Gummy Stem Blight

Gummy stem blight is a fungal disease of cucumbers, melons, pumpkins, squash, watermelons, and other cucurbits.

Gummy stem blight (Didymella bryoniae) is responsible for crop losses of up to 30% when it occurs, but there are steps you can take to prevent it. First, you need to know what it looks like.

Watermelon leaves with severe gummy stem blight symptoms (Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org) CC BY 3.0 US

Gummy stem blight symptoms

Plants infected with gummy stem blight show symptoms on leaves, stems, and fruit. Symptoms can vary, depending on the host plant. Generally, leaves develop circular, water-soaked lesions that start light brown and turn dark. These lesions may develop yellow halos or change shape. When they dry, they may crack and then fall away. Leaf edges (margins) may also turn dark brown.​.

Gummy stem blight on watermelon (David B. Langston, UGA, Bugwood.org) CC BY 3.0 US

Infected stems have water-soaked areas that turn tan. In some cases, those areas evolve into dark cankers that may contain fruiting structures that look like black pepper. These fruiting bodies are called pycnidia [pik-NI-dea]. These cankers often discharge a gummy brown ooze, hence the name. That ooze can collect into reddish-brown or black beads, making it easy to confuse this disease with Fusarium wilt. Plants infected with Fusarium wilt won’t have the black pycnidia seen in gummy stem blight. New seedlings infected with gummy stem blight look like they absorbed too much water and are darker than healthy plants.

Gummy stem blight lesion at the base of a watermelon plant. (Don Ferrin, Louisiana State University Agricultural Center, Bugwood.org) CC BY 3.0 US

Fruits infected with this pathogen are often said to have black rot. But this is not the bacterial black rot disease that affects members of the cabbage family. Fruits infected with gummy stem blight start rotting at the stem end, making it look like an upside-down version of blossom end rot. Fruit rot can also occur from the inside out.

Gummy stem blight disease cycle

Gummy stem blight spores are common in the environment. This disease is both seed- and soil-borne. For infection to occur, there must be enough heat and moisture for fungal spores to germinate. This disease is most likely when humidity levels are above 85% and temperatures are between 68°F and 77°F.

Leaves that stay wet for more than one hour provide the moisture needed for spores to germinate and penetrate healthy plant tissue. Spores also enter through plant wounds created by insect feeding, happy dogs, and other garden visitors. Other diseases, such as powdery mildew, can also weaken a plant, making it easy for fungal spores to take hold. Once it enters a plant, this pathogen moves to the stems, where cankers start forming.

Watermelon stem infected with gummy stem blight (Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org) CC BY 3.0 US

Gummy stem blight management

Remove infected plants and toss them​ in the trash bin. Two- or three-year crop rotations, removing plant debris at the end of each growing season, and maintaining proper plant spacing help prevent gummy stem blight. And keep those leaves dry by avoiding overhead watering. While no resistant varieties are available, you can buy seeds treated for gummy stem blight.