Rhizomania is a serious and continuing virus threat to sugar beet production in the UK. This disease can decrease yields of susceptible varieties by up to 70%. Annual losses due to rhizomania are currently small due to its patchy nature in soils and the switch by growers to 100% use of partially-resistant varieties. Failure of these partially-resistant varieties would cause continued build-up of the disease placing further pressure on the industry.
Rhizomania is caused by the Beet necrotic yellow vein virus (BNYVV) and carried by Polymyxa betae, a soil-borne protist that occurs in all soils where sugar beet is grown in the UK (McGrann et al., 2009). Warm, wet springs and early summers have been particularly conducive to its build up and spread and, once present, the disease cannot be eradicated. The disease is spreading throughout all of the soil types on which sugar beet is grown, and with sugar beet being produced more intensively close to the four British Sugar processing factories, the threat that rhizomania poses to sugar beet production will continue to increase if not controlled.
Experience in other countries suggests that the rate of spread is likely to increase with time. Resistant varieties are widely used but only offer partial resistance and other related crops such as red beet, fodder and leaf beet remain susceptible to rhizomania. Growers need to use the most appropriate varieties and rotations to maximise their sugar beet yield potential in the presence of increasing pressure from rhizomania. This will preserve resistance genes providing a strategy to combat the problem in the future if the virus continues to change or mutate. Adoption of Rz1 varieties is ensuring that growers limit the impact of this disease. However, inappropriate practices can threaten the national rhizomania situation such as the use of susceptible fodder and red beet varieties on known rhizomania-infected fields thereby multiplying the problem for future sugar beet crops or selecting different strains of the virus.
An extremely aggressive strain of the virus capable of overcoming the Holly resistance gene has been identified on 37 fields in East Anglia since 2007 (Stevens et al., 2009; Stevens, 2014, Bornemann et al., 2014). This virulent strain can kill susceptible varieties, and decreases the yield of existing resistant varieties by up to 60% (both root yield and sugar content). There is a need to develop a robust toolkit which will provide the technologies required to monitor, evaluate and apply the most appropriate resistance approach to deal with both the existing and new strains of the rhizomania virus. The development of potentially more durable forms of resistance, such as to the virus vector, would also offer a better and longer-term solution.
The development of new resistance breaking strains of rhizomania and the possible occurrence of Beet black scorch virus in the UK, as well as the identification of new diseases such as stemphylium in the Netherlands highlights the need to maintain a watching brief for new or developing pests and diseases in the UK.
Outcomes / Key Message For Growers And Industry
The aggressive AYPR strain of rhizomania does not appear to be spreading from its current locations.
Previously, rhizomania had a major economic impact on the UK industry, potentially decreasing yields by up to 70%. The development of partially-resistant varieties by the breeders have made a major contribution to protect the yield potential of the UK crop. However, new strains of rhizomania, capable of overcoming varietal resistance, were identified in the UK (e.g. P-type  and the AYPR  strain). Such strains pose a serious threat to current ‘resistant’ varieties, although varieties with an additional resistance gene (Rz1 + Rz2) have been developed and released commercially that yield in the presence of these new strains (e.g. Sandra KWS). If no further sources of novel resistance genes are identified, the likelihood of a future breakdown in rhizomania resistance is high. The project monitors the incidence, distribution and strain variation of the rhizomania virus and assesses any future novel resistance to the virus.
Due to a lack of rhizomania being found this project has been closed.