The fungus produces yellow to brown spots on the upper leaf surface. Under moist conditions, a fluffy, gray fungal growth appears on the underside of the leaf, corresponding to the leaf spots above. Infected leaves may die as the disease spreads from the crown outward. Moist conditions favor downy mildew development. Alternaria Leaf Spot Alternaria leaf spot, caused by the fungus Alternaria cucumerina, affects cantaloupe and cucumber as well as other cucurbits.
Spots merge and defoliation occurs, beginning with the crown leaves.
Weak plants are most susceptible to Alternaria leaf spot. The fungus is spread in wind and rain and is favored by warm, wet conditions. Angular Leaf Spot Angular leaf spot, caused by the bacterium Pseudomonas lachrymans, affects cucumber, squash, and pumpkin. Leaf spots are angular in appearance, being limited by major veins. They are water soaked initially and eventually turn grayish brown.