The planthopper Pentastiridius leporinus is the main vector of Candidatus Arsenophonus phytopathogenicus, a plant pathogenic bacterium associated with the sugar beet disease syndrome ‘basses richesses’ (SBR). The disease first identified in eastern France in1991 has continued to spread in other parts of Europe. SBR causes devastating yield losses. The main disease control strategy currently is through growing tolerant cultivars.
During the past few years, the disease SBR (Syndrome Basses Richesses) has been causing problems in sugar beet growing areas in Germany and Switzerland. The disease leads to significant yield losses and is, therefore, a big problem in affected areas. The disease is caused by bacteria which are spread by the leafhopper (Pentastridius leporinus, figure 1). There is no known way to treat the bacterial infection and to control leafhoppers is a difficult task. The best way to handle this disease is to grow tolerant cultivars that despite some symptom development doesn’t suffer from as much yield loss as susceptible cultivars (figure 2).
Figure 1. Pentastridius leporinus, the vector for SBR disease
First sighting of SBR in France
The disease was first described in Eastern France in the sugar beet growing area of Burgundy south-east from Dijon in 1991. The income loss for the farmers in this area was in 1992 estimated to be approximately 50% on 1.000 ha. The disease caused devastating yield loss in this area for several years. Following the EU sugar market reform in 2006 the sugar factory in this region closed and the sugar beet cultivation ended – acknowledging the virulence of the disease.
Figure 2. Field with natural SBR infection (Germany 2019). Tolerant variety Rhinema grown next to a SBR susceptible variety.
Spread of SBR
The first symptoms of SBR in sugar beet in Germany were seen in Heilbronn in 2008. Since then, outbreaks have been recorded annually – until 2017 when severe symptoms were seen. The disease has since then been seen in at least France, Germany, Switzerland and Hungary and there is a risk of spread to more areas due to the presence of vectors in many other countries.
The disease is associated with two uncultivable phloem-restricted bacterial pathogens; Candidatus Arsenophonus phytopathogenicus (SBRp) and Candidatus Phytoplasma solani. It is not proved which of the two pathogens have the most impact on disease development but it is most likely the bacteria is plays the major role. Both organisms are transmitted by planthoppers belonging to the family of Cixiidae. The flight of these planthoppers is detected from May until September depending on yearly weather conditions and an early attack can lead to dramatic sugar losses. The planthoppers fly to sugar beet fields where they lay eggs on the roots. Nymphs hatching from eggs feed on the sugar beet roots. In the autumn when sugar beet is harvested and wheat is drilled in the same field the nymphs overwinter on the wheat roots. During the following spring the adult hoppers fly to sugar beet fields and spread the bacteria.
The bacteria infect the phloem and cause yellowing of both young and older leaves and brownish discoloration of the taproot vascular tissue. Early symptoms are yellowing and incurvation of old leaves and a new growth of central leaves which appear chlorotic, lancet shaped and asymmetrical. The symptoms often appear late in the season, shortly before harvest, but dramatic yield losses are evident from the significant reduction of sugar content in the taproot.
There is no chemical way to treat this disease and the most effective control is growth of resistant or tolerant varieties. The sugar beet variety Rhinema with its four trait combinations (Rhizoctonia, Beet Cyst Nematode, Cercospora, Rhizomania) has shown good tolerance against the disease and field trials have shown that the sugar content of Rhinema was constantly about 3% higher in locations with natural SBR infection compared to other varieties.
MariboHilleshög has developed, together with the Swedish University of Agricultural Sciences, a qPCR method for detection and quantification of SBRp. Additionally, a quantitative method for the phytoplasma is under development. It is now possible to compare the infection levels between different varieties, which will enable a quicker breeding progress.