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Breaking of Beet necrotic yellow vein virus resistance in sugar beet is independent of virus and vector inoculum densities

“…Clearly the corresponding BNYVV concentrations of the different isolates produced in the Rz1 plants in both assays, strongly supports the resistance-breaking abilities of D, IV, MN and P. The low virus concentrations in rootlets of the Rz1 genotype grown in R and GG corresponded to previous observations by Heijbroek et al (1999), who compared another Rz1 sugar beet genotype in resistance tests (in this study the ELISA tests were performed after 8 weeks) in soil samples from the same geographic areas as R, GG and P soils; similarly a low virus concentration in the R and GG treatments was detected. In general, the virus concentrations in lateral roots of the Rz1 cultivar were higher than those in the double-resistant cultivar, reflecting the known additive effects of the two major resistance genes in greenhouse experiments and field resistance tests (Gidner et al 2005;Liu et al 2005).…”

“…In this study rhizomania-infested soil samples from fields where BNYVV resistance-breaking strains of the virus were observed (Heijbroek et al 1999;Liu et al 2005;Liu and Lewellen 2007 and J. Ayala, personal communication) have been assayed for the first time in a growth chamber study under controlled conditions for BNYVV and P. betae inoculum concentration as well as for P. betae reproduction abilities in lateral roots. All samples were previously tested in parallel in 12-week greenhouse resistance tests and had been shown to induce BNYVV concentrations comparable to susceptible plants in Rz1 cultivars (Pferdmenges et al 2008).…”

“…Although the BNYVV resistance mechanism in sugar beet is not clearly understood, resistant plants are known to confine the virus to the rootlets and limit the spread to the taproot (Heijbroek et al 1999;Scholten et al 1994). Therefore, we wanted to investigate whether infection of lateral roots in both resistant genotypes with isolates from soil samples displaying resistance-breaking abilities led to isolate-dependent systemic virus spread in sugar beet roots.…”

“…Nevertheless, all known major resistant genes against the virus, both on their own or in combination, provide only partial resistance and are unable to prevent BNYVV infection entirely. Usually the cultivars possess the ability to reduce the virus replication in secondary rootlets and inhibit virus spread to the taproot (Heijbroek et al 1999;Scholten et al 1994). Thus it is generally assumed that the inoculum potential in soils increases continuously.…”

“…In Europe, BNYVV-P is the only pathotype that contains an additional RNA 5 (Koenig et al 1986). BNYVV-P shows an enhanced rate of spread in plants due to the additional RNA 5 encoding a second pathogenicity factor (Heijbroek et al 1999;Tamada et al 1996).…”