This disease is caused by the fungus Dendrophoma obscurans . This disease is distinctly different from leaf spot and leaf scorch. Similar spots sometimes develop on the fruit calyx. Normally, one to six lesions develop on a single leaflet. Small black dots, pycnidia of the causal fungus, appear scattered over the center of older spots.
New lesions appear throughout the summer and fall when weather conditions are favorable.
Older blighted leaves may die in large numbers. Click on image for larger version Figure 3. Leaf blight is particularly destructive on slow-growing or weak plants. It seldom damages young, runner plants. The same fungus rarely causes an enlarging, soft, pale pink rot of the stem end of the fruit.