The Water Molds, Wet Weather and Root Rots

Saturated soils are ideal conditions for a group of fungi called the oomycetes, or water molds. Some of these fungi will infect small fruit crops, causing root and crown rots, and even fruit rot. One kind of spore produced by these fungi swims in the soil water solution, looking for a host root. After it finds one, the spore then attaches and infects the root. In the spring, excess water along with cold soils not only stresses roots, but also provides excellent infection conditions for these diseases.

The fungi involved in small fruits are species of Phytophthora and Pythium, which cause diseases such as red stele, black root rot, and leather rot on strawberry, and crown and root rot on raspberries. The extent to which Pythium contributes to black root rot is not known, and even less is known about how to control black root rot. Fortunately, a bit more is known about management of the other diseases.

Susceptibility to some of these diseases varies by cultivar. For example, “Honeyoye’ and ‘Seneca’ are susceptible to red stele, while “Guardian’ and ‘Allstar’ are not. For raspberries, the susceptibility of ‘Titan’ to Phytophthora is well known, while ‘Newburgh’ or ‘Latham’ show some tolerance.

Genetic and Cultural Control: The first defenses against the water molds should be genetic and cultural. If a relatively wet site or heavy soil is the only planting option, then consider using raised beds for small fruits. This reduces or eliminates standing water in the root zone, thus reducing or eliminating the chance for root infection by the Phytophthora or Pythium. Besides raised beds, selecting resistant or tolerant cultivars can greatly reduce root disease problems.

Mulch is a cultural tool used in many small fruit plantings. It can be both good and bad. In strawberries, clean straw mulch over the alleys and in rows will eliminate rain and irrigation splash from soil to plants. This splash is responsible for most of the spread of leather rot disease. On the other hand, experiments show that straw mulch in raspberries appears to hold moisture in the soil longer, and causes much more root rot.

Chemical Control: Generally, chemicals are more effective if used in the early stages of infection, so the sooner the better. By the time plants are showing severe symptoms, treatment will do little good. Fungicides are more of an emergency treatment than standard applications. Their continued use year after year may result in resistance development by the fungi being treated. Consult your local Cooperative Extension center for the latest chemical control information.

By: Daniel Cooley, University of Massachusetts Extension

Updated by: Mary Concklin, UConn IPM. 2012

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