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Gray mold

Botrytis cinerea

Gray mold is the most common fruit rot disease of strawberries in Minnesota. The disease is most prevalent if prolonged cool, wet weather conditions exist during flowering.

Identification

grey-mold

Tan discoloration on fruit from gray mold infection

M.Grabowski, UMN Extension

grey-mold

Gray mold spreading from a spore covered mummy berry to a ripe berry

T. McCamant, Northland Community College

grey-mold

T. McCamant, Northland Community College

Infected blossoms turn from dark brown to black and do not develop into fruit. Fruit rot is most common on ripe strawberries, but a dry, tan spot caused by gray mold can occasionally be seen on white or green fruit. On ripe strawberries, gray mold infections appear as tan, soft, rotted areas that do not have a distinct border but rather fade into healthy tissue. In humid conditions, rotten areas are completely covered with velvety gray powdery fungal spores. Gray mold rot can begin anywhere on the fruit but is most common on the stem end, where the fruit comes in contact with infected flower parts or anywhere the fruit is touched by another infected berry. Ripe fruit quickly becomes completely rotten and covered in powdery, gray, fungal spores. These fruit often remain attached to the plant and dry down into gray or black mummified berries. In wet conditions, mummy berries may be completely covered with gray velvety spores.

Green leaves show no symptoms of infection with the gray mold fungus. Fluffy gray spores can be found growing on the dead, brown leaf surface when leaves are killed by frost or other environmental factors. Plants damaged by winter injury may develop Botrytis crown rot, where leaf and flower stems rot and turn brown at the point where they attach to the crown of the plant.

Important biology

The gray mold fungus overwinters on dead strawberry leaves, infected straw, mummified fruit and occasionally on weeds. Spores form under cool, wet conditions and are blown by wind, splashed by rain or irrigation, or moved by pollinating insects to flowers and other susceptible tissues. The majority of fruit rot infections begin through infection of flowers. Infection is most severe in rainy or humid conditions where flowers remain wet for extended periods of time (>12 hours). Infections may blight blossoms, which become blighted and do not develop into fruit. More commonly, infections grow into the young fruit and remain dormant until the fruit begins to ripen. At this point the fungus rots the fruit and produces powdery, gray spores on the surface of the fruit. Botrytis fruit rot easily spreads to adjoining berries wherever the healthy and rotten fruit touch. Fruit infected with Botrytis fruit rot often remain attached to the plant but dry down to a shriveled mummified berry. New spores are readily produced on mummy berries during wet weather. The gray mold pathogen will colonize young leaves and remain dormant within them. When leaves die naturally, they can become an important source of gray mold spores within the strawberry patch.

Management

Resistance

Honeoye is the only cultivar recommended in Minnesota that has shown partial resistance to gray mold.

Cultural control

Harvest recommendations

Harvest frequently and remove infected fruit from the field throughout the harvest season. Take care to keep diseased fruit separate from healthy fruit as gray mold can spread rapidly even after harvest. Handle berries with care and refrigerate soon after picking.

Fungicides

Fungicides may be needed to protect fruit from gray mold fruit rot in years where rainy wet weather persists while plants are in bloom. In this case, fungicides should be applied during blossom to prevent fruit rot. Read and follow all label instructions. If the season is one characterized by prolonged periods of wet or humid weather, continue spraying at the interval described on the fungicide label until petal drop. If possible watch the weather and spray before rain is predicted. Fungicides with Copper or Captan as active ingredients will reduce gray mold fruit rot in strawberry when applied properly. Fungicide sprays applied to green fruit and during fruit harvest do little to reduce disease and are not recommended.


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