In many areas over the past month, we have received twice the usual amount of rainfall, on top of soil which was already well saturated with winter snow melt. As a result, soils have been soaked for days at a time, if not weeks. These conditions are ideal 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. The excess water we have had in the cold soil not only stresses roots, but 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 isn't 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.
While it is too late for this year, 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, a 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.
This year, because there has been so much water in soils, it may be necessary to use chemicals. Generally these 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. These 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.
Aliette® (fosetyl-A1) is registered to control Phytophthora on strawberries and 'caneberries' (raspberries, blackberries). For strawberries, it may be applied on established beds as a foliar spray for red stele. These applications should start when the plants begin to grow in spring. If wet conditions persist, additional applications should be made at 30 to 60 day intervals. New plants may be dipped in a solution of 2.5 lbs/100 gal. for 30 minutes. Aliette® can also be used as a foliar spray against leather rot. Leather rot should be treated where there has been a significant history of the disease. Start at 10% bloom to early fruit set (2.5 to 5.0 lbs./A), and repeat every seven to 14 days while wet weather continues.
Ridomil® (metalaxyl) may also be used against Phytophthora in blueberries as well as strawberries and caneberries. It comes as a 2-E liquid or a 50WP. For either formulation, treatment focuses on the soil rather than the plant, though for leather rot on strawberry the treatment is foliar. Consult the label for more specific directions.
Daniel Cooley, University of Massachusetts Extension