This article was written by Dr. Francesca Rotondo.
Gummy Stem Blight, caused by Didymella bryoniae, is a significant disease affecting cucurbit crops like squash, pumpkins, cucumbers, and watermelons. It can impact plants at any growth stage. The disease is also a concern for greenhouse-grown cucumbers, affecting all above-ground parts of the plant.
The pathogen can be seed-borne or survive on organic debris from previous crops. The fungus spreads through two types of spores—ascospores, which are windblown, and pycnidiospores, spread by splashing water. Infection is highly dependent on moisture and temperature, with optimal conditions being 61 to 75°F. With warmer temperatures and changing climate conditions, the incidence and severity of gummy stem blight may increase due to more favorable conditions for the fungus.
The symptoms on the leaves include dark yellow or reddish-brown lesions appear, often starting at the leaf margins and progressing inward, leading to leaf death. On fruits the symptoms vary by crop, with winter squash often showing black rot. This rot can affect the rind, flesh, and seed cavity, with a noticeable growth of white and black fungus. Lesions on fruit are water-soaked and may lead to decay, especially at the site of attachment.
The disease management relies on chemical control (pages 128-129 of the MidWest Vegetable Production Guide) and cultural practices:
- Use disease-free seeds.
- Deeply plow crop residue after harvest.
- Rotate crops to avoid growing cucurbits in the same location for 3 to 4 years.
- Apply protective fungicides regularly.
- Avoid fruit wounding during harvest and store at 45 to 50°F to prevent postharvest rot.
- Resistant cultivars are currently unavailable.
