Phytophthora root rot is now regarded as a major cause of declining red raspberry plantings. Blackberries and black raspberries appear to be less susceptible than red or purple raspberries. Wet soil conditions favor the development of the disease and it can often be observed in low-lying areas of a field. Declining plants previously diagnosed as suffering from "wet feet" or winter injury are usually infected by this root rot.

Symptoms

Leaves of infected canes yellow prematurely or appear scorched along the margins and between the veins. Infected canes have weak lateral shoots and are stunted. The root systems of affected canes must be examined to diagnose Phytophthora root rot. Infected roots have a characteristic red-brown color, whereas healthy roots appear white. A distinct line can be seen where infected and healthy tissue meet.

Disease Cycle

Phytophthora root rot is caused by at least eight different species of soilborne fungi belonging to the genus Phytophthora. Resistant spores of the fungus can persist in the soil for a number of years. They are resistant to environmental extremes and chemicals. The fungus requires high levels of moisture and cool temperatures for reproduction. The fungus infects plants in the spring and during the onset of dormancy.

Disease Management

Good soil drainage and proper cultivar selection are necessary for controlling Phytophthora root rot. Plant all brambles in well-drained soil or in raised bed plantings. Highly susceptible cultivars include Titan, Ruby, Hilton, Canby, Taylor, Reveille, and Festival. Latham and Newburgh are the least susceptible and are the safest choices. No raspberry is immune, although black and purple raspberries are somewhat resistant.