Gummy Stem Blight (GSB), caused by the fungus Didymella bryoniae, is a destructive disease affecting muskmelons and other cucurbits. It is also referred to as "black rot" due to the characteristic dark lesions it causes on fruits. The disease can infect plants at any growth stage, from seedlings to mature vines, and manifests symptoms on leaves, stems, and fruits, but not on roots. GSB can lead to significant yield losses, especially under warm, humid conditions, and is often spread through infected seeds, organic debris, and volunteer cucurbit plants. Understanding its symptoms, infection cycle, and effective management practices is essential to control its impact.

Symptoms:

• Circular brown or tan spots on leaves, often covering large areas.
• Lesions on leaves and stems, leading to splitting and canker formation.
• Stems exude a brown, gummy substance.
• Vine wilting and stem death occur in advanced stages.
• Fruits develop small, water-soaked lesions that enlarge, exude gummy substances, and display black fruiting bodies.
• The disease is commonly referred to as "black rot" due to its dark lesions on fruits.

Infection and Spread:

• GSB infects cucurbits at all growth stages, except roots.
• Early symptoms include chlorosis (yellowing of leaf margins) and necrotic spots on cotyledons.
• The pathogen thrives in warm, humid conditions (16°C–24°C, 85% RH), with symptoms appearing 7–12 days post-infection.
• Infection is facilitated by wounds caused by pests (e.g., aphids, cucumber beetles), mechanical damage, or other diseases like powdery mildew.

Sources of Inoculum:

1. Seeds: May harbor GSB without visible symptoms. Use GSB-free seeds or treat seeds before planting.
2. Organic Debris: Residues from previous cucurbit crops can harbor the pathogen. Implement crop rotation and plow debris deeply into the soil.
3. Volunteer Plants: Wild cucurbits (e.g., balsam pear, wild citrons) serve as reservoirs for the pathogen.

Management Practices:

1. Seed Treatment: Use treated seeds or disinfectant solutions to eliminate seed-borne inoculum.
2. Crop Rotation: Rotate cucurbits with non-host crops for 2–3 years.
3. Sanitation: Remove and destroy infected debris and eradicate wild cucurbits before planting.
4. Scouting: Regular field inspections help identify infections early, allowing timely interventions.
5. Fungicides: Apply preventive fungicides such as Chlorothalonil and Mancozeb, alternating chemistries to prevent resistance.
6. Postharvest Care: Avoid wounding fruits during harvest and store at 7°C–10°C to prevent black rot development.
7. Biological Control: Limited success due to environmental dependency, but can complement fungicide applications.

By following an integrated approach involving seed health, rotation, sanitation, and fungicide use, GSB can be effectively managed in muskmelon and other cucurbits.

Image Source: Plant Village and University of Florida