EFFECT OF SUGAR-BEET CULTIVARS WITH DIFFERENT LEVELS OF RESISTANCE TOBEET NECROTIC YELLOW VEIN VIRUS ON TRANSMISSION OF VIRUS BY POLYMYXA-BETAE

The effect of resistance of sugar beet cultivars to beet necrotic yell ow vein virus (BNYVV) on virus content of resting spore clusters of th e vector Polymyxa betae was studied in controlled environments and in naturally infested fields. The total number of resting spore clusters formed in roots of a partially resistant and a susceptible cultivar di d not differ when assessed 6 and 12 weeks after inoculation with virul iferous resting spores. Transmission experiments showed that in partia lly resistant plants, having a low virus content in the roots, the pop ulation of resting spores formed was less viruliferous than that in su sceptible plants with a high virus content. Consequently, growing a re sistant cultivar can be expected to delay the build-up of virus inocul um in soil. In a trial field sampled in 1991, the inoculum potential o f BNYVV (most probable number of viruliferous P. betae propagules) in soil was lower after growing a partially resistant cultivar than after growing a susceptible one. On the other hand, in four sites sampled i n 1990, inoculum potential in soil was hardly increased by growing sug ar beet and was not significantly affected by the cultivar grown.