Michelle Grabowski, UMN Extension Educator
M.Grabowski, UMN Extension
Photo 1: Rotten strawberry with characteristic dusty gray spores of gray mold
As the new crop of strawberries is being harvested, many gardeners may be dismayed to see that their long awaited berries have spots of soft brown rot, often covered in dusty gray spores. The source of the problem is a common fungal pathogen known as Botrytis cinerea, or gray mold.
Gray mold is prevalent this year because of the cool wet weather in Minnesota during the days that the strawberry plants were flowering and growing green fruit. The gray mold fungus thrives in high humidity and temperatures between 59 - 71F. The longer the leaves of the plant stay wet, the more likely the fruit are to rot.
The gray mold fungus survives from year to year in old plant debris. Fungal spores form on old dead leaves and are blown onto the strawberry blossoms.
Gray mold reduces the strawberry crop in two ways. Often one or more flowers within a cluster will turn completely brown and dry up, never producing fruit. These flowers are victims of blossom blight. In some cases the gray mold fungus infects the flower but does not cause blight. Instead the fungus waits until ripe fruit develops. It then infects the fruit and causes soft brown rot to form. As the fruit ripens, the rot spreads until eventually the entire fruit is rotten. If left on the plant, this fruit may become a dry brown mummy berry. Most fruit rot starts with a flower infection, but fruit can also become infected through contact with another rotten berry, soil or dead leaves. Gray mold can also spread after harvest when a rotten fruit sits in a bowl next to healthy fruit.
The best way to reduce problems with gray mold in a home strawberry patch is to reduce humidity and leaf wetness in the patch and to clean up old infected plant debris. The following steps will help keep gray mold in check.
· When choosing a location for the berry patch, pick a site with good air movement and good drainage. Orient rows to prevailing winds so that leaves and fruit dry quickly.
· Control weeds to reduce humidity and shading in the bed.
· Do not apply nitrogen fertilizer before harvest. This results in a dense canopy that shades the fruit and increases humidity.
· Pick fruit frequently and early in the day. Pick only firm healthy fruit.
· Handle berries with care and refrigerate soon after picking
· Remove infected fruit from the field frequently throughout the harvest season.
· For June bearing strawberries, mow the beds after harvest. Rake up and remove all leaf debris. For more details on renovation of June bearing strawberries read Time to Renovate.