The fungus can infect the leaves of muskmelon, pumpkin, summer and winter squash, and gourds and the fruit of pumpkin, butternut, acorn, and processing "pumpkins." Although affected fruit are unlikely to rot in storage or to cause other fruit to rot, they may be unmarketable because of the raised white spots caused by the disease.
Symptoms of Septoria leaf spot are similar on all the cucurbits infected. The spots are very apparent on muskmelon and butternut squash leaves, but are also identifiable on other cucurbits. Spots are normally circular or occasionally irregular, beige to nearly white in color, measuring 1 to 2 mm in diameter or occasionally larger on the upper leaf surface. A narrow brown border surrounds the spot and, with age, the lesion may crack. When the disease first appears in the spring under moist conditions, the spots appear with or without a white speck surrounded by a much larger brown water-soaked border, giving the appearance of a different disease. The distinguishing sign on older spots is the presence of small, black, specklike fruiting bodies called pycnidia embedded within the tissue. Not all spots will contain pycnidia, but some may contain up to eight or more. The black specks can be seen with an unaided eye.