Phytophthora root rot on raspberries is caused by several species of soil-borne fungi in the genus Phytophthora. Root rot is most common on red raspberries, although purples and blacks may also be affected. Phytophthora has been identified as the cause of the decline of stands of red and purple raspberries previously thought to be suffering from winter injury or "wet feet."
Root rot is most commonly associated with heavy soils or portions of a planting where water accumulates, such as lower ends of rows, dips in the field, etc. Infected canes are often stunted with weak lateral shoots and leaves that yellow prematurely or scorch along the margins and between the veins. Severely infected canes wilt and die as the weather turns warmer before harvest. Over time, fewer new canes emerge from within the diseased patches. New canes may become infected, wilt and die within the first year.
It is important to identify the cause of the wilting symptoms since other entities may cause similar symptoms. Winter injury, cane borers, anthracnose and cane blights can also cause cane dieback. To diagnose Phytophthora root rot, dig up canes which are wilting but not dead, and scrape off the outer layer of tissue (epidermis) from the main roots and crown. On healthy plants this tissue will be white, but on diseased plants, this tissue will be red-brown. Often a distinct line can be seen between the diseased and healthy tissue, especially on the crown.
Since this fungus requires free water to spread, the disease is more severe during periods of excess moisture. If water remains standing and oxygen is depleted from the root zone, the plant becomes less capable of resisting invasion by the fungus.
- Avoid saturated soils. Plant raspberries in well drained soils or on raised beds. While root rot is usually a problem on red and purple raspberries, black raspberries and blackberries may also be susceptible if environmental conditions are favorable (wet soils).
- Avoid contamination of planting site. Phytophthora is usually introduced into an area by contaminated soil from runoff water, garden equipment, previous crop plants (apple or stone fruits) or symptomless stock plants.
- Plant resistant varieties. Resistant red varieties include Latham and Newburgh; resistant purples include Royalty and Bradywine. Titan and Hilton are very susceptible to root rot.
- Chemical control. For the homeowner, Ridomil and Alliete can be use to drench the soil. Fungicides can be applied in the spring or fall, but Ridomil has cannot be used on raspberries within 45 days of harvest. Fungicides are virtually useless if the other control measures aren't practiced as well.
This article originally appeared in the June 28, 1996 issue, p. 112.