Fruit & Vegetable Magazine

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Ontario potato growers should beware of white mould


August 3, 2021
By Eugenia Banks, Ontario Potato Board

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Lush canopies and wet weather provide the best conditions for the development of white mould, a yield robber that needs to be monitored closely before serious losses occur, according to Eugenia Banks of the Ontario Potato Board.

The white mould fungus, Sclerotinia sclerotiorum, overwinters in the soil as black sclerotia that can survive for several years. The sclerotia germinate in early summer, forming small, pink to beige mushroom-like structures called apothecia. Each apothecium releases millions of spores (ascospores), which are the primary source of inoculum. The ascospores are ejected into the air and are dispersed by wind throughout the field and into adjacent fields.

The peak period of ascospores release coincides with full bloom of potatoes. Ascospores infect potatoes if they land and germinate on stems, leaves or blossoms. 

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The incidence of white mould is reduced with the application of fungicides. White mould infects many crops, including beans, peas and alfalfa. Crop rotation with non-susceptible hosts such as corn or small grains that are weak hosts minimizes the incidence of white mould.

Symptoms of white mould (Sclerotinia sclerotiorum)

All photos are courtesy of Eugenia Banks.

Lesions on leaves initiate on spots where blossoms fell.

White, cottony mycelium develops on infected stems.

Dried-up lesions give the stems a bleached appearance.

Black sclerotia of irregular shape form in dead stems.

Varieties with large canopies provide best conditions
for white mould development if the weather is wet.
Always open up the canopies and check the lower part of the plants to detect white mould.

White mould mycelium can develop on dead leaves or stems, whereas late blight cannot survive on dead plant parts – it survives only on living tissue.