Phytophthora Root Rot
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, which can often be observed in low-lying areas of a field. Declining plants previously diagnosed as suffering from "wet feet" or winter injury usually are infected by this root rot.
Symptoms show up primarily in wet sections of the field. Death of plants may be sudden or gradual. 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. Severely infected fruiting canes wilt and die as the weather grows warmer before harvest. Few canes are produced, in contrast to when the cause is of aboveground origin (e.g., winter injury, cankers, borers) where normal numbers of primocanes are produced. The root systems of affected canes must be examined to diagnose phytophthora root rot. This is done by digging up plants that are wilting but that have not yet died and scraping away the outer surface (epidermis) of the main roots and crown. 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.
Phytophthora root rot is caused by at least eight different species of soilborne fungi belonging to the genus Phytophthora. Depending on the species of phytophthora causing the infection, inoculum may already have existed in the planting site, or may have been brought in on infected plants. Once phytophthora is present, saturated soil is necessary for spread of the disease. A certain period of flooding may be necessary in order for infection to take place. 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.
Use only clean planting stock. Good soil drainage and proper variety selection are necessary for controlling phytophthora root rot. Plant all brambles in well-drained soil or in raised-bed plantings. At no time should water be standing in the field. Highly susceptible varieties include Hilton, Ruby, and Titan. Canby, Cumberland, Festival, Munger, Reveille, and Taylor also appear to be very susceptible. Bristol, Cherokee, Jewel, Latham, and Newburgh are the least susceptible and the safest choices if berries are on marginal sites. Black and purple raspberries are relatively, but not completely, resistant. No raspberry is immune, although black and purple raspberries are somewhat resistant.