Karen Rane, Plant Disease Diagnostician
These leaf spots on cucumber are the symptoms of anthracnose, caused by the plant pathogenic fungus Colletotrichum orbiculare. The round, tan lesions are 2-10 mm in diameter, and often develop holes or cracks in the center. Lesions can form on leaf blades, petioles, stems and fruit. The spore-bearing structures (acervuli) of the causal fungus are found in the necrotic lesions. They are tan or light orange in color and contain numerous spores in a slimy matrix. A diagnostic feature of this disease is the presence of dark brown hairs, or setae, in the acervuli. The setae are visible with a hand lens or dissecting microscope.
The fungus can infect muskmelon and watermelon in addition to cucumber. The pathogen survives the winter in infected plant residues. The fungus can also be associated with seed. As with most fungal diseases, long periods of leaf wetness favor disease development. Spores are splashed from leaf to leaf, and plant to plant, during irrigation or rain events. Several disease cycles can occur in a single growing season, resulting in defoliation of severely infected plants.
Management of anthracnose in cucurbits involves both cultural and chemical measures. There are several cucumber cultivars that are resistant to anthracnose –this information is usually listed in the descriptions found in seed catalogues. Remove infected plant debris from the vegetable garden to reduce the amount of spores available for causing infection the following season. Purchase new seed each year, since the fungus can overwinter on seed from infected fruit. Avoid overhead irrigation, which promotes long leaf wetness periods and aids in the spread of the fungus from plant to plant. If anthracnose has been a problem in the past, consider using a protectant fungicide to protect uninfected leaves. Remember, fungicides will not cure leaves with lesions. Always follow label directions when using any pesticide.
Click on the small image to view a larger image.
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Anthracnose, caused by Colletotrichum orbiculare, on cucumber leaves |
Close-up of anthracnose leaf lesions (Photos by Peggy Sellers) |
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| Setae (dark hair-like structures) of Colletotrichum orbiculare in an anthracnose leaf lesion (magified 100X) (Photos by Peggy Sellers) |
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Plant and Pest Digital Library Project, Purdue University, West Lafayette, Indiana.