Effect of conditions during storage of infested soil on infection of bait plants by Polymyxa betae and beet necrotic yellow vein virus

Infectivity of resting spores ofPolymyxa betae in soil stored air-dry or moist was determined by assessing infection of bait plants that were exposed to the soil. Storage of soil under air-dry conditions at room temperature resulted in a delayed onset of germination of resting spores compared to germination in soil stored under moist and cool conditions, as inferred from the infection of the bait plants. Bait plants had to be exposed for more than 12 h to flooded infested soil before germination and infection had occurred. However, when soil was prewetted for 24 h before exposing bait plants, germination, infection and transmission of beet necrotic yellow vein virus (BNYVV) were accomplished within 12 h, but only with the moistly stored soil. When resting spores isolated from roots were stored for 4 and 8 weeks under dry conditions at 22°C, germination of viruliferous spores, as measured by detection of BNYVV in bait plants exposed for 48 h to the spores, was less than that of spores stored in moist soil at 22°C. Approximately 100% of bait plants were infected after exposure to resting spores that were frozen in demineralized water or stored cool (5°C) in water or moist soil for 42 weeks. Air-dry cool storage for 42 weeks resulted in a low percentage of infection. Storage conditions of soil influence the results of bioassays for detection of rhizomania when short baiting periods are applied, whereas differences in infectivity were not detected using a bioassay with long duration.