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An investigation of the significance of resistance-breaking BNYVV pathotypes for the UK sugar beet industry - PH0145

Description
Rhizomania is a serious disease of sugar beet which although widespread in Europe, has been contained within a limited area in the UK. The UK has ‘protected zone status’ for rhizomania disease and is therefore obliged to maintain a range of control measures. The aim of the statutory action is to prevent the further spread of rhizomania within the UK or to the UK from other countries. These measures are economically justifiable only until effective disease control is available. Currently, the only method for control of rhizomania is the use of resistant sugar beet cultivars which yield as well as the currently used susceptible cultivars. As part of the strategy to control rhizomania in the UK some of the available partially resistant cultivars have been grown on known rhizomania infected farms but only on fields believed to be free of rhizomania.

BNYVV is the causal agent of rhizomania disease of sugar beet. The virus is rod shaped and has a multipartite genome which is normally made up of 4 RNA segments, designated RNA 1, 2, 3 and 4. RNA 1 codes for the RNA polymerase, RNA 2 for the coat protein gene and genes involved in virus movement, RNAs 3 and 4 are involved in virus movement and transmission by the vector, Polymyxa betae. Recently, extra species of RNA have been found in some isolates. A species called RNA 5 has been described by Tamada in Japan (1), by Miyanishi from China (2) and by Koenig from Pithiviers in France (3). Koenig (4) has recently reported that the RNA 5 from the Pithiviers isolate is similar to the Japanese RNA 5 but has a nucleic acid sequence that is distinctly different. The full nucleic acid sequences for these RNA 5s are available from genomic libraries. The A and B pathotypes appear to be of similar pathogenicity. However, Tamada (5) has reported that the BNYVV isolate containing RNA 5 from Japan causes a breakdown in the type of resistance found in sugar beet cultivars such as Rizor. It is unclear whether the resistance found in the partially resistant cultivars such as Ballerina, Rebecca and Rosanna would be similarly compromised. If so, this would almost take the situation back to that of the early 1980s when no resistance to rhizomania was available in sugar beet.

The P pathotype, from France, containing RNA 5, as described above, however, does appear to be potentially more pathogenic and could ‘break down’ some types of resistance in beet cultivars in a similar way to that described in Japan. During the 1999 survey a significant number of outbreaks were reported in these crops. It was uncertain whether this was due to the innate variability of the level of resistance in Beet necrotic yellow vein virus (BNYVV), the causal agent of rhizomania, in the UK.

There is a need to establish how serious this new pathotype is likely to be for the beet industry in the UK, particularly whether the P pathotype will cause more severe losses than the common pathotypes in susceptible and partially resistant cultivars.

The aim of this project was to develop a diagnostic test for the new pathotype, to assess its occurrence in UK crops and to evaluate the likely effect of this new pathotype on some of the partially resistant cultivars of beet being used in the UK. The milestones for the project were to:

1. Obtain and propagate RNA 5 isolates.
2. Develop a Polymerase Chain Reaction(PCR) test to RNA 5.
3. Test a range of UK BNYVV infected beets for the P pathotype.
4. Compare resistance levels in different sugar beet cultivars.

In conclusion, RNA 5 isolates from France and Japan and China have been obtained and successfully propagated under quarantine conditions. A completely new RT-PCR test, R-T5 has been designed using the sequence information available, optimised, developed, evaluated and successfully used to detect BNYVV RNA 5 in French, Japanese and Chinese isolates.

A range of known infected beet from the UK 2000 annual Rhizomania survey have been tested using R-T5 for the presence of the P pathotype-none was detected.

The ability of the BNYVV P pathotype to break down resistance present in cultivars of sugar beet such as Ballerina and Rebecca was assessed under quarantine glasshouse conditions by conducting two different experiments. In one experiment to measure yield loss, where a selection of well-known semi-resistant sugar beet cultivars were grown to maturity, two of the cultivars, Rebecca, semi-resistant and the susceptible cultivar Roberta, showed a marked yield loss compared to healthy controls and UK normal, RNA 2 BNYVV. Cultivars Rosanna and Beta 805 had a higher yield in P pathotype soil than in UK soil and could benefit from being tested further, perhaps also in field conditions to explore any potential they might have, should P Pathotype ever reach the UK in future years.

The second experiment to assess the ability of the P pathotype to break down resistance, tested 20 cultivars of sugar beet from different plant breeding firms. Replicates of the cultivars were grown in P pathotype soil for 6 weeks and then ELISA tested and results compared to the same cultivars grown as controls in uninfected and normal BNYVV infected UK soil. The results showed all cultivars tested were infected when grown in P pathotype soil, whereas some of the cultivars did show resistance when grown in UK BNYVV infected soil. Cultivars Beta 805 and Rosanna were also included in this experiment and became infected when grown in P pathotype soil.

The results obtained from the project have provided a new, highly sensitive test which can detect the P pathotype, should suspect beets be found, together with useful information on the risks from the new BNYVV pathotype for use by MAFF Plant Health Division in deciding its future policy on rhizomania. The information will also benefit the sugar beet industry and the plant breeding industry, providing tools that are necessary to allow the development of cultivars of sugar beet resistant to the P pathotype.

Objective
1. To develop a PCR test for RNA 5, based on conventional or Taqman PCR systems (the latter would be preferable should it become necessary to conduct larger surveys yielding samples for test).
2. To assess whether isolates of BNYVV currently present in the UK contain RNA 5.
3. To assess the ability of BNYVV P pathotypes to break down resistance present in cultivars of sugar beet such as Ballerina and Rebecca.
Project Documents
• Final Report : An investigation of the significance of resistance-breaking BNYVV pathotypes for the UK sugar beet industry   (343k)
Time-Scale and Cost
From: 1999

To: 2001

Cost: £29,965
Contractor / Funded Organisations
Central Science Laboratory
Keywords
Plant health              
Plant Pests and Diseases              
Plants and Animals              
Fields of Study
Plant Health