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White mold on tomatoes

White mold (aka timber rot) has been observed occasionally on Long Island in tomatoes growing in the field and in high tunnels.  Other crops observed affected in the area include pepper, lettuce, and cabbage.

Diagnostic feature for white mold is the black, hard structures resembling rat droppings the pathogen forms on or in diseased tissue (sclerotia).  These are inside tomato stems that are white and dry because of the disease.  Sclerotia are how the pathogen survives for many years.  It is worthwhile, especially in high tunnels, to remove affected plant tissue and thus the pathogen’s survival structures.

Important facts to know about this disease pertinent for management include:

  • The pathogen is monocyclic and thus does not spread from an infection site during a season.
  • The pathogen can survive for years in soil.
  • Many plants are susceptible.

Most fungal pathogens infecting foliar plant tissue produce spores that are dispersed (typically by wind or splashing water) to healthy plant tissue where they germinate, infect, and initiate a new infection cycle.  This cycle is repeated several times within a cropping period (polycyclic).  The white mold pathogen (Sclerotinia sclerotiorum) does not produce spores like this on diseased plant tissue.  Therefore fungicides are only recommended before, not after, symptoms develop.  The spores (ascospores) responsible for starting the single infection cycle are produced by the pathogen from small mushroom-like structures (apothecia) developing from sclerotia near the soil surface when soil is cool (50 – 68 F) and wet (near saturation).  Ascospore germination and infection occurs during long periods (16 – 48 hours) that foliar tissue is wet.  Often dying or dead tissue, such as senescing flowers and leaves, are infected first.

Where white mold is confirmed to have occurred in the past, consider applying Contans to soil before planting.  It is a biological fungicide that acts on sclerotia.  Endura is a fungicide that can be applied to foliage.

Also use cultural management practices that minimize wetness periods, such as wide row spacing, trellising, drip irrigation, and orienting rows parallel to the main wind direction. Manage weeds.  Till deeply to bury sclerotia where removing affected plant tissue is not feasible.  Effective rotation is challenging due to the pathogen’s wide host range and ability to survive in soil as sclerotia for at least 5 years.

For more information, see White Mold in The Plant Health Instructor.

Please Note: The specific directions on pesticide labels must be adhered to — they supersede these recommendations, if there is a conflict. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.

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