Disease of the Month - Gummy Stem Blight

  Hort Snacks - November 2018
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 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
    • May also be seedborne
  • 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
  • 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
  • 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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For more information about the content of this document, contact Robert Spencer.
This information published to the web on October 30, 2018.