Several soil-borne fungi belonging to the genus Phytophthora cause root rot in plants in the Rubus genus, which include raspberries and blackberries. These berries grow on canes, so they are also called caneberries. Phytophthora weakens the plants and stunts their growth, often causing them to collapse and die.
Berry plants will show a lack of vigor and grow poorly. Canes that look healthy may suddenly decline and collapse in late spring or summer. The leaves will look red, orange or yellow and the edges may look scorched. The infections often strike in patches, spreading along a row. Infected canes wilt and die.
If scraped, the outer surfaces of roots look reddish-brown. As the rot continues, the roots turn dark brown. The roots and crown lack fibrous roots.
Conditions for Infection
Phytophthora fungi thrive in moisture. They are often found in heavy clay soils or in parts of a field or garden that are slow to drain, including dips and the bottom ends of rows.
How Phytophthora Spreads
Phytophthora fungi are found as dormant spores in the soil or as mycelium (threadlike growths) in infected roots. Reproductive structures called sporangia form on the roots.The sporangia expel swimming spores into water-saturated soil. When the spores find a root, they attach themselves to it.
Roots in standing water that has been depleted of oxygen are unable to resist infection. The potential for an epidemic exists when plants are repeatedly allowed to sit in standing water. Infection can occur throughout the growing season, but most often in spring and fall.
Do not plant caneberries that show symptoms of infection in a nursery.
Plant clean berry plants in soil that is not infected. Plant berries in soil that drains well. Amend heavy clay soil if necessary.
Plant caneberries on raised beds.
There are cultivars of raspberries that resist Phytophthora root rot. Check your local agricultural extension service to learn what resistant varieties grow well in your area.
Laboratories and greenhouses are now able to propagate raspberry cultivars through tissue culture so that they don't come into contact with potentially infected soil. These plants are more likely to be free of Phytophthora fungi than those grown in soil.
Horticulturalists at the University of California recommend treating the soil before planting caneberries with fungicides containing the active ingredient 1,3-Dichloropropene. Apply 48 gallons per acre. Do not reenter the area without protective clothing for five days.
Apply fungicides containing the active ingredient mefenoxam on caneberries in the fall before it rains. Apply ¼ pint per 1,000 feet of row. Do not enter the area without protective clothing for 48 hours. Do not harvest berries until 45 days after application.
Apply fungicides containing the active ingredient Fosetyl-Al in the fall after harvest and before it rains, also three to four weeks later if necessary. Apply five lbs. per acre. Apply when the shoots are one to three inches tall in the spring and three to four weeks later if necessary. Do not enter the area without protective clothing for 12 hours. Do not harvest berries until 60 days after application.