Survival and spread

Title: Target spot of several vegetable crops
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00066903/00001
 Material Information
Title: Target spot of several vegetable crops
Physical Description: Book
Creator: Pernezny, Ken
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1988
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066903
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
        Page 1
    Survival and spread
        Page 2
        Page 3
        Page 4
        Page 5
Full Text

Plant Pathology Fact Sheet

Target Spot of Several Vegetable Crops

Ken Pernezny and Gary W. Simone, Professor, Plant Pathology, Everglades Re-
search and Education Center,Belle Glade; and Professor (Retired), Plant Pathol-
ogy Department, Gainesville, Florida. 1988; Revised November 1999.
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean


Target Spot is the name often used for
vegetable diseases caused by the fungus
Corynespora cassiicola. It is a relatively "new"
disease in Florida, being first reported on cu-
cumber from the Immokalee area in 1967.

The name target spot derives from the
ringed or bull's eye appearance that is some-
times seen in lesions caused by C. cassiicola.
However, concentric rings are not always
readily apparent in target spot lesions, and not
all lesions with concentric ringing are caused
by C. cassiicola. It is often necessary to examine
suspected target spot lesions for the character-
istic spores of the causal fungus to ensure that
a correct diagnosis is made. The pathogen can
be induced to sporulate from diseased tissue
after incubation in a high humidity chamber for
24-48 hours. In some cases, growth of C. cassiicola
from pieces of suspect tissue on agar media in
the laboratory is needed to verify that target
spot is indeed the problem.

The Pathogen

Historically, C. cassiicoia has had numer-
ous other names including some in the genera
Helminthosporium and Cercospora. The fungus
belongs to the class "Fungi Imperfecti"
(Deuteromycotina), and as this name suggests,

only an imperfect or asexual stage is known to
exist. The vegetative stage consists of a
branched, pale to mid-brown colored myce-
lium. The individual fungal strands are typi-
cally septate (having crosswalls) and give rise
to spore-bearing structures (conidiophores)
with as many as nine cylindrical cells per stalk
(see Fig. 1A). The asexual spores conidiaa) borne
from these conidiophores are variable in size,
shape, and color. Conidia may be borne singly
or in chains, they may be club-shaped to cylin-
drical and straight or curved, and range in color
from a pale olive to a dark brown. The diag-
nostic feature of Corynespora under the dissect-
ing microscope is the iridescent appearance of
the conidiophores. Under the compound mi-
croscope, the conidia will contain from 4-20
pseudo-crosswalls (pseudosepta) that appear
to divide the spore but do not extend to the
outer spore walls (see Fig. 1 B). The base (the
widest part) of the conidium also has a charac-
teristic protruding peg hilumm) that can aid in

Plant tissue from the field may support
little superficial fungal growth on the tissue
since C. cassiicola grows largely intercellularly
in the plant. Normally, the tissue should be in-
cubated in a high humidity chamber to induce
sporulation or cultured on an agar medium for
isolation and fungus identification.


Survival and Spread

The Corynespora pathogen has several
means for survival and spread in the field. It
may survive up to 2 years in crop debris. The
wide host range of this fungus also contributes
to survival of the fungus in Florida. Strains of
Corynespora from different hosts have been
found often but not always to be cross infective
to other hosts. Corynespora has been proven to
be seedborne in soybean, but seedborne sur-
vival in vegetable crops is not known at this
time. The primary means of field spread is by
air-disseminated conidia that have a primary
release period in midmorning. Greatest disease
severity from Corynespora appears to occur in
seasonal periods when the temperature is ap-
proximately 280C (820F).


The number of host plants attacked by
C. cassiicola totals over sixty species, with new
hosts reported on a regular basis. This patho-
gen has worldwide distribution. In Florida, the
fungus is known to attack such diverse crops
as papaya, passion-vine, soybean and such
common ornamentals as Aphelandra sp., Ficus
spp., Hydrangea sp., and Ligustrum spp. Among
the vegetable crops, C. cassiicola infects pepper,
cowpea, cantaloupe (muskmelon), yellow
squash, and KY type snap beans. Target spot is
a particular problem on cucumbers and toma-
toes in Florida.

On cucumbers, the disease starts as
small, yellow leaf flecks that gradually enlarge
to about 1 cm (0.4 in) across and become angu-
lar. Individual mature lesions are very light tan
with a thin brown margin (Fig. 2). Lesions may
coalesce, with the development of large circu-
lar areas of dead tissue which dry and tear out.
Small, elongate target spot lesions may occa-
sionally occur on cucumber petioles and stems.
Target spot, especially in the early stages, is
difficult to distinguish from angular leaf spot

and downy mildew, two common foliar dis-
eases of cucumber. In late stages, the disease
can be confused easily with anthranose of cu-
cumber. Again, readers are encouraged to seek
microscopic examination of lesions for signs of
specific pathogens in order to make the proper

On tomato leaves, the disease first ap-
pears as small necrotic lesions with light brown
centers and dark margins. Some varieties show
a pronounced yellow halo around these leaf
spots. Later, somewhat circular lesions about 1
cm in diameter develop with sunken tan to light
brown centers. Individual lesions often coalesce
and cause a general blighting of leaves (Fig. 3).
Symptoms also occur on flower and fruit stalks
and stems.

On tomato fruit, a succession of symp-
toms is observed. Small, brown, slightly sunken
flecks are observed first (Fig. 4A). As fruits
mature and the disease progresses, lesions be-
come larger and darker. Coalescence of lesions
result in large pitted areas (Fig. 4B). Advanced
disease on fruit appears as large and deeply
sunken lesions, often with visible dark gray to
black growth of the fungus in the center (Fig.
4C). A recessed zone of healthy looking tissue
will usually surround the zone covered with
fungal growth.

In artificial inoculation trials conducted
at the Tropical Research and Education Center
in Homestead, very slight wounding (as from
fine sand particles abrading the fruit surface)
was essential for reproduction of the fruit symp-
toms observed in the field. It is likely that wind-
blown sand is important in outbreaks of target
spot on tomato fruit in the field.

Target spot symptoms, especially in the
early stages, can be readily confused with two
other tomato diseases, bacterial spot and early
blight. Again, microscopic examinations of tis-
sue samples for spores of the Corynespora fun-
gus may be needed for proper disease identifi-


Target spot is a recent discovery on snap
beans. So far, observations are limited to KY
type beans (beans with a pod similar to a pole
bean, but with plant growth similar to a bush
bean). Leaf spots appear primarily on lower
leaves. The spots are relatively large and have
distinct bull's eye patterns (Fig. 5).


Use of plant resistance in cucumber to
control target spot has been a long-standing
control practice in Europe for over 15 years.
Plant resistance to this fungus has also been
documented in soybean and tomato. Due to the
sporadic occurrence of target spot in Florida,
however, breeding efforts for vegetable culti-
vars have not concentrated on this particular

rigure In. I Ie iiuigus LUrynct purtu
cassiicola; conidiophore with linked cylin-
drical cells. Magnification approximately

Currently, target spot is controlled pri-
marily by periodic applications of protectant

Because of frequent changes in pesti-
cide registrations, currently recommended
fungicides can be identified by the University
of Florida Cooperative Extension Service. It
should be noted that tank-mix sprays of
copper fungicides and maneb do not provide
acceptable levels of target spot control. In the
past, several outbreaks of target spot of
tomato have been correlated with frequent
use of copper/maneb tank-mixes, primarily
for bacterial spot control, to the almost total
exclusion of other fungicides. Correct diagno-
sis of the cause of tomato foliar lesions obvi-
ously is needed if proper fungicide choices
are to be made.

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not extend from side wall to side wall. Mag-
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Figure 2. Target spot lesions in leaf of cu-

Figure 4A. Progressive target spot damage to
tomato fruit: (A) early flecking stage.

Figure 3. Blighting of tomato leaf due to
advanced target spot development.

Figure 4B. Progressive target spot damage to
tomato fruit: (B) development of pitted areas.

Figure 4C. Progressive target spot damage to
tomato fruit: (C) appearance of large, deeply
sunken lesions with dark growth of

Figure 5. Target spot lesions in leaves of KY
type snap beans.

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