Leather rot occurs sporadically in Minnesota. The disease infects flowers and fruit at all stages. Infected strawberries have a distinctively unpleasant odor and a strong, bitter taste. Infection of a few ripe berries that are processed into jam can ruin the whole jar with this off-taste.
Infections on green fruit are typically tan to brown areas but occasionally the infected area remains green outlined by a brown margin. As the disease progresses these unripe berries become completely brown and have a rough leathery texture. Infection of ripe fruit may cause little to no color change, or the infected area may become pale, purple or brown. Rot on ripe fruit becomes dry and leathery over time. Both ripe and unripe infected fruit eventually dry down into fruit mummies.
The leather rot pathogen is an oomycete; commonly called a water mold. These unique pathogens thrive in wet conditions and produce three types of spores. Oospores are tough resting spores that form in mummified berries and can survive many years in soil. These germinate when soils are saturated to produce sporangia and then zoospores. Zoospores are swimming spores that move through a film of water on the plant or soil to reach susceptible fruit and flowers. Zoospores only need two hours of moisture on the plant surface to start an infection. Once infected, sporangia are produced on fruit and are splashed by rain or irrigation to infect other fruit. The leather rot fungus thrives in areas where water stands for awhile after a rain event.
- Choose a location with good drainage or improve drainage before planting through adding organic matter to soil and redirecting water away from the area. Strawberries can also be planted on raised beds to improve drainage.
- Utilize straw mulch to keep berries from contacting soil and any puddled water. Mulch will also reduce splashing of spores from the soil up onto fruit and flowers.
- 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 dry quickly after irrigation.
- Pick fruit frequently and remove over ripe and diseased berries from the field.
- There are no fungicides available to home gardeners that are effective in preventing leather rot.