Phytophthora root rot on raspberry plants is caused by several related species of the soil-borne fungi Phytophthora fragariae. It thrives in moist conditions around the roots of strawberry canes and appears in heavy, clay soils, in dips in a field, at the lower end of rows and other places that are slow to drain.
The affected roots of the raspberry canes rot and lack fiber. After hot, dry weather leaves can turn yellow, orange or red, and they may begin scorching at the edges. The plant grows slowly. Berries remain small and wither before ripening. New roots may appear above rotting ones but they are weak and don’t spread laterally. These new roots often become infected during cold, wet weather in the fall and winter.
Phytophthora infection usually occurs in patches and may spread along a row of canes. Canes that seemed healthy may suddenly decline and die in late spring or summer.
To diagnose, dig up canes that are wilting but not dead. Scrape off the outer layer of the main roots and crown. Healthy roots will be white; infected roots will be reddish-brown.
Contact your local agricultural extension service to find out what Phytophthora-resistant varieties of raspberries are appropriate for your area. It is possible to buy raspberry cultivars that have been propagated in laboratories using tissue culture techniques. There is little risk that those plants will introduce Phytophthora fungi into a site that has not been infected.
Plant canes in soil that has not grown raspberries or strawberries for several years. Do not let water collect around plants. Plant raspberries in a bed amended with gypsum and raised 12 inches around the soil. Slope the soil from the base of the plants to the alleys between plants or install drain tiles in the field.
In western Washington state raspberry growers report some success in suppressing Phytophthora by turning the ground around the plants with a rototiller in mid-July through mid-September after which they irrigate the soil to capacity and cover it with clear plastic. This practice, called solarization, aims to kill the fungi with the heat of the sun.
Fungicides containing the active ingredient metalaxyl are labeled by the Environmental Protection Agency for controlling Phytophthora. Drench the soil with the fungicide in the spring or fall, according to instructions on the fungicide label.
Fungicides will do little good if other control measures are not also followed. Do not use fungicides containing metalaxyl within 45 days of harvesting raspberries.