Here is a post from the Plant Pathogen Detective, Brenna Terry:
The healthy neighbor
Brenna is right about her diagnosis.
Plants infected with Alternaria cucumerina usually develop circular spots or lesions on the oldest or crown leaves near the center of the hill. The number of spots increases rapidly in warm, humid weather, later spreading to the younger leaves toward the tips of the vines. At first the lesions are small, circular, and somewhat water-soaked or transparent. They enlarge rapidly until they are 1/2 inch or more in diameter, turning light brown when mature. Definite concentric rings may often be seen in the older leaves, round to irregular spots, giving them a target like appearance. Spots may merge, blighting large areas of the leaf (as seen in Brenna’s second picture). The leaves commonly curl, wither, and fall prematurely.
We’ll perform Alternaria infections at the end of this week and then we’ll learn more about this devastating fungus! Feel free to share stories and photos of your sick plants!