Why are my strawberries rotting on the plant?

How to tell which rot it is

Look closely at a few damaged berries. Gray mold starts as light brown, firm spots, usually at the cap end or where a berry touches a rotten one, and soon grows a dusty gray fuzz. Leather rot turns green berries brown and leathery inside and out, and ripe ones dull pink to lilac with a bitter taste. Anthracnose leaves round, slightly sunken spots 1/8 to 1/2 inch across, often with salmon-pink ooze in the center in wet weather. Slugs chew ragged holes with smooth edges and leave silvery slime trails, mostly on berries resting on the ground.

Why it happens

Gray mold is the usual culprit. University of Connecticut IPM calls it the most important disease of strawberry fruit. The fungus is present almost everywhere and favors cool, wet weather between about 59 and 77°F. It gets in through dying flower petals, then often waits inside the green fruit until it ripens. Ripe berries left on the plant, berries touching each other, crowded rows, weeds, and overhead watering all keep fruit damp and help it spread. Iowa State University Extension notes that nitrogen fertilizer in spring promotes lush, leafy growth that keeps the planting humid.

The other rots start from the soil. Leather rot, caused by Phytophthora cactorum, is splashed onto fruit by rain or infects berries lying on wet soil, and UConn notes it can infect after as little as an hour of wetting at about 60 to 80°F. Anthracnose favors warm, humid weather, survives in plants and debris, and is more severe on day-neutral and late-bearing varieties. Slugs and snails feed at night and on cloudy days, and University of California IPM notes that they are serious pests of ripening fruit close to the ground, such as strawberries.

What to do now

Pick ripe berries often, and pick every rotten or damaged one too, carrying them out of the bed in a separate container rather than dropping them on the ground. Iowa State advises removing berries with gray mold from the planting. Tuck a thick layer of straw under the plants so fruit sits on dry mulch instead of soil. Water at the base in the morning so leaves and fruit dry by evening. For slugs, lay boards raised about an inch off the ground and scrape off the slugs that gather underneath each morning, or scatter iron phosphate bait thinly as the label directs and keep pets away from it.

Next season

Prevention comes down to airflow and keeping fruit off wet soil. Plant in well-drained soil, space plants widely in narrow rows, keep weeds down, and skip spring nitrogen fertilizer on varieties that bear one main crop a year. UConn notes that straw mulch works better than plastic against leather rot and anthracnose, because plastic lets berries sit in water and bounces spore-filled raindrops higher into the plants. Start with disease-free plants, and if anthracnose was a problem, move strawberries to a new spot for two to three years. Clear dead leaves and old fruit from the bed, since gray mold gets its start on dying tissue.