How to Manage Pests
UC Pest Management Guidelines
Phytophthora and Pythium Root Rots
Pathogens: Phytophthora drechsleri and Pythium aphanidermatum
(Reviewed 11/05, updated 11/05, corrected 12/09)
In this Guideline:
SYMPTOMS AND SIGNS
Symptoms for Phytophthora and Pythium root rots are different, but management of the two diseases is the same. Plants with Phytophthora root rot appear wilted in the early stages of disease development and eventually wilt permanently, especially when hot, dry conditions prevail. Initial infection occurs at the base of lateral roots, causing a small necrotic lesion. As the disease progresses, it appears as a wet root rot and advances upward toward the crown. Rotted tissue turns brown with a distinguishing blackish margin adjacent to healthy tissue.
Pythium root rot is a wet rot that causes the taproots of mature beets to become brown to black. As the disease progresses foliage wilts, leaves yellow, and older lower leaves die. Older leaves may have blackened water-soaked lesions at the base of the petiole.
COMMENTS ON THE DISEASE
Phytophthora drechsleri and Pythium aphanidermatum are soilborne fungi. The disease is most common in fields where sugarbeets are exposed to excessive soil moisture, particularly following heat or moisture stress. Optimum temperatures for disease development are 82° to 88°F (28° to 31°C). Stand reduction can occur if seedlings are infected and stressed or when older plants are infected later in the season. Heavily infected sugarbeet crops have a low sugar concentration and high level of impurities at harvest.
Provide adequate field drainage and prevent excessive seepage from irrigation canals. Most importantly avoid overirrigating, especially during periods of high temperatures. In soils where drainage is a problem, plant in raised beds, use sprinkler irrigation, and rotate to nonsusceptible crops to reduce inoculum potential. Carefully adjust cultivating and thinning equipment to reduce mechanical injury to feeder roots.
UC IPM Pest Management Guidelines:
S. Kaffka, Plant Sciences, UC Davis
Acknowledgement for contributions to Diseases:R. T. Lewellen, USDA, Salinas
C. A. Frate, UC Cooperative Extension, Tulare County