What causes brown spots or blotches on leaves of cucumbers, cantaloupes, and watermelons?
Small to medium-sized tan to brown spots on the leaves are probably symptoms of the disease Alternaria leaf blight, caused by the fungus Alternaria cucumerina. Cantaloupe (muskmelon or netted melon) and watermelon are most commonly affected, but the disease can occur on cucumber, pumpkin, and squash.
Anthracnose, caused by the fungus Colletotrichum orbiculare, is another one of several diseases that can also destroy foliage of these vine crops. Gummy stem blight, caused by the fungus Didymella bryoniae, can also cause similar symptoms.
Remove and destroy all infected plant residues. Rotate the crops in your garden. Purchase certified disease-free seed. Avoid wetting foliage when watering plants. Protectant fungicides applied before disease symptoms appear can reduce disease severity. Contact your local Extension office for fungicide recommendations.