Target Leafspot (Corynespora cassiicola)
Z. Abul-Hayja and P. H. Williams
Department of Plant Pathology
University of Wisconsin, Madison
Cucumis sativus L. (Cucumber)
Target leaf spot (2)
Corynespora cassiicola (Berk. and Curt.) Wei
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):
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
Institute of Food and Agricultural Sciences
University of Florida, Box 909
Sanford, Florida 32771
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).
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.
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.
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:
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.
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.
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.
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.
Cucumber seedlings are indexed 2-3 hours after removal from the dew chamber. They are classified into resistant and susceptible plants.
Susceptible check is Gy 14. Resistant check is Expo.