Diseases, Bureau of Plant Industry, U.S. Department of Agriculture"--T.p.
Funding: Bulletin (University of Florida. Agricultural Experiment Station) ;
 Record Information
Bibliographic ID: UF00027364
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000923502
oclc - 18173113
notis - AEN4053

Table of Contents
    Title Page
        Page 301
    Credits
        Page 302
    Table of Contents
        Page 303
        Page 304
    Introduction
        Page 305
        Page 306
        Page 307
    Downy mildew
        Page 308
        Page 309
        Page 310
    Bacterial diseases
        Page 311
        Page 312
        Page 313
    Damping-off
        Page 314
        Page 315
        Page 316
        Page 317
    Anthracnose
        Page 318
    Gray mold
        Page 319
        Page 320
    Bottom rot
        Page 321
        Page 322
        Page 323
    Tip-burn
        Page 324
    Mosaic
        Page 325
        Page 326
    Seeding
        Page 327
    Alternaria blight of escarole
        Page 328
    Southern blight
        Page 328
    Leaf spots
        Page 329
    Other diseases
        Page 330
    Seeded sterilization
        Page 330
        Page 331
    Seed disinfection
        Page 332
        Page 333
Full Text



Bulletin 195


UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
In Cooperation with the Office of Vegetable and Forage Diseases,
Bureau of Plant Industry, U. S. Department of Agriculture.



DISEASES OF LETTUCE, ROMAINE,

ESCAROLE, AND ENDIVE

By
G. F. WEBER
Associate Pathologist, Florida Agricultural Experiment Station
and
A. C. FOSTER
Pathologist, Vegetable and Forage Diseases, Bureau of Plant.
Industry, U. S. Department of Agriculture


Fig. 123.-Drop disease in the lettuce field.

Bulletins will be sent free upon application to the
Agricultural Experiment Station
GAINESVILLE, FLORIDA


April, 1928







BOARD OF CONTROL
P. K. YONGE, Chairman, Pensacola E. L. WARTMANN, Citra
E. W. LANE, Jacksonville J. T. DIAMOND, Secretary, Talla-
A. H. BLANDING, Leesburg hassee.
W. B. DAVIS, Perry J. G. KELLUM, Auditor, Tallahassee
STATION EXECUTIVE STAFF
WILMON NEWELL, D. Sc., Director ERNEST G. MOORE, M. S., Asst. Ed
JOHN M. SCOTT, B. S., Vice-Director IDA KEELING CRESAP, Librarian
S. T. FLEMING, A. B., Asst. to Di- RUBY NEWHALL, Secretary
rector K. H. GRAHAM, Business Manager
J. FRANCIS COOPER, B. S. A., Editor RACHEL MCQUARRIE, Accountant
MAIN STATION-DEPARTMENTS AND INVESTIGATORS


AGRONOMY
W. E. STOKES, M. S. Agronomist
W. A. LEUKEL, Ph. D., Asso.
C. R. ENLOW, M. S. A., Asst.*
FRED H. HULL, M. S. A., Asst.
A. S. LAIRD, M. S. A., Asst.
ANIMAL INDUSTRY
JOHN M. SCOTT, B. S., Animal
Industrialist
F. X. BRENNEIS, B. S.A., Dairy
Herdsman
CHEMISTRY
R. W. RUPRECHT, Ph.D., Chemist
R. M. BARNETTE, Ph. D., Asst.
C. E. BELL, M. S., Asst.
.H. L. MARSHALL, M. S., Asst.
J. M. COLEMAN, B. S., Asst.


BRUCE MCKINLEY, B. S. A., Asst.
M. A. BROKER, M. S. A., Asst.
ECONOMICS, HOME
OUIDA DAVIS ABBOTT, Ph. D., Chief
L. W. GADDUM, Ph. D., Asst.
C. F. AHMANN, Ph. D., Asst.
ENTOMOLOGY
J. R. WATSON, A. M., Entomologist
A. N. TISSOT, M. S., Asst.
H. E. BRATLEY, M. S. A., Asst.
HORTICULTURE
A. F. CAMP, Ph. D., Asso. Hort.
M. R. ENSIGN, M. S., Asst.
HAROLD MOWRY, Asst.
G. H. BLACKMON, M. S. A., Pecan
Culturist


J. B. HESTER, B. S., Asst. PLANT PATHOLOGY
COTTON INVESTIGATIONS G. F. WEBER, Ph. D., Asso.
W. A. CARVER, Ph. D., Asst. K. W. LOUCKS, B. S., Asst.
M. N. WALKER, Ph. D., Asst. ERDMAN WEST, B. S., Mycologist
E. F. GROSSMAN, M. A., Asst. VETERINARY MEDICINE
RAYMOND CROWN, B.S.A., Field Asst. A. L. SHEALY, D.V.M., Veterinarian
ECONOMICS, AGRICULTURAL D. A. SANDERS, D. V. M., Asst.
C. V. NOBLE, Ph. D., Ag. Economist E. F. THOMAS, D. V. M., Lab. Asst.
BRANCH STATION AND FIELD WORKERS
W. B. TISDALE, Ph. D., Plant Pathologist, in charge, Tobacco Experiment
Station (Quincy)
ROSS F. WADKINS, M. S., Lab. Asst. in Plant Pathology (Quincy)
JESSE REEVES, Foreman, Tobacco Experiment Station (Quincy)
J. H. JEFFERIES, Superintendent, Citrus Experiment Station (Lake Alfred)
W. A. KUNTZ, A. M., Assistant Plant Pathologist (Lake Alfred)
R. L. MILLER, Assistant Entomologist (Lake Alfred)
W. L. THOMPSON, Assistant Entomologist (Lake Alfred)
GEO. E. TEDDER, Foreman, Everglades Experiment Station (Belle Glade)
R. V. ALLISON, Ph. D., Soils Specialist (Belle Glade)
J. H. HUNTER, M. S., Assistant Agronomist (Belle Glade)
J. L. SEAL, Ph. D., Assistant Plant Pathologist (Belle Glade)
H. E. HAMMAR, M. S., Field Assistant (Belle Glade)
L. O. GRATZ, Ph. D., Associate Plant Pathologist (Hastings)
A. N. BROOKS, Ph. D., Associate Plant Pathologist (Plant City)
A. S. RHOADS, Ph. D., Associate Plant Pathologist (Cocoa)
STACY O. HAWKINS, M. A., Field Assistant in Plant Pathology (Homestead)
D. G. A. KELBERT, Field Assistant in Plant Pathology (Bradenton)
R. E. NOLEN, M. S. A., Field Assistant in Plant Pathology (Monticello)
FRED W. WALKER, Assistant Entomologist (Monticello)
E. D. BALL, Ph. D., Associate Entomologist (Sanford)

*In cooperation with U. S. Department of Agriculture.















CONTENTS


INTRODUCTION .......................

D ROP .............- ...- ...... .....

DOWNY MILDEW ...............

BACTERIAL DISEASES ...................

DAMPING OFF -....-......-..

ANTHRACNOSE ... .....-.........

GRAY M OLD ........ ..........................

BOTTOM ROT ......... ... ..........

TIP BURN ............. .......

M OSAIC ........................... .

SEEDING ............ --. ............

ALTERNARIA BLIGHT OF ESCAROLE.

SOUTHERN BLIGHT ................

LEAF SPOTS ......... .........- ......

Alternaria sp. ............ ...

Septoria lactucae Pk. ...........

Cercospora lactucae Stev ....

OTHER DISEASES .................

Rio Grande disease ............

Fusarium wilt .................

Ozonium blight .............

Brown blight .............-

SEEDBED STERILIZATION .............

SEED DISINFECTION ........... -


Page

................ .................... ...................... 303

.-...- ..-........- .......-.. ... .. .............. ..... 304

-.-.......-.-.. -- ........ ... ........ ... 308

........... ..... ..................... ........... ... 311

..................................... .................... 314

........ .......-........--....... .. ....--. .. 318

..... .......................... ...... ...... .......... 319

.......... .............................. ................. 32 1

-.-..-............ .. .. .... .... .............. .... 324

-.... --..-. -.. --.. --.. -....... ...-....--------- ... 325

.. -.....-..-.........- --- ----- -----.....-- 327

... .. ....................................... 328

...... -.-.......-.-- ....- ....... ........... ... 328

.....-.-.....- ..- ..-.- ..-.- ..- ...-.- ......... 329

.. ..... ..... .. 330


... -... -..........- ....- .-.. .......... ... .. 3 3 0

.. ...... ............. .................... ... .... 330
-------------- 330



.... ............... .................... ...... ... ..... 330
--.- -- -- --- ---- ----. .........-... 330


..... ................ 330
...- ..-.- ........ ...........- .......- ......-- .....- ....-.-.-.... 3 3 0

- ---. -. -. .-.--- - -- -- -.- ----. -.- .-.---- 3 3 0

........... .. -... ..- ..- ..- .. .. ...... ........ 330

.....-.-......- ......-..- ...... ............. 332












DISEASES OF LETTUCE, ROMAINE, ESCAROLE,
AND ENDIVE
By G. F. WEBER AND A. C. FOSTER*
INTRODUCTION

The production of lettuce in Florida has been decreasing dur-
ing the past five years. In 1922 there was a production of over
3,000 cars, and in 1927 the production was less than 1,000 cars.
This decrease in production may be attributed primarily to two
causes; the heavy losses sustained by growers due to the preva-
lence of diseases in the field and decay in transit, and a decline
in the market price for the Big Boston type of lettuce.
The market price is regulated principally by the supply of
lettuce on the markets and the condition of that commodity at
the time it is purchased by the retail trade. Because of the suc-
culent tender condition of lettuce at all times it is more subject
to decay than almost any other vegetable, and its freedom from
decay at arrival on the markets is determined mainly by the way
it is handled at time of harvest, the way it is packed in crates
or hampers and the methods of loading in refrigerator cars.
Proper refrigeration and precooling of the car is almost essen-
tial to insure good condition on arrival.
It is the purpose of this bulletin to describe the diseases of
lettuce and related crops and give in so far as they are available
control measures. Unfortunately, many of the diseases of these
plants cannot be satisfactorily controlled or prevented in the
field, but by careful handling of the crop at time of harvest and
by following the advice given as to handling, crating and pre-
cooling, it is thought that the suggestions offered will in many
cases greatly reduce the losses incurred by the growers.
The scope of the bulletin is limited to the diseases found in
the field and those which subsequently in transit cause decay
of lettuce, Lactuca sativa var. capitita and the closely related
plants, Romaine, Lactuca sativa romaina; Endive, Cichorium
endiva; and Escarole, Cichorium intybus. These plants are grown
during the same season and in the same localities in the state.
Lettuce is far more important commercially than the other
three combined. This crop is followed successively in importance
*Weber, G. F., Associate Plant Pathologist, Florida Agricultural Ex-
periment Station; Foster, A. C., Pathologist, Vegetable and Forage Dis-
eases, Bureau of Plant Industry, United States Department of Agriculture.






Florida Agricultural Experiment Station


which are microscopical and are scattered by wind and rain.
The production of apothecia usually begins in the fall and con-
tinues during late winter. They are usually found in refuse piles
in the woods where the previous season's debris from the field
has been dumped, and also in the field itself, especially when a
spring crop of lettuce or celery has been grown in this soil.

SYMPTOMS
The first symptom on a plant that is attacked by the drop fun-
gus is a general lack of turgidity. The older leaves appear to be
limp and wilted, gradually the wilting increases, involving more
of the leaves toward the heart of the plant until all of the loose
leaves not in the head are prostrate on the ground. The heart
leaves sink into a soft mass. This condition is rapidly followed
by a drying and browning of the infected tissues.
Upon removing from the soil a plant which shows the first
wilting symptoms one can usually find a more or less cottony de-
velopment of the fungus itself around the main stem of the
plant in the cavities between
the petioles of the leaves and
Sthe surface of the soil. The
Main stem is completely
girdled and rotted through
S. b and in most instances will
S" break off close. to the soil
line when the head is pulled
up.
U At the time the wilting ap-
pears on the plant the scle-
rotia have not yet developed,
but with the drying out of the
decayed tissues numerous
Fig. 125.-Sclerotia of various shapes coal black bodies irregular in
taken from a diseased lettuce plant. size and shape develop on the
surface or within the tissues of the diseased head. These scle-
rotia might be compared to seeds of a plant in that they are
able to preserve the fungus even though they become dried out
and are broken up. They retain their vitality for considerable
time and when conditions during the following season or year
favor their development they are able to produce the fungus
which in turn attacks the plants. The season favorable for their







Bulletin 195, Diseases of Lettuce, etc.


development is also favorable for the development of the host
plant, and consequently the growth of the two at the same time
under the same favorable conditions makes the control of this
disease extremely difficult.

CONTROL OF DROP
The application of fungicides is not to be recommended, prin-
cipally because these plants are usually consumed without cook-
ing in different salad preparations, and also because the fungus
is soil inhabiting and attacks the plant from the soil line where
it cannot be reached with a spray solution.
It is possible, however, to gain considerable control of drop
by the practice of strict sanitation. The entire plant should be
removed from, the soil at the time it shows the first symptoms of
wilting. Such timely removal will prevent the scattering of the
later developing sclerotia in the soil and thus remove the source














Fig. 126.-Upon germination, the sclerotia develop apothecia. The mush-
room-like disks contain the spores of the fungus.

of the spread of the fungus for the following season. It is espe-
cially advantageous to practice sanitation on new ground, on
sections that are just being prepared for the cultivation of crops
attacked by the disease. This practice, though not adhered to at
the present time, will undoubtedly be an important factor where
truck crops follow each other successively.
To further eliminate the disease, a very definite system of
crop rotation should be followed where possible. If the disease
is especially severe, the soil should not be planted to crops at-
tacked by the parasite, but if it is absolutely necessary to plant






Florida Agricultural Experiment Station


crops, grow for a fall or spring crop those that are not so severe-
ly attacked, such as peppers. Romaine is not so severely at-
tacked as Big Boston, and because of the upright nature of its
growth may be found to be a desirable substitute. On badly in-
fested soil it is especially important to discontinue the growth
of lettuce, celery, cabbage, eggplants and beans. Seedbed treat-
ment is also extremely important since drop is spread from
seedbed to the field at transplanting time.
Seedbed treatment is one of the most important and advisable
practices to follow before attempting to grow lettuce. The im-
portance of this will be realized when the grower recognizes that
practically all of the lettuce diseases may have their origin in
the seedbed. If healthy plants are set in the field, the quick
growth of lettuce will usually bring it to maturity before dis-
eases having their origin in the field can cause heavy losses.
The treatment of the seedbeds (see page 331) either with live
steam, by the inverted pan method, or with formaldehyde not
only kills off many of the important soil inhabiting organisms,
but also kills weed seeds and nematodes or root-knot organisms,
and leaves the soil in a better physical condition.

DOWNY MILDEW
Downy Mildew is caused by the fungus Bremia lactuca Reg.
It is found in most of the lettuce growing sections of the United
States, both under glass and in the field. The occurrence of this
disease in Florida is widespread, and in most sections it is con-
sidered common.
The fungus attacks all of the specific host plants dealt with in
this publication and, because of the extensive planting of lettuce,
causes greater losses with this crop than all of the others. It has
also been found on a number of the wild host plants belonging
to the genera Lactuca, Sonchus, Gnaphalium, etc., which abound
in Florida in waste places during most seasons of the year, thus
acting as centers of inoculum from which the fungus is spread
to the cultivated crops. It would be somewhat difficult to make
an estimate of the losses resulting from the presence of this
disease principally because it is followed up by numerous other
organisms that do considerably more damage once they have
entered the host plant.
Downy mildew is widely distributed in all sections of Florida,
growing in the seedbeds and in the field, and does its greatest


308







Bulletin 195, Diseases of Lettuce, etc.


damage while the commodity is in transit. Direct losses from
downy mildew in the seedbed are seldom serious, but its occur-
rence there is of considerable importance, since it is spread from
there throughout the field at transplanting time. Losses in the
field are not of considerable importance in this state, and only
occasionally are plants so severely attacked at cutting time, or
slightly before, as to cause them to be discarded. In transit the
portions of the plant that have been weakened or killed by the
downy mildew fun-
gus serve as active
centers for the de-
velopment of other
organisms that de-
velop rapidly from
the time the plants
are packed in the
hampers to the time
they are sold on tne
market. This transit
loss is usually
charged up to the
secondary organisms
and very little re-
sponsibility is placed
upon the presence of
downy mildew at
shipping time.
The disease ap-
pears in the seed-
beds frequently in
August or Septem-
ber, but is never
abundant at this
time. The fall crop Fig. 127.-Enlarged portion of lettuce leaf,
of lettuce develops showing downy mildew fungus.
rapidly and it is usually intended that this crop be marketed for
the Christmas trade. The downy mildew increases in importance
as the weather becomes cooler so that it is an important factor
on the early crop. During the winter months the disease becomes
more important and occasionally causes considerable loss to the
growers. As the weather becomes warmer in the spring the dis-






Florida Agricultural Experiment Station


ease appears to be somewhat held in check, and the later crops
are usually free from it.
In fields that have been cut over and in which numerous
plants have developed seed stalks, considerable amounts of the
disease are present, often on the looser leaves and the flowering
heads in considerable abundance. This later occurrence, however,
is of little direct importance, since lettuce seed is not grown in
Florida. However, infected seed stalks act as centers of distribu-
tion for the spores.
SYMPTOMS
The downy mildew fungus attacks mainly the older leaves,
but may be found on any parts of the plant above ground. It is
first evident by the production of a yellowish area on the upper
surface of the leaves, and in most instances the corresponding
area on the lower surface is covered by a dense, white or gray-
ish, fluffy growth of the fungus which produces the spores.
This growth, which is easily detected, is close to the epidermis of
the leaf and can be easily removed by brushing the finger over
the area. These spots gradually enlarge and, when there are
several of them on a single leaf, often coalesce, or grow together.
These areas gradually die and become brown, causing the death
of the leaves. The fungus also produces spots on the stems of the
seed stalk where they are readily seen as irregular, white patches
or blotches. The fungus attacks the older leaves first, gradually
spreading to the younger growth.

CONTROL OF DOWNY MILDEW
No definite control measures can be recommended, but a few
simple and inexpensive precautions such as the spraying of the
seedlings the first week they are up with a 4-4-50 Bordeaux mix-
ture, and a later spraying four days before transplanting to the
field, will materially check the spread of the disease. It is essen-
tial that both lower and upper surfaces be sprayed thoroughly,
or results cannot be expected. Sanitary measures should be
taken in dealing with this disease. Plants that have not been
harvested and seed stalks should be destroyed, so that the fun-
gus will not be able to develop on them. Keep the fence rows
and uncultivated areas clean of any of the wild host plants al-
ready mentioned.







Bulletin 195, Diseases of Lettuce, etc.


To avoid further loss during transit, carefully strip off all of
the diseased leaves when packing the lettuce.

BACTERIAL DISEASES
The bacterial diseases of lettuce are several in number, as
follows: bacterial wilt caused by Bacterium vitians Brown; mar-
ginal blight, Bacterium marginale (Brown) ; rot, Bacterium viri-
dilividum Brown; black rot, Bacillus lactuca Vog; rosette
Aplanobacter rhizoctonia Thomas. The diseases caused by these
organisms have been grouped together under the head of bac-
terial diseases principally because of convenience in handling in
this paper and because their general symptoms are so similar.


Fig. 128.-Bacterial decay of central leaves of young lettuce plant.

It is necessary to have a specialized training on the identifica-
tion of these specific diseases to be able to make correct deter-
minations from the symptoms in the field. These diseases are
all found on lettuce. None of them have been found on escarole,
romaine or endive. These diseases are common in the United
States in most sections where lettuce is grown either under
glass or in the field. Their occurrence in Florida is as widespread






Florida Agricultural Experiment Station


as growing of lettuce. They are common and cause great" dam-
age annually in the larger lettuce growing centers.
The host plant may be affected by the disease in all stages of
its development from seedling to maturity and in transit. In
many fields the loss caused by bacterial troubles is considerable.
The greatest losses usually are local in single fields rather than
of an epidemic nature in the lettuce growing areas. For instance,
in a 10-acre field in 1924 the disease commonly spoken of as
"rot" or "black rot" caused an 80 percent loss of plants that
were mature enough for harvest.
The losses in the field are only a portion of the ultimate loss
due to bacterial organisms. It is an exceptional shipment of head
lettuce that does not suffer some loss in transit from bacterial
diseases. It is generally understood that only healthy heads are
cut and packed, but it is almost an impossibility to detect all
infections that may be on the plants and for this reason excep-
tionally large losses caused by bacterial troubles develop in
transit.
These troubles are present in lettuce fields throughout the
whole season from the time of planting of seedbeds in the late
summer until the last lettuce is cut in the early spring. It is
not definitely known how the disease is carried through unfav-
orable periods but it is a general belief that the organisms are
capable of remaining viable in green manure not thoroughly dis-
integrated and in the debris left in the fields following harvest-
ing and that they are in a suitable position to infect plants that
are set in the field for the following crop. There are also a
number of wild plants closely related to lettuce that grow along
the roadsides and ditch banks throughout the whole year which
may readily act as hosts to these organisms when cultivated
crops are not grown. The organisms are spread during the grow-
ing season principally by wind and water. This is particularly
true where overhead irrigation is practiced. Their spread also
may be accounted for to some extent by cultural practices such
as transplanting, cultivating and harvesting.

SYMPTOMS
In general, the common names of bacterial diseases are some-
what descriptive of their general appearance in the field. The
wilt disease signifies a general wilting of the plant, a systematic
collapse of the leaves beginning with the older ones and continu-







Bulletin 195, Diseases of Lettuce, etc.


ing to the youngest. This diseases is generally considered as a
dry rot or wilt, and not a wet rot, the organism in most cases
attacking the lower leaf petioles or the main stem.
The next disease, called "marginal blight," is common on let-
tuce, affecting the edges of the leaves, causing them to turn
brown or blackish over an area several millimeters in depth
along the entire edge of the blade. This disease is more of a dry
rot and in many cases might be confused with tip-burn. The
margin of the leaf becomes dry and if the subsequent season is
relatively dry this portion of the leaf falls away, leaving a jag-
ged edge.
Another disease, spoken of as "rot" or "black rot," is char-
acterized by a wet slimy decay and a distinct brownish discolora-
tion. This bacterial disease is probably more important in Flor-
ida than all the others combined. Plants that are attacked break
down rapidly and eventually become a loose, wet mass on the
ground. This disease is often difficult to detect in the field in its
early stages. It affects leaves inside the head that are completely
covered over with apparently healthy leaves. It can be detected
in the field by slight pressure on the head.
The last one of the diseases, commonly known as "rosette," is
not common in Florida lettuce fields and from its general ap-
pearance would be quite difficult to separate from physiologi-
cally stunted plants.
CONTROL
Control methods for bacterial diseases have not been formu-
lated, principally because of the extreme difficulty involved in
applying corrective measures. The usual methods for disease
control cannot be put into practice effectively because the host
plant growing close to the ground does not permit the applica-
tion of fungicides. The closeness to the soil also conserves the
moisture among and under the plants, providing a favorable en-
vironment for the development of parasitic organisms. The sec-
ond important reason why fungicides are not used is because of
the discoloring of the plants by the residue from the fungicides.
These plants are consumed while still green and fresh, and any
foreign material on the foliage reduces their salability on the
market.






Florida Agricultural Experiment Station


Consequently, the principal recommendations to be made at
this time are to be classed as precautions such as, the choice of
soil, drainage, previous rotation, seed treatment or certification,
seedbed sterilization, general sanitation in the seedbed, at trans-
planting time and in the field, care in cultivation, in applying
fertilizer and in the process of harvesting, packing and prepara-
tion for shipment.
Plants showing leaf spots caused by parasitic organisms or
blemishes due to mechanical injury are detrimental to first class
packs. The best disease-free seed should. be secured from the
most reliable seed dealers.
Seedbeds should be free from parasitic organisms. Treat them
with live steam or formaldehyde solution.
Practice sanitation by removing all diseased plants from the
field.
DAMPING-OFF
Damping-off is a very common trouble of lettuce plants, es-
pecially in the seedbed. It is caused by several organisms, each
independent or in association with one or more of the others.
The organisms most often found concerned with this disease are
Sclerotinia sclerotiorum (Lib.) Mass., Pythium debaryanum
Hesse, Corticium vagum B. & C., and Botrytis cinerea Pers.
This disease is usually very common in seedbeds of all kinds
in the United States. In Florida it has been found wherever
plants are grown in seedbeds and is of considerable importance.
Under favorable conditions practically all seedlings of truck crops
are subjected to this disease, and the prevention of it is tied up
mostly with cultural practices. It has been found on all types of
soils from heavy muck to light sandy, and during all seasons of
the year. It is most favored in its development, however, by soil
high in humus content, which is very wet during warm seasons.
Many growers overlook the presence of this disease as some-
thing necessarily associated with the growing of young plants.
They take it for granted that there is to be a certain mortality
and do not go further in thought or action as to control. For
this reason its importance is often overlooked. In Florida 10 to
15 percent of seedlings are killed off annually because of damp-
ing-off, which can, in most cases, be controlled.






Bulletin 195, Diseases of Lettuce, etc.


SYMPTOMS

Seedlings attacked by damping-off first show a slight droop-
ing at the tips of the cotyledons. At this time examination of the
stem will reveal a water-soaked area at or near the surface of the
soil. In a few hours the wilting is quite pronounced and is usu-
ally followed by the enlargement of the lesions on the stem to
such an extent that it is not strong enough to hold the plant
erect. When a seedling is killed by one or more of these organ-
isms, it falls prostrate on the ground and is rapidly overgrown
by one or all of them.
















Fig. 129.-Soil disinfection assures the grower of a good seedbed. Damping-
off took practically all of the seedlings in the untreated bed in the
foreground.

Where the seeds have been planted broadcast, the disease has
a tendency to develop more or less in circular patches, killing all
of the plants within the circumference, gradually enlarging and
including more plants on the edge. In Florida these fungi live in
the soil for considerable lengths of time, usually the year around.
The Rhizoctonia stage of Corticium vagum is encountered in
the seedbeds probably more often than the other organisms. In
mild attacks this fungus causes a characteristic browning of the
stems at and slightly above the soil line. This attack results in a
distinct stunting of the plants and in many seedbeds is the direct
cause of the uneven development of the young plants. When the
attack is slightly more severe a number of the leaflets become in-
volved and often include all but the undeveloped bud leaf. Plants






Florida Agricultural Experiment Station


in this condition often survive the attacks of the fungus and
develop eventually into average plants. Often this recovery is
associated with a change of environmental conditions, such as
temperature, moisture or aeration. When the attack is fatal to
the seedlings, they may be killed from the time of seed germina-


Fig. 130.-Bare spots in escarole seedbeds caused by damping-off in the
early seedling stages.

tion until they are past the transplanting stage. The fungus is
evident on the dead seedlings in the form of numerous charac-
teristic chocolate brown undulating, silk-like threads of the size
and appearance of coarse spider web.
The disease caused by attacks of Sclerotinia, Botrytis and Py-
thium are quite similar in their general characteristics. They






Bulletin 195, Diseases of Lettuce, etc.


are very destructive, especially on extremely young seedlings.
Once a plant is attacked, it eventually succumbs. The killing of
the young plants is rapid, and usually a 24-hour period will be
sufficient between infection and death of the plants. These or-
ganisms seldom cause a stunting of plants. In seedbeds planted
broadcast, the fungi kill the plants in circular areas, gradually
spreading from the original point of infection in the center.

CONTROL
Seedbed diseases can be controlled by seed and soil steriliza-
tion. There are very few instances where lettuce diseases have
been directly attrib-
uted to infected seed.
However, it would be
a precautionary meas-
ure to treat seed be-
fore planting them.
The seedbed can be
sterilized with either
live steam, formalde-
hyde or other com-
mercial disinfectants
at nominal cost and
thc soil thus rid of disease
S- organisms. Necessary pre-
F i cautions must be taken fol-
lowing soil disinfection, so
that it is not reinfected by
tools with which the soil is
Worked or by laborers
walking on unsterilized and
then on sterilized soil.
./ Once a seedbed is infect-
ed the development of the
disease can be controlled
to a certain extent by the
Fig 131.-Lesions caused by rhizoctonia proper regulation of irri-
on stems of lettuce seedlings, at trans- gation waters. Subirriga-
planting time. tion is to be desired or irri-
gation around the edges of the raised beds. Water should never
be applied to the surface of the beds themselves and the plants






Florida Agricultural Experiment Station


should never be sprinkled. When a small area in the seedbed be-
comes infected it would be profitable to remove the soil in it
with a small amount of the surrounding uninfected soil from
the bed and drench the spot with a formalin solution.

ANTHRACNOSE
Anthracnose is caused by the fungus Marssonina panattoni-
ana (Berl.) Mag. It has been known in America for the past 35
years and at the present time may be considered as generally
scattered over the lettuce growing regions of the country. It is
common in Florida and well distributed throughout the state
and has been found on all of the varieties of lettuce usually
grown. It does not, however, cause a disease that is of consid-
erable economic importance even though, wherever it is looked
for, it can usually be found. It is seldom that one can find a plant
that shows more than one or two affected leaves. It also has been
found on several wild hosts closely related to these cultivated
plants that grow in waste places around the fields. The disease
is favored in its development by the cooler parts of the season,
it being more important in late fall and winter than early fall
and late spring. Moisture relations favorable for the develop-
ment of the lettuce plants are also favorable for the develop-
ment of this disease. The spores of the fungus are developed in
considerable numbers on the diseased areas and are scattered
considerably by wind, rain and running water so that their dis-
tribution becomes quite general in a field during a season.

SYMPTOMS
Anthracnose manifests itself first in the form of elongated
water soaked areas along the midribs on the lower sides of the
older leaves. These spots are slightly yellowish in color and as
they enlarge this discoloration becomes more pronounced. They
gradually enlarge along the midrib and form the spores in defi-
nite spots over the surface of the affected area. The lesions on
the blades of the leaf are more circular than the elongated spots
on the midribs. The affected tissue of the blades of the leaves
is rapidly killed, becomes brown and often falls away, leaving
irregular holes in the blade.
Although this disease does not include considerable portions
of the plant itself, it is important in transit in that it opens the







Bulletin 195, Diseases of Lettuce, etc.


way for secondary organisms which rapidly involve considerable
portions of single heads and result in loss in transit from slimy
soft rot.
CONTROL
The most important phase of the control of anthracnose and
losses from secondary organisms involves sanitation in the field
at harvest time. The primary loss is negligible. When the plants
are cut and packed, old infected leaves at the base of the plants
should be removed.
Soil sterilization should be practiced in the seedbed. All debris
left in the field after the cutting of the crop has been completed
should be collected and disposed of by burying in some waste
place away from
the fields. n

GRAY MOLD
Gray mold is
caused by the f
fungus Botrytis
cinerea P er s. h p c
This disease is
common in the
United States -'
where these crops
are grown. It is
well distributed
over the State of
Florida, growing
wherever the
host plants are
grown, but it has
never been of
considerable im-
portance in this
state during any
single season nor
in Fig. 132.-Lettuce plant showing an attack
in any definite of the gray mold fungus.
locality. The fun-
gus is found on all of the hosts mentioned herein from early fall
to early spring and has been found to attack these hosts at all







Florida Agricultural Experiment Station


stages of their development. It has been most common, however,
on lettuce and romaine.
It is important on the host plants in the field about harvest
time and causes considerable loss in transit when infected plants
are included in the pack. The fungus is found in the seedbeds,
where it is important primarily because of the possibility of
transmitting it from there to the field. The disease develops
during the whole growing season.
The fungus produces sclerotia on old plants that are left in
the field at cutting time. These sclerotia act in the same way as
the sclerotia of the fungus causing "drop." They remain viable
during unfavorable seasons and periods when no host plants are
grown. When conditions are right for the development of this
disease on the plants these sclerotia germinate and develop my-
celium and spores of the fungus which attack growing plants,
causing the disease.
SYMPTOMS


Water-soaked areas on the lower leaves are first indications of
the presence of gray mold. These water-soaked areas rapidly
enlarge, often be-
a coming slightly yel-
Slowish, and in 24 to
3 6 hours a fine
r grayish growth of
mycelium appears
over the affected
surface. This
S4 growth of the fun-
S. gus is essential to
Sthe production of
P spores which appear
.4 in the form of small
S globular masses.
These spores are
easily detached and
Fig. 133.--Sclerotia of Botrytis cinerea Pers. de-
veloped in pure culture. scattered by the
wind and rain, thus
spreading the disease. Often plants are attacked in local areas,
causing only a portion of the plant to wilt down, but usually
the infection is quite general, causing the plant to wilt in a


320







Bulletin 195, Diseases of Lettuce, etc.


fashion similar to "drop," but not so rapidly. Following the wilt-
ing of the plant and its involvement by the fungus it becomes
overgrown by the fruiting mycelium, which produces mil-
lions of spores. Fol-
lowing this develop-
ment numerous small
black sclerotia are
formed which re-
main after the host
tissue becomes dead,
brown and dried.
CONTROL
The most impor-
tant recommendation
to be made in con- Fig. 134.-Upon germination, the sclerotia of
trying gy mld gray mold develop mycelium and spores.
trolling gray mold is
sanitation in which infected plants are removed from the fields.
To prevent losses in seedbeds, spraying the young plants with
2-4-50 Bordeaux mixture is of considerable importance. Rota-
tion is necessary in fields that show considerable amounts of
the disease consistently year after year.

BOTTOM ROT

This trouble, which is common in Florida, is caused by the
fungus Corticium vagum B. & C. It has been reported from some
of the eastern states but generally has not been considered com-
mon. In Florida this disease is found in all lettuce growing re-
gions and in most places is of some importance, depending on the
amount of moisture present. It has been found to attack all of
the hosts dealt with here and in addition about 50 other host
plants in the state, including seedlings of almost all of the truck
crops and annual flowering plants. The most destructive time
of the year is during the winter growing season, because of the
large acreage. It is more common on muck than sandy soils.
Naturally it is more important economically on the muck lands,
because 90 percent or more of the truck crop plants are grown
on this type of soil.
This disease is not of considerable economic importance even
though occasionally it is severe in local areas. It first appears








322 Florida Agricultural Experiment Statzon


attacking young plants in the seedbed, where it causes consider-
able loss, depending principally upon the moisture present and


.-
*.' -,.


Fig. 135.-Rhizoctonia damping-off and stunting in seedbeds of endive
(upper), lettuce (center), and romaine (lower.)


t 1 -* .
r ..







Bulletin 195, Diseases of Lettuce, etc.


the method of applying moisture to the seedbed. Here it causes
a definite damping-off of seedlings. When the fungus attacks
plants that have been transplanted to the field it is found most
common on the roots and main stem, sometimes also on the low-
est leaves that are in contact with the soil. Certain fields dur-
ing the past season showed as much as 5 percent loss from this
disease at harvest time.
SYMPTOMS

A description of the damping-off phase of bottom rot is given
under that heading elsewhere. The fungus attacks the roots of


Fig. 136.-Last stages of bottom rot of lettuce caused by Corticium
vagum B & C.

the growing plants, turning them brown and killing them, in
which instance secondary organisms rapidly cause their disinte-
gration. As new roots are put out the fungus successively at-
tacks them and gradually the plant becomes stunted and finally






Florida Agricultural Experiment Station


begins to wilt, showing an appearance similar to "drop" as al-
ready described. The wilting is typical of dry weather caused
partially from lack of water and probably because of the pos-
sible toxic effect of the fungus on the plant. On removing such
a diseased plant from the soil, the disease can not be mistaken
because of the scarcity of roots and the stubby development of
the main stem.
CONTROL
Soil sterilization will control bottom rot but such treatment
is expensive and is not to be recommended except in the seedbed.
Drainage and irrigation are important factors and should be
adequate to handle and control soil moisture.
Spraying or dusting is not to be recommended, since neither
has proven entirely satisfactory on plants that are half grown
or more. Sanitation and rotation are important in the control of
this disease. Plants that show the presence of bottom rot should
be removed from the field and other crops should be grown be-
tween lettuce crops in an endeavor to prevent the further spread
of the disease.
TIP-BURN
This is a physiological condition of lettuce plants which has
been found during the later stages of their development, being
most serious at maturity or harvest time. The cause is not at-
tributed to organisms, parasitic or otherwise, but rather to the
unfavorable environment in which the plant has been growing.
This disease is common in the United States where these crops
are grown. It is statewide in occurrence and occasionally of con-
siderable importance in Florida. It is more common on lettuce
and romaine than on endive and escarole.
Losses from this disease are not important when the lettuce
crop is considered in the aggregate, although local fields have
been almost complete losses. This condition is generally brought
about by fluctuating temperature and moisture conditions. For
instance, suppose that for several days the weather is favorable
for the growth of the plant, accompanied by high humidity.
During this time considerable new tissue develops. Then the
weather quickly changes to bright sunshine with less humidity.
This results in rapid evaporation from the leaves and dying of
the newly developed tissue around the edges of the leaves. This






Bulletin 195, Diseases of Lettuce, etc.


tissue rapidly turns dark brown to black, resulting in the pro-
duction of the disease.
SYMPTOMS
Tip-burn is characterized by the dark brown discoloration of
the margin of the leaves. This discoloration develops rapidly and
is readily detected on the plants in a field as the earliest symp-
toms. In reality it is the killing of plant tissue by drying or burn-
ing and does not develop further. This condition usually is found
on the looser leaves of the head, although some of the younger
leaves completely within the head show disease. It is more com-
mon on head lettuce than on leaf lettuce. Very little of the lat-
ter is grown in Florida.
CONTROL
Good cultural methods are important in the control of tip-
burn. Both irrigation and drainage should be well regulated in
conformity with the atmospheric conditions. Applications of
fungicides, rotation and sanitation have no connection with this
trouble, whereas cultivation may be exceedingly important.

MOSAIC
Mosaic is caused by what is generally known as one of the
filterable "viruses." It is more common on lettuce and romaine
and less common on escarole and endive. It has been observed
on these plants in Florida for several seasons and may be con-
sidered of more importance in the Sanford section than in any
other trucking section of the state. It is not common, even
though it is quite generally distributed over this section. The
damage it does is slight and by most growers is not considered
of much importance.
The disease itself is exceedingly difficult to detect by the ordi-
nary layman and especially before the plant has started to grow
after being transplanted to the field.
The general host range of the mosaic disease in Florida is
not known at the present time, although it has been found af-
fecting a large number of different plants. This list of host
plants includes most of the cultivated truck crops and a large
number of wild plants growing in the edges of the fields. It is
often thought that these wild plants act as a source of infection
for the cultivated plants. They show symptoms of the disease to






Florida Agricultural Experiment Station


some extent during almost every month of the year. It is not
thought, however, that soil type has any relation to the occur-
rence of this disease. It occurs on the cultivated crops developing
from late fall until early spring.

SYMPTOMS
A diseased plant is easily detected when mosaic is severe be-
cause of a distinct green or green and yellow mottling, which


Fig. 137.-Lettuce plant showing characteristic mosaic symptoms.

usually is found on the leaves previous to heading time. The
plants in many instances are extremely stunted. The leaves de-
velop loosely in relation to each other and, in some instances
where the disease is severe, are somewhat blasted and seem






Bulletin 195, Diseases of Lettuce, etc.


wilted. Plants that show less distinct mottling are often of a
yellowish color, appearing as though they were almost glazed
on the surface. The leaves are somewhat harder and more brit-
tle than healthy leaves. Plants that are severely attacked by
this disease often fail to head.

CONTROL
Plant only seed secured from the most reliable seed dealers.
Remove from the field plants that show mosaic. Control insects
which spread the disease in the field, with especial attention to
aphids. Care should be exercised in the handling of diseased
plants both in the process of removing them from the field and
during cultivation. The application of fungicides has no bear-
ing on the control of this trouble. Sanitation, rotation and culti-
vation are secondary factors in control.

SEEDING
Lettuce growers in the State of Florida are usually well ac-
quainted with the seeding of supposedly head lettuce varieties.
The term "seeding" almost explains itself, inasmuch as it refers
to the development of lettuce plants that produce seed stalks
early in their growth instead of rounding into heads. This phe-
nomenon has been noted particularly in fields of lettuce that
have been planted in the late summer and early fall and which
develop under exceptionally high temperatures.
There are other probable causes that might hasten the de-
velopment of the seed stalks in lettuce plants, such as environ-
mental conditions, namely, lack of moisture or lack of plant food.
It may also be attributed to a certain extent to cultural methods
such as the doubling up of the main roots at the time of planting
or the setting in the field of plants that have stood too long in
the seedbed. There is a third possibility that the seed planted
may not have been true to type or the best strain of the desired
variety.
CONTROL
Avoid planting too early in the fall, thus escaping high tem-
peratures. Practice good cultural methods, relating to moisture
supply, application of fertilizer and setting and cultivation of
plants.






Florida Agricultural Experiment Station


ALTERNARIA BLIGHT OF ESCAROLE
The leaf spots caused by Alternaria sp. on this host plant have
occasionally been found affecting large areas in certain fields.
The disease itself is not considered important, although it has
caused in certain instances more damage to this crop than any
other disease. The fun-
gus attacks the blades
of the leaves, forming
small circular spots on
them. The centers of
these spots gradually
dry out, become sunken
and turn black. Over
their surface the fungus

are distinctly of the
genus Alternaria. There
is a possibility that with
further study it will be
found that this organ-
ism is the same as the
one that causes a sim-
ilar leaf spot on lettuce.
CONTROL
Control measures for
Alternaria blight of es-
carole have not as yet
c been worked out. The
application of 4-4-50
Bordeaux mixture is rec-
ommended in seedbeds
Fig. 138.-Leaf spot of escarole caused by and up until the plants
Alternaria sp.
are about half grown.
Spraying later may cause discoloration of the plants until har-
vest time, which is objectionable on the market.

SOUTHERN BLIGHT
This disease caused by Sclerotium rolfsii Sacc., has been ob-
served only a few times in the state on any of these host plants.






Florida Agricultural Experiment Station


ALTERNARIA BLIGHT OF ESCAROLE
The leaf spots caused by Alternaria sp. on this host plant have
occasionally been found affecting large areas in certain fields.
The disease itself is not considered important, although it has
caused in certain instances more damage to this crop than any
other disease. The fun-
gus attacks the blades
of the leaves, forming
small circular spots on
them. The centers of
these spots gradually
dry out, become sunken
and turn black. Over
their surface the fungus

are distinctly of the
genus Alternaria. There
is a possibility that with
further study it will be
found that this organ-
ism is the same as the
one that causes a sim-
ilar leaf spot on lettuce.
CONTROL
Control measures for
Alternaria blight of es-
carole have not as yet
c been worked out. The
application of 4-4-50
Bordeaux mixture is rec-
ommended in seedbeds
Fig. 138.-Leaf spot of escarole caused by and up until the plants
Alternaria sp.
are about half grown.
Spraying later may cause discoloration of the plants until har-
vest time, which is objectionable on the market.

SOUTHERN BLIGHT
This disease caused by Sclerotium rolfsii Sacc., has been ob-
served only a few times in the state on any of these host plants.






Bulletin 195, Diseases of Lettuce, etc.


Fig. 139.-Leaf spot of lettuce caused by Alternaria sp.
The disease produced is very typical of the diseases produced by
this fungus on other truck crops. Because it is so scarce no fur-
ther details need be given here.

LEAF SPOTS
Several 1 eaf
spots have been
collected on let-
tuce growing in
the state. They
are of no particu-
Sla r importance
from the view-
point of the
growers, inas -
S much as they
cause no losses
and are in most
cases entirely
overlooked unless
one is examining
plants for their
Fig. 140.-Leaf spot of lettuce caused by Septoria occurrence. They
lactucae (Peck). are of some im-
portance in transit, however, since they open a way for secondary
organisms to attack the plants and cause considerable decay.






Florida Agricultural Experiment Station


These leaf spots are distinct and are caused by the organisms
Alternaria sp., Septoria lactucae Pass. and Cercospora lactucae
Stevenson. The early symptoms of these diseases are quite sim-
ilar and it is often difficult to tell them apart until after they


Fig. 141.-Leaf spot of lettuce (left) and romaine caused by Cercospora
lactucae (Stevenson).

have enlarged, except by the presence of the fungus and exam-
ination of the spores. Of the three listed, Alternaria sp. is un-
doubtedly the most common.

OTHER DISEASES
There have been listed on lettuce in other portions of the
United States diseases such as Rio Grande disease, Fusarium
wilt, Ozonium and Brown blight. Since these diseases have not
occurred in Florida it is not important that they be taken up in
general discussion in this paper.

SEEDBED STERILIZATION
It is necessary to combat certain diseases of lettuce in the seed-
bed, because that is where they first appear and cause consider-






Florida Agricultural Experiment Station


These leaf spots are distinct and are caused by the organisms
Alternaria sp., Septoria lactucae Pass. and Cercospora lactucae
Stevenson. The early symptoms of these diseases are quite sim-
ilar and it is often difficult to tell them apart until after they


Fig. 141.-Leaf spot of lettuce (left) and romaine caused by Cercospora
lactucae (Stevenson).

have enlarged, except by the presence of the fungus and exam-
ination of the spores. Of the three listed, Alternaria sp. is un-
doubtedly the most common.

OTHER DISEASES
There have been listed on lettuce in other portions of the
United States diseases such as Rio Grande disease, Fusarium
wilt, Ozonium and Brown blight. Since these diseases have not
occurred in Florida it is not important that they be taken up in
general discussion in this paper.

SEEDBED STERILIZATION
It is necessary to combat certain diseases of lettuce in the seed-
bed, because that is where they first appear and cause consider-







Bulletin 195, Diseases of Lettuce, etc.


able loss. Even when the conditions are such that little loss oc-
curs in the seedbeds, there is still the danger of carrying the dis-
ease to the field where it might cause damage when conditions
are favorable for its development. Then it would be in a position
to spread rapidly over the field. In sterilizing the seedbed a thor-
ough job should be done. Two efficient methods of sterilizing soil
to be used as a seedbed are given below.

SOIL STERILIZATION WITH FORMALDEHYDE
After the soil selected for the seedbed has been well stirred and
loosened, apply a formaldehyde solution made of 8 pints of for-
maldehyde and 50 gallons of water at the rate of one gallon to
one square foot of soil surface. The solution is best applied with
a common garden sprinkler. In that way there is little danger of
its running into low places before soaking into the soil.
After the solution has been applied the surface of the soil
should be thoroughly covered with a tarpaulin, canvas, or bur!ap
bag. This covering prevents too rapid evaporation of the dis-
infectant. The covering should be left on the soil for at least two
days. After that it can be removed and the soil stirred to
hasten evaporation of the formaldehyde. Ten days after apply-
ing the solution and eight days after uncovering and stirring it,
the soil can be prepared and the seed planted.

SOIL STERILIZATION WITH STEAM
This process necessitates the use of a steam boiler of consider-
able capacity and a rectangular galvanized iron pan large enough
to cover a considerable area but small enough to be moved by four
men. The pan should have sides six to 10 inches high with sharp
edges, so that it can be inverted and pushed down into the soil.
In this position it is connected to the boiler by steam hose so that
steam from the boiler is discharged into this pan until a potato
of medium size buried six inches in the soil under the pan is
cooked. Then remove the pan to another portion of the bed and
repeat the process. This method is more satisfactory than the
formaldehyde method, but it is considerably more expensive.
For more information inquire of the Experiment Station.







Florida Agricultural Experiment Station


SEED DISINFECTION
Seed disinfection is recommended to all lettuce growers and
should be practiced with every planting. The process is easy and
takes only a short time and the results are profitable. In treating
the seed all pathogenic organisms adhering to the seed, such as
bacteria and fungi, are killed. The seed themselves are not harmed
and will germinate almost as quickly and produce as strong plants
as those which have not been treated.
The disinfection process is as follows: Place the seed in a cloth
bag, tie the top securely, leaving plenty of space in the bag for
the seed, for instance twice as much room as the seed occupy.
Submerge the bag of seed in a solution of corrosive sublimate,
strength 1:1,000, for 10 minutes. During this time it is well to
move the bag around in the solution, using a short stick. This will
insure the removal of air bubbles so that all seed will come in con-
tact with the disinfectant. After the 10 minute period is up, re-
move the bag of seed from the disinfectant and rinse in several
changes of clear water. After the seed are thoroughly rinsed,
spread them out in the shade to dry. When dry the seed are ready
to plant.
The container should not be of metal because some of the mer-
cury in the corrosive sublimate unites with the metal, thus weak-
ening the solution and corroding the vessel. Instead, use wooden
containers such as pails and half barrels or earthenware crocks.
The disinfectant can be purchased from local druggists and can
be obtained in the form of dry crystals or tablets. An ounce of
the crystals dissolved in 71/2 gallons of water makes a 1:1,000
solution. When smaller quantities of the disinfectant are desired
it is advisable to use tablets. One tablet dissolved in one pint of
water gives a solution of the strength of 1:1,000. If a gallon of
the disinfectant is desired, dissolve eight tablets in a gallon of
water.
Corrosive sublimate is a deadly poison when taken internally.
Consequently it should be kept away from children and farm ani-
mals. When the solution is made up it looks like water, being
tasteless and odorless, yet deadly poisonous. Necessary precau-
tions should be taken in disposing of the liquid after it has been
used so that nothing will drink it.







Bulletin 195, Diseases of Lettuce, etc. 333

Seed may be treated without soaking by the use of different
mercurichlorophenol* compounds prepared in the form of dusts.
The seed are disinfected by thoroughly mixing certain amounts
of them with a specific amount of the dust. The above dusts can
be purchased from most seed and spray machine dealers.

*There are a number of mercurichlorophenol compounds, sold under
trade names, used for the treatment of seed of various kinds.




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