Many red raspberry plants are exhibiting symptoms of Phytophthora root rot after the recent period of excessive rainfall and saturated soils. Movement of soil or water from an infected planting will spread this disease. Phytophthora first appears in low-lying spots and then typically expands outward. With the recent high temperatures following the rainy period, infected primocanes (new vegetative canes emerging from the soil) wilt rapidly and die. Near the soil surface, canes appear water-soaked. Infected floricanes (fruiting canes) may also wilt or produce weak lateral shoots with yellow leaves or interveinal chlorosis. Phytophthora-infected roots have a reddish brown color when the bark is lightly scraped. This symptom is easily found at the interface of infected and healthy tissue.
The best control is to avoid planting in low-lying areas. A sloping site or a narrow raised bed with rapid water drainage away from the root zone is a good preventative measure against Phytophthora. Some cultivars, such as ‘Latham’ and ‘Newburgh’, are resistant to somewhat resistant this root rot, while ‘Festival’, ‘Heritage’, ‘Reveille’, and ‘Taylor’ are moderately to highly susceptible. ‘Titan’ and ‘Hilton’ cultivars are extremely susceptible to Phytophthora. For chemical control, mefenoxam may be applied in early spring (delayed dormant stage) when the tips of the buds show green. Another application can be made after harvest. Mefenoxam fungicide is available as Ridomil Gold® SL (liquid) or GR (granular). Do not apply Ridomil within 45 days before harvest. Other products containing phosphorous acid are available for commercial producers. These products are systemic and are rapidly absorbed into the leaves and translocated to the crown and roots of the plants. Other control recommendations can be found at: http://www.ag.purdue.edu/hla/Hort/Documents/ID-169-2010.pdf.
While Phytophthora is one problem with raspberries, the rapid transition from cool to hot, steamy weather will create other problems. Raspberry canes will cease rapid growth and will become stunted, as they are not well adapted to hot weather conditions. At temperatures over 85ºF, fruit size is diminished and has a short shelf-life. To maintain raspberry quality, harvest the fruit early in the morning when temperatures are cooler and refrigerate them immediately.
REVISED: October 8, 2013