Alternaria Leaf Blight
Alternaria leaf blight in mirlitons, caused by the fungus Alternaria alternata, occasionally occurs in the U.S.A. The disease manifests as yellow, irregular spots that appear on leaves, often with a brown center. It’s often a secondary infection when the vine becomes stressed and weakened by excessive rains. It frequently affects new mirlitons in the early stages of growth.
Since it primarily affects leaves, it is not lethal and the vine will survive, but it can reduce the canopy.
It’s a waterborne fungus, so rain and overhead watering will spread it. Avoid overhead irrigation and remove and destroy infected plant material promptly to reduce the source of inoculum.