1.11.13
LEVELS OF BEET NECROTIC YELLOW VEIN VIRUS AMONG RESISTANT AND SUSCEPTIBLE SUGAR BEET CULTIVARS IN RHIZOMANIA-INFESTED FIELD PLOTS

GC WISLER, RT LEWELLEN, JL SEARS, HY LIU and JE DUFFUS

USDA-ARS, 1636 East Alisal Street, Salinas, California, USA

Background and objectives
Rhizomania, an economically important disease of sugar beet, is caused by the beet necrotic yellow vein furovirus (BNYVV). BNYVV is vectored by Polymyxa betae. Viruliferous cystosori survive in soil for many years. Control of rhizomania includes avoidance of infested fields by testing soil for the presence of BNYVV prior to planting, soil fumigation, and use of resistant cultivars. Many sugar beet cultivars have been bred with varying degrees of resistance to rhizomania. Previous studies [1, 2] showed that resistant sugar beet cultivars differ in levels of BNYVV detected in the roots. Because viruliferous P. betae remains in soil after harvest and survives until the next crop is planted, it is important to plant varieties which would not contribute to increasing inoculum levels of BNYVV. The purpose of this study was to evaluate virus content in representative commercial and experimental sugar beet cultivars developed for production in the USA that range in host-plant reactions to rhizomania from uniformly susceptible to highly resistant.

Materials and methods
Field trials were conducted at the USDA-ARS Research Station in Salinas, California, where rhizomania tests have been made on infested land since 1984 when BNYVV was identified in California. Tests were planted on 1 May 1997 in a split-plot design with eight cultivars randomized into three harvest dates (July, August, October) and eight replications. From each plot, nine randomly selected beets were analysed for a total of 72 plants per cultivar and 576 at each harvest date. Each beet was scored for rhizomania disease on a scale of 0-9 (where 0=no symptoms), weighed, and tested in TAS-ELISA for BNYVV. At final harvest, beets were evaluated for root weight and percentage sucrose. Plates were coated with a polyclonal antiserum made to the cloned coat protein of BNYVV, and the monoclonal antibody and conjugate were provided by Agdia, Inc.

Results and conclusions
Differences in absorbance (A405 nm) values measured among the eight cultivars closely corresponded to a dosage effect and to the frequency of the Rz allele that conditions resistance to BNYVV. A diploid (Rzrz) hybrid had a significantly lower value than a similar triploid (Rzrzrz) hybrid. Cultivars that segregated (Rzrz:rzrz) had higher absorbance values than uniformly resistant (Rzrz) hybrids. For all cultivars, differences were observed among harvest dates, with progressively lower absorbance values measured as the season progressed. Absorbance values were significantly positively correlated with rhizomania disease index scores, and negatively correlated with individual root weight, plot root weight and sugar yield. This information is useful in resistance breeding and evaluation programmes and for the sugar industry in consideration of cultivar choice, inoculum production and rotations for future cropping.

References
1. Asher M, Kerr S, 1996. British Sugar 64,19-22.
2. Tuitert G, Musters-Van Oorschot PMS, Heijbroek W, 1994. European Journal of Plant Pathology 100, 201-220.