Jake Jones, Extension Agriculture Agent, Kent County; jgjones@udel.edu
Gummy Stem blight is caused by Stagonosporopsis spp. and can occur on cucurbit crops and seedlings. It can be introduced from infected seed or seedlings, highlighting the importance of greenhouse and field sanitation. Gummy stem blight can cause symptoms on the leaves, stems, and vines and is also called black rot when fruits are infected. Leaf lesions in watermelon often start at the leaf margin and are brown, circular, and have concentric rings. Symptoms on watermelon vines often include cracking and a gummy ooze, but this can also occur in anthracnose and low pH conditions. Stem cankers can girdle the stems and cause wilting a few weeks after infection. A diagnostic sign of gummy stem blight is the black fruiting bodies (pycnidia), which can be found in the lesions. Black rot of watermelon fruit only occurs if the vines are severely infected. Early symptoms on fruit are circular and greasy in appearance but eventually will coalesce and become brown/black in color. Gummy stem blight can be confused with anthracnose but the leaf lesions of anthracnose are irregularly shaped in watermelon and the stem cankers are spindle-shaped.
Optimal conditions for gummy stem blight infection in watermelon is 75°F temperature and prolonged moisture with leaf wetness lasting 1-10 hours.
Fungicide resistance is an important consideration for gummy stem blight. In the US, the disease has shown resistance to boscalid, azoxystrobin, thiophanate-methyl, penthiopyrad and low levels of resistance to tebuconazole. Currently, DMI, AP, SDHI, and PP fungicides (tebuconazole, difenoconazole, cyprodinil, fluopyram, fludioxonil, etc.) are relied upon with pre-mixtures common and tank mixed with chlorothalonil or mancozeb to manage fungicide resistance in gummy stem blight. You can refer to the Mid-Atlantic Commercial Vegetable Production Recommendations for a list of recommended fungicides: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/commercial-crops/vegetable-crops/midatlantic-vegetable-recommendations/.
Figure 1. Circular lesion of gummy stem blight in watermelon with concentric rings.
Figure 2. Watermelon stem lesion with gummy exudate, a symptom of gummy stem blight.