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Ministry of Agriculture, Food and Rural Affairs

White Mold

White mold showing sclerotia in stem White mold symptoms on tomato stem Fungal bodies (sclerotia) in white mold lesion White mold on tomato stem- black sclerotia are visible White mold symptoms on tomato stem
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Beginner

Scientific Names
Sclerotinia sclerotiorum, Sclerotinia minor

Identification

  • On young transplants:
    • Water-soaked lesions develop, followed by a prolific, white fungal growth on the stem
    • Small black sclerotia may appear on the stem
  • On older plants:
    • Lesions may occur anywhere on the stem
    • The stem becomes soft and bleached or light grey
    • During cool, moist weather, a cottony growth appears on the stems
    • Black sclerotia may be found on or within the stem
    • Parts of the plant above the affected area may collapse and die
    • Infected fruit rots completely

Often Confused With
N/A

Period of Activity
White mold is usually first noted about the time of flowering.  It is uncommon, except in very wet seasons and therefore not usually a serious concern.  The disease develops rapidly at 20- 25°C (68- 77°F), and not at all below 5°C (41°F) or above 30°C (86°F).

Scouting Notes
The presence of black sclerotia (fungal bodies) in the affected areas is diagnostic.  The sclerotia can resemble rat droppings.

Thresholds
None established.

Advanced

Scientific Names
Sclerotinia sclerotiorum, Sclerotinia minor

Identification
White mold may affect young transplants.  Water-soaked lesions develop, followed by a prolific, white fungal growth on the stem.  Small black sclerotia may appear on the stem.

On older plants, lesions may occur anywhere on the stem.  The stem becomes soft and bleached or light grey.  During cool, moist weather, a cottony growth appears on the stems.  Black sclerotia may be found on or within the stem.  The stem may collapse and die.  Infected fruit rots completely.

Often Confused With
N/A

Biology
White mold overwinters as sclerotia.  Moist soil conditions cause the sclerotia to germinate and release spores which then infect the crop.  These Sclerotinia species are weak pathogens and do not usually infect healthy plant tissue.  They often invade through dead or injured tissue, or spent flower blossoms.

Period of Activity
White mold is usually first noted about the time of flowering.  It is uncommon, except in very wet seasons and therefore not usually a serious concern.  The disease develops rapidly at 20- 25°C (68- 77°F), and not at all below 5°C (41°F) or above 30°C (86°F).

Scouting Notes
The presence of black sclerotia (fungal bodies) in the affected areas is diagnostic.  The sclerotia can resemble rat droppings.

Thresholds
None established.

Management Notes

  • White mold can attack over 360 species of plants.
  • Practice a 3– 4 year crop rotation away from susceptible crops (cucurbits, edible beans, soybeans, carrots and lettuce). 
  • The inclusion of a cereal crop in the rotation will significantly reduce the amount of sclerotia in the soil.
  • Dense canopies and poor air circulation, that allow foliage to remain wet for extended periods, encourage the development of white mold.
  • Sclerotinia species can also cause a collar rot of transplants.