2004b Sugarbeet Research and Extension Reports. Volume 35, Page 212 LACK OF CONTROL OF APHANOMYCES ROOT ROT ON SUGARBEET BY GREEN MANURE CROPS AND SOIL SOLARIZATION

Aphanomyces cochlioides (= A. cochlioides) is a soilborne “water mold” that causes seedling stand loss and chronic root rot of older sugarbeet plants when soil is warm and wet. Unusually wet summers since 1993 have resulted in an increase in prevalence and severity of Aphanomyces diseases on sugarbeet. This pathogen produces thick-walled oospores in infected roots which survive in soil for years, even when a sugarbeet crop is not grown. Current control measures for Aphanomyces damping-off and root rot include: early planting (to avoid warm, wet soils favorable for infection), seed treatment with the fungicide Tachigaren (hymexazol), planting partially resistant varieties, water management (installing tiles or ditches to improve soil drainage, cultivating to dry soil), and weed control (A. cochlioides infects several common weed species, e.g., pigweed, lamb’s-quarters, kochia). When fields have high potential for disease, producers are advised to avoid planting sugarbeet because if the season is wet and warm, control options are inadequate and do not result in an economic return. Since current disease control options are limited in effectiveness, other strategies such as green manure crops and soil solarization (singly and in combination) are being explored. Green manure crops suppress several soilborne pathogens and pests on many crops including: sorghum sudan grass for Verticillium wilt on potato, buckwheat for scab on potato, oilseed radish for the sugarbeet cyst nematode, and oat for Aphanomyces root rot on pea (5). Solarization also reduces disease by “pasteurizing” soil through a process where wet soil is covered with clear polyethylene plastic to capture solar energy and increase soil temperatures, ideally between 97 to 122 °F in the upper 12 inches (3). Equipment is available for large-scale solarization of fields and is applied most effectively in geographic areas with intense solar radiation. Solarization also can be effective in temperate regions when combined with green manure crops (4), reduced dosages of chemicals, or biological control organisms (3). Recent research in The Netherlands has shown that incorporation of fresh broccoli or grass into soil followed by tarping with plastic reduced several soilborne fungal pathogens and nematodes by rapidly depleting oxygen and not by thermal inactivation (1, 2).