Squash Anthracnose

Colletotrichum species of fungi

Anthracnose on cucumbers [Credit: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org]

Anthracnose on cucumber foliage [Credit: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org]

Host Plants:

On Crops: Watermelons, muskmelons and cucumbers

Where Found:

Worldwide in areas with warm, humid summers

Description:

The first symptoms of this leaf-spot disease are brown spots with black margins on leaves, especially tender new growth. The first spots often form near a leaf vein, and quickly become numerous at which point the leaf withers and dies. More spots often can be found on stems, and stem spots tend to be elongated rather than round. In wet weather, mature spots (fungal colonies) produce masses of salmon-colored spores with a jelly-like appearance. Cucurbit anthracnose is most likely to spread in warm, damp weather with temperatures just above 75F (24C).

Damage:

Badly affected leaves turn brown and drop off. If wet weather persists, melon and cucumber fruits develop soft dark spots that become sunken rot spots.

Preventing Problems:

Because anthracnose is a wet weather disease, growing cucumbers and melons at proper spacing, so sunlight reaches all the leaves and air circulates freely, is a sound preventive measure. Avoid low places with repeated heavy dews when choosing a place to grow melons, and use mulch to keep soil from splashing onto new leaves. Many varieties offer some genetic resistance. This disease can be carried on seed, so buy only disease-free seed and do not save seeds from infected plants.

Managing Outbreaks:

If the damage is limited and dry weather is predicted, you can wait in hopes that dry conditions put an end to the problem. If more than one third of a plant’s leaves are lost to disease and the immature fruits show suspicious spots, it is probably a waste a time to wait on the crop. Pull up the plants and compost them.

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