Phytophthora Root Rot

Field symptoms of Phytophthora root rot are first noticed when new primocanes (first-year canes) wilt and the shoot tips die back. Floricanes (second-year canes) of affected plants have weak lateral shoots. Leaves turn yellow or scorch from the margins. Often severe wilt and dieback occur during the first hot spell of the season. Roots and crowns are dark brown in color and lack fibrous roots. If the outer surface is scraped from the crown or main roots of recently wilted plants a reddish brown color can be seen with a distinct line where infected and healthy tissues meet. Infected tissue will eventually turn dark brown as the tissue decays.

Phytophthora is a soilborne pathogen that survives in the soil as a resting spore (oospore). When soils become saturated with water for prolonged periods, infectious motile spores (zoospores) are released into the soil and can infect raspberry roots or crowns. Phytophthora species other than P. fragariae var. rubi may be involved in root rot of raspberry. (Generally, Phytophthora does not cause economic damage to blackberries.) Not all root rots are due to Phytophthora. Raspberry roots are very sensitive to excessive moisture in the soil for long periods of time. Root death from lack of air can also occur and result in similar foliar and root decay symptoms.

Control is best achieved by planting in noninfested soils that have good drainage. Avoid low-lying areas that collect excessive water or clay soils that are poorly drained. The use of raised beds can improve drainage as can proper irrigation management; this in turn can reduce disease incidence and severity. Carefully monitor and control irrigation. Use clean plant stock, plant in noninfested soils, and use cultivars suitable for local conditions that are less susceptible to root rots.

Many blackberry cultivars appear to be highly tolerant to Phytophthora, whereas red raspberries are in the main fairly susceptible, with the varieties Latham, Killarny, Caroline, and Nordic most tolerant and the varieties Ruby, Heritage, and Polana most susceptible.

Proper site selection with good water management and the use of clean stock and appropriate cultivars are acceptable management tools in an organically certified crop.

Preplant fumigation can reduce initial disease inoculum to allow for plant establishment in heavily infected sites, though the pathogen will recolonize the area with time. Properly timed fungicide applications may also reduce disease incidence in established plantings.