| Causal Organism: Didymella bryoniae
Crops Affected: members of the Cucurbit family including cucumbers, squash, pumpkins and melons – field or greenhouse
Disease Cycle:
- Fungal pathogen
- May survive for years as dormant vegetative material (mycelia) or as chlamydospores on plant debris
- The disease is spread in water splashes as conidia (asexual spores) or on the wind as ascospores (sexual spores)
- Spores may also be spread physically by hands, clothing and knives
- Pathogen develops in humid conditions, with ample free moisture on the surface of leaves
- The wetness period significantly impacts the rate of infection and the expansion of lesions
- All plant parts may be infected
- Infection occurs directly through leaves (through cuticle or trichomes) or through flower scars
- Wounding can also contribute to disease development and spread, particularly in older plant parts
Symptoms/Damage:
- Initial symptoms on stems - pale brown lesions may be observed where wounds have occurred (points where leaves or fruit have been removed)
- Lesions progress to become dotted and then covered with tiny lack fruiting bodies (pycnidia or perithecia)
- An gummy amber-coloured sap may be observed flowing out of lesions that have cracked open
- Such lesions may result in girdling of the stem, and subsequent wilting of plant parts above that point
- Fruits can be infected internally and externally
- Internal rots will result in a tapering of the fruit at the blossom end and internal discolouration, followed by a blackening due to an increase in external fruiting bodies
- External rots starts as irregular spots that go from yellow to grey, then brown
- Lesions are soft, wet and sunken, often with some gummy sap at the centre of them
- These lesions are often observed in storage
- Leaf lesions usually start at the tips, being pale yellow turning brown, developing backwards in a V-shaped pattern
- Lesions may have a lighter halo around the edge
- Lesions may also resemble to stem or fruit lesions, developing as circular spots
Conditions Favouring Disease Development:
- High humidity and free moisture on leaf surfaces are required for spore production, leaf infection and the spread of lesions
- Optimum temperature range is the mid-20°C, however the disease can develop at temperatures between 5°C and 35°C
- Wounding of older plants parts
Management:
- Ensure good sanitation practices are in place
- Remove crop debris (bury and compost to speed up decomposition)
- Wash and disinfected all greenhouse surfaces
- Prevent or minimize extended periods of leaf wetness, as well as ensuring that humidity does not build up
- Avoid situations where rapid changes in temperature result in condensation on plant surfaces
- In a greenhouse situation, transition nighttime to daytime temperatures prior to sunrise to allow plants to warm somewhat
- Ensure good ventilation of greenhouses
- Exercise care in harvesting, to reduce wounding
- Use knives for harvest, rather than pulling
- Disinfect cutting tools regularly, in higher disease pressure situations
- Ensure plants have adequate levels of nutrients to be healthy
- Avoid excessive vegetative growth
- Promote good air circulation, through pruning, as well as by removing wilted or damaged plant parts
- Only harvest and store undamaged produce
- Cool harvested produce quickly, and store at cooler temperatures (10-12°C) with plenty of air movement
- Avoid storing with ethylene-producing products
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