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.
Gray mold spreading from a spore covered mummy berry to a ripe berry
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.
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.
Honeoye is the only cultivar recommended in Minnesota that has shown partial resistance to gray mold.
- Choose sites with full sun, good soil drainage and air circulation. This fungus requires long periods of continuous wetness to infect plants, thus any practice that promotes quick drying of leaves and fruit will reduce disease.
- Remove weeds to improve air circulation around plants. This will also remove any weeds infected with gray mold to prevent spread of the disease to the strawberry plants.
- Plant in rows or narrow beds, no wider than 12-18 inches to promote good air movement in and around plants. Patches grow with time as new runners are produced. Use renovation to maintain narrow beds.
- Renovate strawberry beds every year after harvest. Renovation is described in Strawberries for the Home Garden.
- Following renovation, rake and remove old leaves.
- Irrigate through drip irrigation or a soaker hose. If overhead sprinkling is your only option, water early in the morning on a sunny day so leaves and blossoms dry quickly.
- Apply nitrogen fertilizers after renovation. Avoid early spring applications of nitrogen which encourage overly lush growth that reduces airflow promoting a moist microclimate in the canopy.
- In patches with a history of gray mold, remove and discard all straw in early spring approximately when lilac flower buds appear. Place fresh straw or other organic mulch between beds to reduce rain splash and weeds and improve air movement around berries.
- Avoid wounding plants. Wounds facilitate entry of the pathogen.
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 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.