Disease of the Month - Sclerotinia Rot

  Hort Snacks - January 2017
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 A.k.a. White Mould, Cottony Soft Rot, (Lettuce) Drop,

Causal Organism: Sclerotinia sclerotiorum

Crops Affected: Carrots, lettuce, celery, beans, cole crops, potatoes, peas, cucurbits (pumpkins, cucumbers, squash, melons), solanaceous crops (tomato, pepper, eggplant), various weed species (e.g. cruciferous), canola, soybeans, etc. (over 400 host species)

Disease Cycle:

  • Soil-borne sclerotia (hard mycelial bodies) produce mycelium, which infect plant tissues
    • Sclerotia can survive for over 10 years in the soil
  • Sclerotia can also produce little mushrooms (apothecia), which release spores
    • Spores can infect weak tissues in high and prolonged humidity conditions
  • Disease develops at temperatures above 0C, but prefers 13-18C for optimum growth.
  • High humidity and free moisture also contribute to disease infection
  • May initially cause damping off or petiole infections, with leaves and crown areas becoming infected, especially those in contact with the soil
  • Infected tissues typically turn dark brown and a characteristic white, cottony mycelium covers it
  • Some crops exhibit a wilting of infected tissues (e.g. lettuce)
  • Black sclerotia (hard, mycelial, overwintering bodies – not spores) form in diseased tissues later on, sometimes within tissues
    • Size of these will vary, but are often large (may resemble rat droppings or Tic Tacs
    • May be flattened, rounded, oval or disc-like
  • In storage (specifically carrots and cabbage), a watery soft rot is first observed, followed by darkened tissues, which gradually are covered with the white mycelium, with sclerotia forming in the middle of the mycelial growth
  • This disease can be distinguished from similar diseases by the cottony white mycelium and the black sclerotia
    • Bacterial soft rots do not form mycelium and are slimy
  • Rotate to non-host crops; maintain a 3-5 year rotation to reduce soil borne inoculum
  • If possible, remove infected debris from growing areas to reduce inoculum
  • Control weeds, as they are both alternate hosts and can contribute to increased relative humidity
  • Ensure good air circulation within the canopy
  • Ensure good soil drainage
  • Research has found that controlling canopy density can significantly reduce disease levels
    • Use reduced rates of nitrogen
    • Use varieties that are more upright or that have thinner tops
    • Trim the canopy (reduce leaves up to 40%) to ensure good air movement
      • Trimming also severs the connection between older, weak leaves that lie on the soil surface, reducing crown infection
  • Rapidly cool harvested crops and store crops such as carrots at constant 0C
  • Ensure storage areas and containers (e.g. pallets, etc.) are properly sanitized, to remove any sclerotia that may adhere to surfaces
    • If possible, avoid use of storage equipment (bins, pallets, etc.) that are made of organic materials
  • Preventative chemical sprays are available to protect various crops. Follow label instruction for maximum impact
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on January 3, 2017.