NC State and USDA Cucumber Disease Handbook
Target Leafspot (Corynespora cassiicola)
Z. Abul-Hayja and P. H. Williams
Department of Plant Pathology
University of Wisconsin, Madison
Host:
Cucumis sativus L. (Cucumber)
Disease:
Target leaf spot (2)
Causal Agent:
Corynespora cassiicola (Berk. and Curt.) Wei
Corynespora melonis
Description of Pathogen:
Appearance in Culture:
On Czapek's agar and potato dextrose agar under light: thin hairy
growth, greenish brown on the surface, black in the bottom.
Microscopic Appearance (2):
Mycelium:
Septate, pigmented.
Spores:
Conidia on conidiophores, single or in chains, variable in shape,
obclavate to cylindrical, straight or curved, subhyaline to olivaceous
brown,4-20 pseudoseptate, 40-220 µm long, 9-22 µm thick.
Sources of Corynespora casiicola:
P. H. Williams
Department of Plant Pathology
1630 Linden Drive
University of Wisconsin
Madison, Wisconsin 53706
J. Strandberg
Institute of Food and Agricultural Sciences
University of Florida, Box 909
Sanford, Florida 32771
Pathogenic variability:
Host Range:
Extremely wide host range including cucumbers, cowpea, soybean,
sesame, okra, cotton, tomato, and other cultivated and weed species
(2). An isolate from cucumber did not cross-inoculate other hosts,
and likewise isolates from the other hosts did not infest cucumber
(4).
Pathogenic Variability:
No reports on pathogenic stability or breakdown of resistance.
Stability in Culture:
Loss of virulence was reported in continuous transfer of colonies
(1, 3). Transition to mycelial types, but no loss of virulence
after 10 weekly transfers was noted (7). No loss of pathogenicity
was noted after 2 years of periodic transfer of conidia or storage
under oil at 4°C.
Inoculum Increase:
Initial Inoculum: Transfer mycelium with agar from storage; let
grow on Czapek's agar (80 g glucose/1) under light for 6-8 days.
Spores are then used for subsequent inoculum increase.
Increase: A spore suspension is made from a 6-8 day old culture
by adding few milliliters of water and tapering the spore gently
using a rubber policeman. The suspension is filtered aseptically
through one layer of cheesecloth into a sterile test tube. The
spore concentration is measured using a hemacytometer and then
adjusted to 2 x 104 - 2 x 105 spores/ml. Czapek's agar slants
(80 g glucose/1) are inoculated using 0.1 ml spore suspension
by spreading it over the entire slant surface using a wire loop.
Inoculated slants are incubated under cool white fluorescent light
at 25°C for 6-8 days. Spores are collected from the agar
surfaces using distilled water in two washings. The suspension
is passed through one layer of cheesecloth with continuous agitation.
The spore concentration is determined using a hemacytometer and
then adjusted to 8 x 106 spores/ml.
Long-Term Storage:
Storage: Store under oil at 4°C. Seed small Czapek's agar
slants with C. cassiicola spores. Let grow under light for 6-9
days. Fill the tubes with sterile mineral oil. Store at 4°C.
Retrieval from Storage: Aseptically transfer a block of mycelium
from oil onto Czapek's agar media and incubate under light at
25°C.
Sources of Resistance:
Cultivars:
No American cultivars with resistance to C. cassiicola
are yet available. Resistance so far was derived from PI lines
from The Netherlands and from other known Dutch resistant material.
Plant Introductions:
PI lines reported to contain some resistant plants are: 277741
and 255936. Available at USDA, Plant Introduction Service, Ames,
Iowa.
Inheritance of Resistance:
J. Strandberg believes it is recessive. Andeweg says it is dominant.
Resistance obtained from Royal Sluis was single dominant.
Growing Host for Inoculation:
Cucumber seeds are sown in flats (52 x 36 x 7 cm) filled with
sterile moist vermiculite. Flats are preferably kept on a heating
pad and covered with newspaper to prevent cooling by evaporation.
The temperature is best kept at 32°C inside the flats to
insure fast and uniform seed germination. Three to 4 days after
germination is optimal for inoculation.
Inoculation:
Three to 4 days after emergence, the seedlings are inoculated
by placing 0.01 ml drops of spore suspension (as prepared in 7.2)
on the abaxial surface of one cotyledon of each plant.
Post-inoculation Environment:
Inoculated seedlings are incubated in a 24-28°C dew chamber
for 48 hours.
Quantifying Host Resistance:
Symptom Development: Water soaking starts to develop 24 hours
after inoculation in susceptible cotyledons. When the plants are
taken outside the dew chamber, susceptible cotyledons develop
slightly sunken, rough surface lesions on the abaxial surfaces.
Resistant cotyledons do not produce lesions.
Disease Rating:
Cucumber seedlings are indexed 2-3 hours after removal from the
dew chamber. They are classified into resistant and susceptible
plants.
Controls:
Susceptible check is Gy 14. Resistant check is Expo.
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