Rhizomania, caused by Beet necrotic yellow vein virus (BNYVV), is vectored by Polymyxa betae. The disease can only be controlled by growing partially resistant sugar beets, which quantitatively reduce virus replication and spread. None of the known major resistance genes (Rz1, Rz2, Rz3), alone or in combination, are able to prevent BNYVV infection entirely. Here we report for the first time the identification of a Spanish soil, containing an Atype BNYVV with RNA 1-4, displaying Rz1 resistance-breaking abilities comparable to soils from the USA and to those from France containing the French (Pithiviers) P-type BNYVV with RNA 5. A resistance test with several soil samples vs. different sugar beet cultivars was conducted under standardised conditions. Sugar beets were analysed after 12 weeks of greenhouse cultivation for taproot weight, BNYVV and relative P. betae content. The soil samples from Spain, France and the USA produced high virus contents and strong rhizomania symptoms in Rz1 plants, indicative of resistance-breaking abilities. In addition, all resistance-breaking soil samples produced detectable virus concentrations in plant lateral roots of the Rz1+Rz2 cultivar, and plants grown in the Spanish soil sample also had reduced taproot weight and displayed severe rhizomania disease symptoms. Additionally, the main pathogenicity factor P25, responsible for the formation of BNYVV symptoms, showed high sequence variability in the amino acid tetrad at position 67-70. The results suggest the geographically independent selection of BNYVV resistance-breaking isolates following the uniform cultivation of Rz1-containing sugar beet cultivars.
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.
Rhizomania (beet necrotic yellow vein virus, BNYVV) represents an important sugar beet disease, which is transmitted by the biotrophic plasmodiophoromycete Polymyxa betae. As long as the disease is not controlled it can lead to yield losses up to 90%. To date yield losses due to BNYVV infestation are inhibited by cultivating resistant sugar beet genotypes, which restrict the virus replication and translocation from infected hair-roots to the taproot. The BNYVV resistance is provided in marketable sugar beet varieties by two major resistance sources (Rz1 and Rz2 which either occur singular or in combination). But meanwhile on sugar beet genotypes carrying one (Rz1) as well as two resistance genes (Rz1+Rz2) resistance breaks could be observed at several BNYVV A-type infected sites in the USA and in Spain. To confirm these observations a 12 weeks greenhouse resistance test with three different cultivars (two partial resistant genotypes containing either Rz1 or Rz1+Rz2 resistance sources as well as a susceptible genotype) under standardized conditions with naturally infested soils from 6 locations was performed. The single resistance (Rz1) was compromised in soils from Spain (D), France (P-type, RNA-5 containing), and the USA (IV and MN); in reference soils from Italy (R, A-type) and Germany (GG, B-type) Rz1 resistant sugar beets were not affected. Overcoming of Rz1+Rz2 resistance after 12 weeks could only be observed in D soil. Over and above the genomic region that encodes for the pathogenicity factor (P25) of the BNYVV RNA3 from beets grown in all soils was analysed.Previously suggested correlation between "valine" on position 67 of P25 and a higher virulence could not be confirmed. Isolates in one of the soils as well as experiments previously published, where overcoming of resistance could be observed, contain several other aa 67 than valine. Analyses of additional soil borne pathogens using ITS sequencing and database comparison showed the presence of three pathogens (Rhizoctonia solani Keskin, Fusarium sp., Pythium sp.). Synergism between BNYVV, Rhizoctonia solani Keskin and Pythium sp. could lead to severe virus symptoms and weight reductions particularly in the Spanish soil.To determine if resistance breaks are correlated with the BNYVV inoculum concentration a "Most Probable Number"(MPN) -tests was conducted where same soils as in the resistance tests were examined. Thereby, D soil revealed the highest BNYVV density, the GG soil on the other hand displayed 520 times lower MPN. In order to obtain information on the aggressiveness of particular virus isolates an additional MPN with Rz1+Rz2 genotypes was performed. Within this test D, IV, MN and P resulted again in high BNYVV densities even able to infect Rz1+Rz2 plants after 4 weeks cultivation. These results give strong evidence that high inoculum doses are not responsible for the observed resistance breaks. To prove this conclusion another experiment with normalised inoculum added to sterile soil was carried out. Within this test three time harvests were co...
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