Jake Jones, Extension Agriculture Agent, Kent County; jgjones@udel.edu
Gummy Stem blight caused by Stagonosporopsis spp. is a major disease of cucurbits in our region. High humidity, rainfall, and overhead irrigation, along with temperatures around 75°F for watermelon and cucumber and 65°F for cantaloupe are favorable for disease development. The disease can develop from multiple sources: infected seeds, infected cucurbit crop debris where the pathogen overwinters, or from windblown spores.
Gummy stem blight can cause symptoms on the leaves, stems, and vines and is also called black rot when fruits are infected. Leaf lesions often start at the leaf margin and are brown, circular or triangular, and have concentric rings. Water-soaked lesions are also common and on hosts other than watermelon. The centers of leaf lesions may fall out. Pumpkin leaf symptoms are often less distinguishable since they appear as marginal necrosis then wedge-shaped necrotic areas and are similar to those found in cucumber and squash.
Symptoms on vines often include cracking and a reddish-brown gummy ooze, especially in watermelon and cantaloupe. 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 fruit can display symptoms pre or post-harvest. Symptoms pre-harvest in cantaloupe are light brown lesions with incomplete netting surrounded by a green halo. On watermelon greenish tan to black circular lesions first appear on the blossom ends. Black decaying tissue appears in cucumbers at the blossom end and in pumpkins anywhere on the fruit. Symptoms are unique in butternut squash, appearing as orange to white lesions with concentric rings. Hubbard squash does not display symptoms pre-harvest but along with the other cucurbits can develop symptoms post-harvest.
Gummy stem blight can be confused with anthracnose but the leaf lesions of anthracnose are irregularly shaped in watermelon, the stem cankers are spindle-shaped, there are no concentric rings, and orange to pink spores are found on black hairs (setae). Black rot can be confused with blossom-end rot, caused by calcium deficiency and water stress but there will be no sporulation from this abiotic disorder and lesions will only appear on the blossom end.
Management of gummy stem blight begins with purchasing disease free seed/seedlings and monitoring seedlings for symptom development. If symptoms develop in seedings, dispose of infected seedlings and nearby plants.
Protectant fungicides can be used during the season to manage the disease, although 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. Tank mix 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/.
Sanitation is important to reduce the overwintering of the pathogen in infected crop debris and tillage post-harvest reduces pathogen survival. Finally, rotate to crops other than cucurbits for at least 2 years. Properly managing gummy stem blight is the first step to managing black rot later in the year. Careful harvesting to reduce damage to rinds in winter squash and pumpkins will also help reduce the incidence of black rot post-harvest.
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.
Figure 3. Black rot symptoms on watermelon fruit.