Alternaria Blight Blight, caused by the fungus Alternaria cucumerina , infects melon, squash, cucumber, pumpkin, and watermelon. Wet weather and temperatures of 60-90 F favor blight. Melon is quite susceptible and most commonly infected. Spots develop first on older leaves near the center of the hill. The spots are circular and appear somewhat water soaked.
Rapid defoliation occurs when weather favors the disease.
Severe attacks occur only when plants have been weakened by poor soil fertility, poor growing conditions, other diseases, or a heavy fruit set. Alternaria leaf spot on muskmelon leaf. Photo by Clemson University. The blight fungus survives the winter in diseased cucurbit crop refuse. The fungus is spread by wind.