Anthracnose is a fungal disease of corn, cucumber, beans, peppers, squash and tomato. It can spread very quickly in warm (80 degrees F), wet weather, especially if air circulation is poor. Fortunately for California gardeners it doesn’t thrive in our hot dry summers.
This disease first appears as small, variously colored, circular spots (those on watermelon are angular) on the older leaves, though it eventually spreads to younger leaves, stems, pods and fruit. The spots enlarge and merge, getting darker until the leaves drop off and the plant is defoliated (or the stem is girdled) and dies. Sunken, round, water-soaked spots appear on fruit.
Anthracnose prevention is easier than cure. Remove diseased plants promptly to minimize its spread. Keep the plants off of the ground on stakes or cages to provide good air circulation. The spores overwinter on volunteers and crop debris, so clear up the beds in fall and rotate your crops. The spores are most often spread via water, when soil containing spores are splashed onto the plants by rain or irrigation. You can reduce this by mulching around the plants and by using drip irrigation. They may also be spread on the hands if the gardener, so don’t touch wet plants (especially not after removing infected plants). Some crop varieties are resistant to Anthracnose.
Anthracnose can also be carried on the surface of the seed, in which case treat them with hot water (127 degrees F for 25 minutes) or bleach solution (1 part bleach to 10 parts water for 30 minutes) to kill the spores.
Image: David B. Langston, University of Georgia, Bugwood.org