Institut für Pflanzenschutz in Ackerbau und Grünland
Phytoplasmas are bacteria-like, cell wall-less organisms. They usually spread via vectors from the locust order (Cixiidae family), plant propagation material and seeds. Phytoplasma Candidatus Phytoplasma solani (PHYPSO, Stolbur for short) causes the blackwood disease of grapevines (Bois Noir), but also attacks other crop species such as celery, carrots and lettuce, where it can lead to yield losses of up to 80 %. Field bindweed and stinging nettle have been identified as alternative host and reservoir plants. Until 2019, PHYPSO was classified as a quarantine pest (Directive 2000/29/EC Annex II/AII d8 EPPO status: A2) and limited to individual findings. Proteobacteria form the largest and most diverse grouping within the domain of bacteria. In contrast to phytoplasmas, only a few plant pathogenic γ-proteobacteria species are known, including the phloem-limited Candidatus arsenophonus phytopathogenicus (ARSEPH, common name: SBR). As with phytoplasmas, locusts from the Cixiidae group are among the most important known vector organisms. Transmission within a host population occurs almost exclusively horizontally. Within the vectors, multiplication takes place in the digestive tract. In the case of ARSEPH and the main vector Pentastiridius leporinus, the occurrence on sugar beet has been known for years and represents a major problem for the production sector.The first detection of PHYPSO on potatoes in Germany was in 2006. In response to this change of host, the JKI carried out initial investigations into the transfer of the pathogen from grapevine to potatoes. This work showed that the same pathogen that causes black vine blight is capable of causing stolbur on potatoes. In addition, the first variety trials were set up to identify resistances. The results of these studies contributed significantly to a more precise description of the symptoms on potatoes. These include the formation of air nodules, thread sprouts, anthocyanin discolouration of the upper leaves and rubber nodules. In collaboration with the Max Rubner Institute, it was also possible to establish the strongly quality-reducing significance of rubber tubers for industrial processing.The first reports on the increased occurrence of the above-mentioned pathogens on potatoes and sugar beet were published in 2022 and show an associated occurrence with the reed glassy-winged cicada on sugar beet in southern Germany. The disease SBR in sugar beet has been known for years and is widespread in neighbouring countries, but the additional occurrence of the Stolbur pathogen in infections with strong symptom expression is a new phenomenon. In potatoes, however, these mixed infections do not appear to cause an increase in symptoms compared to pure Stolbur infections. Since the first appearance of SBR and Stolbur on potatoes in combination with the vector P. leporinus in southern Germany, a continuous progression has been documented. This spread is accompanied by locally strong infestation fluctuations and epidemic-like outbreaks. Infections can be identified by visible symptoms in the field and must be confirmed using specific PCR methods. Genetic factors that allow a clear differentiation of pathogen isolates on potatoes, sugar beet and grapes are not yet known.Strategies for sustainable control of the pathogens and vectors in the field have not yet been able to guarantee protection. The use of insecticides against the vectors is not sufficiently effective and active substances with a direct effect on phytoplasmas or γ-proteobacteria are not known. The transmission of pathogens through infected daughter tubers to the next generation is considered a critical factor, but in the past it was considered possible but of very minor importance. It is assumed that due to the massive occurrence of the pathogens in the infested regions, varietal resistance alone cannot provide sufficient protection against yield and quality losses. The aim of the project is to establish structures and methods at the JKI Institute A in Braunschweig for scientific studies on the detection and transmission of the pathogens PHYPSO and ARSEPH on daughter tubers. To this end, it is necessary to test and standardise the suitability of various molecular diagnostic methods. These methods are to be harmonised with the diagnostic laboratories of the official plant protection services by carrying out a comparative laboratory test. All sequence data generated in the process should be published and be available for use in phylogenetic studies.
Federal Ministry of Food and Agriculture
