Phytophthora diseases a threat to strawberries during excessively wet conditions

If your strawberries are susceptible to leather rot, improving field drainage and maintaining straw mulch between the fruit and the soil are the key control methods.

Leather rot, caused by the oomycete Phytophthora cactorum, can cause considerable losses in years with lots of rain during early fruit development. Fruit is susceptible at all stages of fruit development and entire fruit clusters may be killed. Small green berries become brown and hard. Larger green berries may appear brown and leathery while mature berries turn off-pink or light brown and have a pungent, chemical-like odor. I sometimes call them “Neapolitan” berries because they may have a gradation of beige, pink and brown colors. The strong odor distinguishes this disease from other fruit rots.

Leather rot is primarily a problem in fields with little straw cover or areas with puddles of water, so that the berries are in direct contact with the soil and standing or splashing water. Oospores of Phytophthora cactorum can survive in the soil for many years and germinate under wet conditions in the spring. Swimming zoospores can reach berries directly or get splashed onto the fruit from the soil.

The key control methods are maintaining a good layer of straw mulch between the fruit and the soil and improving drainage in the field. If the disease nevertheless appears, the following fungicides are effective against leather rot: Ridomil Gold, Abound, Cabrio, Pristine, Captan, and phosphites like ProPhyt and Phostrol. Abound, Cabrio, Captan and Pristine are also effective against other fruit rots like anthracnose, but only Pristine is effective against Botrytis gray mold as well. Other fungicides may be needed for control of Botrytis gray mold during bloom.

Although red stele, caused by Phytophthora fragariae, is rare in Michigan due to our predominantly sandy soils, it may flare up on heavier soils this year, especially if there is standing water for several days. Symptoms include rapid collapse and death of plants, particularly in wetter portions of the field, and rotted roots. When roots are cut open, the central vascular cylinder (the stele) is reddish brown while the root cortex may still be creamy in color. Growers who have had red stele in their fields in the past may decide to apply Ridomil Gold or phosphites preventively. Good drainage is the best management technique, but this may not be feasible in all sites.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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