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

Abstract

Beet necrotic yellow vein virus (BNYVV) is transmitted by Polymyxa betae to sugar beet, causing rhizomania disease. Resistance-breaking strains of BNYVV, overcoming single (Rz1) or double (e.g. Rz1+Rz2) major resistance genes in sugar beet have been observed in France and recently in the USA and Spain. To demonstrate if resistance-breaking is dependent on inoculum density, the inoculum concentration of BNYVV and P. betae in soil samples where resistance-breaking had been observed was estimated using the most probable number (MPN) method. The MPN-values obtained displayed highly significant differences with respect to the virus concentration in various soils and did not correlate with the ability to overcome resistance. Virus quantification in susceptible plants demonstrated that soils containing resistance-breaking isolates of BNYVV did not produce higher virus concentrations. The MPN assay was repeated with Rz1+Rz2 partiallyresistant sugar beets to see if the resistance-breaking is concentration-dependent. There was no correlation between soil dilution and increased virus concentration in Rz1+Rz2 plants produced by BNYVV resistancebreaking strains. Determination of the absolute P. betae concentration by ELISA demonstrated that all resistance-breaking soil samples contained elevated concentrations. However, the calculation of the proportion of viruliferous P. betae did not show a positive correlation with the resistance-breaking ability. Finally resistance-breaking was studied with susceptible, Rz1 and Rz1+Rz2 genotypes and standardised rhizomania inoculum added to sterilised soil. Results from these experiments supported the conclusion that resistance-breaking did not correlate with virus concentration or level of viruliferous P. betae in the soil.

Detection and molecular characterization of Polymyxa betae, transmitting agent of sugar beet rhizomania disease, in Iran

“…P. graminis isolates were very diverse in their ITS rDNA sequences (Legrève et al, 2002;Kanyuka et al, 2003). The lack of diversity in this region of P. betae has already been observed in other studies (Pferdmenges & Varrelmann, 2009;Smith et al, 2011Smith et al, , 2013. Our work with Iranian isolates supports the finding that this part of the genome is highly conserved in P. betae.…”

Host Effect on the Genetic Diversification of Beet necrotic yellow vein virus Single-Plant Populations

“…The original soil inoculum was infested with a BNYVV population composed of one predominant haplotype, named hO, encoding the WT amino acid A 67 C 68 D 135 p25-motif. Previous studies indicate that BNYVV strains carrying this p25 signature were virulent in rz1rz2 but avirulent in the analyzed Rz1 and Rz2 sugar beet genotypes (3,25,32). By contrast, RB variants able to overcome Rz1 encoded valine at position 67 (1,3,25), whereas the capability to cause rhizomania in Rz2 plants has not been associated to any specific mutation.…”

Occurrence of resistance‐breaking strains of Beet necrotic yellow vein virus in sugar beet in northwestern Europe and identification of a new variant of the viral pathogenicity factor P25

“…Three reference sugar beet genotypes, with and without major BNYVV resistance genes Rz1 and Rz2 , employed in numerous previous studies, were used for this work (Liu et al ., ; Pferdmenges & Varrelmann, ; Pferdmenges et al ., ; Koenig et al ., ; Bornemann & Varrelmann, ): one Rz1‐ resistant genotype ( Rz1rz1 ), one Rz1 + Rz2 genotype, which carries the two major resistance genes Rz1 and Rz2 ( Rz1rz1 + Rz2rz2 ), and a susceptible genotype ( rz1rz1 ). The different genotypes were used both in field trial and greenhouse assays.…”