Gummy stem blight (GSB), caused by the fungal pathogen Didymella bryoniae, is a serious disease that affects butternuts and other cucurbits. This disease impacts seedlings, foliage, stems, and fruit, leading to significant yield losses and post-harvest challenges if not properly managed.
Dormant period
After infecting a host plant, D. bryoniae produces overwintering spores that can survive extreme dry conditions for up to three years. These spores are deposited in the soil or remain in plant debris left in the field. New outbreaks typically begin when these dormant spores are carried to susceptible host plants via wind or water splash under favourable environmental conditions.
Sources of inoculum
Infected Seed
- Infected seeds can result in seedling infections soon after emergence or may only express symptoms later in the plant’s life cycle.
- Sakata conducts rigorous testing of all cucurbit seed to ensure it is free of bryoniae, as this pathogen is a quarantine concern in certain parts of the world.
Spores from neighbouring fields
- Overwintering spores in the soil, plant debris, or weeds from previous infections can spread to new plants.
- Diseased plants in neighbouring fields also act as a source of inoculum.
Infected water
Spores of D. bryoniae can spread through irrigation runoff, floodwater, or water splash, contaminating healthy plants.
Infected soil and tools
- Farming tools and equipment used in infected fields can carry spores, spreading the pathogen to new fields or healthy plants.
Symptoms
Seedlings
- Seedlings exposed to spores either in the environment or from the seed can develop symptoms soon after emergence.
- Symptoms include water-soaked brown lesions, damping off and wilting.
Leaves and stems
- Leaves initially show chlorosis (yellowing) on the margins, which progresses to brown necrotic lesions.
- Stems develop brown lesions that may crack and exude a gummy substance.
Fruit
- Butternut fruit is most susceptible just before colouring begins.
- Fruit of watermelon and melon can develop black rot, which is associated with infection in the vine. The disease originates from the inside of the fruit and progress to the rind.
- Spores penetrate the fruit rind and remain latent or grow slowly beneath the epidermis.
- At harvest, fruit may appear asymptomatic or show small lesions. Symptoms can develop in storage under humid and mild temperatures (20–22°C).
Characteristic lesion: In butternut, spores cause unique “fossilised” concentric circle lesions on the fruit rind, which may be tan, orange, or white in colour.
Note: Post-harvest fruit dips are ineffective in preventing disease development during storage.
Secondary Spread
Once lesions form, they actively release spores, causing external infections on healthy leaves, stems, and fruit. Secondary spread occurs through spores dispersed by wind, water, or farm activities.
Disease Management
Cultural practices
- Remove cucurbit weeds and volunteer plants from crop waste.
- Crop rotation. (2 – 3 year rotation)
- Ensure proper field sanitation by removing all debris after harvest.
Irrigation and Field Design
- Use drip irrigation instead of overhead watering to minimize water splash.
- Avoid excessive irrigation and ensure well-drained fields.
Disease and pest control
- Manage powdery mildew and insect pests, as their presence can increase GSB incidence.
- Scout fields regularly for symptoms and remove infected plants promptly.
By implementing these practices, growers can effectively manage gummy stem blight and reduce its impact on butternut crops.