Septoria leaf blight

Septoria leaf blight (Septoria cucurbitacearum) is a fungal plant pathogen of Cucurbits. It thrives under cooler, humid conditions of 16-19°C (61-66°F).

Symptoms
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 1-2mm 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. Under moist conditions, long, thin, needlelike conidia are released. Small (1-2mm) erumpent whitish spots appear as a rash on the surface of infected butternut, acorn, and pumpkin fruit. Pycnidia of Septoria are not often found on these whitish spots, but other organisms including cucurbit anthracnose (Colletotrichum orbiculare) and black dot (Colletotrichum coccodes) sometimes invade these lesions. Unless other diseases such as anthracnose or gummy stem blight are involved in fruit infection, fruit rots caused by Septoria are not likely to occur. Growers who have tried to market fruit with the small white speckles have had some of their produce rejected.

Cultural
Following a 2-year rotation will eliminate most disease carryover. Requires cool temperatures and summer rains to spread to fruit.

Chemical
Apply a broad-spectrum fungicide at the first sign of foliar infection.

Similar conditions
Septoria leaf blight symptoms are often confused with :
 * Angular leaf spot
 * Scab
 * Anthracnose
 * Alternaria