Grapevine yellows (GY) are serious phytoplasma-caused diseases affecting viticultural areas worldwide. At present, two principal agents of GY are known to infest grapevines in Germany: Bois noir (BN) and Palatinate grapevine yellows (PGY). Disease management is mostly based on prophylactic measures as there are no curative in-field treatments available. In this context, sensor-based disease detection could be a useful tool for winegrowers. Therefore, hyperspectral imaging (400–2500 nm) was applied to identify phytoplasma-infected greenhouse plants and shoots collected in the field. Disease detection models (Radial-Basis Function Network) have successfully been developed for greenhouse plants of two white grapevine varieties infected with BN and PGY. Differentiation of symptomatic and healthy plants was possible reaching satisfying classification accuracies of up to 96%. However, identification of BN-infected but symptomless vines was difficult and needs further investigation. Regarding shoots collected in the field from different red and white varieties, correct classifications of up to 100% could be reached using a Multi-Layer Perceptron Network for analysis. Thus, hyperspectral imaging seems to be a promising approach for the detection of different GY. Moreover, the 10 most important wavelengths were identified for each disease detection approach, many of which could be found between 400 and 700 nm and in the short-wave infrared region (1585, 2135, and 2300 nm). These wavelengths could be used further to develop multispectral systems.

“Flavescence dorée” (FD)-related phytoplasmas are widespread in alder in Germany and their transmission to grapevine represents a high risk for FD outbreaks when the primary vector, Scaphoideus titanus, becomes present in the future. Therefore, the potential role of the Deltocephalinae leafhopper species in transmitting FD-related phytoplasmas from alder to grapevine was studied in extensive transmission trials conducted between 2017 and 2020. The transmission capacity of autochthonous Allygus spp. and the invasive Orientus ishidae captured on infected alder trees was tested under controlled conditions using various test designs, including grouped insects and single-insect studies. The latter experiments were analyzed in terms of survival probability, transmission success and phytoplasma load in the insects, measured by quantitative PCR. A minimum inoculation titer (MIT) required for successful transmission to alder was defined for both Allygus spp. and O. ishidae. While Allygus spp. exhibited slightly better survival on Vitis vinifera compared to O. ishidae, the latter displayed higher phytoplasma loads and greater transmission success. Although all species were capable of infecting alder seedlings, O. ishidae was able to transmit 16SrV-phytoplasmas directly to single grapevines. Infective adults of O. ishidae were captured from the beginning of July until the end of August, while Allygus spp. were only considered infective towards the end of the season. Thus, O. ishidae likely poses a higher risk for FD transmission from alder to grapevine, albeit at a very low level, as only five out of 90 transmission trials to V. vinifera were successful.

Flavescence dorée (FD) and Bois noir (BN) are the principal grapevine yellows diseases in Europe caused by distinct phytoplasmas: FD by 16SrV phytoplasmas (FDp), BN by Candidatus Phytoplasma solani. FDp is spread epidemically by the introduced Nearctic Deltocephalinae Scaphoideus titanus and is listed as a quarantine pest in the European Union (Regulation (EU) 2019/2072). Black Alder (Alnus glutinosa) is a common asymptotic host of 16SrV phytoplasmas in Europe and considered the original host of FDp (Malembic-Maher et al. 2020). Palatinate grapevine yellows (PGY) transmitted from alder to grapevine by the Macropsinae Oncopsis alni (Maixner et al. 2000) is not transmissible by S. titanus, unlike isolates transmitted by the autochthonous Deltocephalinae Allygus spp. and the invasive Orientus ishidae (Malembic-Maher et al. 2020). Germany is considered free from FD in grapevine and from its vector. A single case in a nursery in 2014 was eradicated (EPPO 2017). Since S. titanus was detected in 2016 in the neighboring French Region of Alsace, monitoring of FD was carried out in Germany. It was focused on vineyards within a distance of 100 m from stands of alder trees. A geodata-based risk map (Jalke 2020) was used to identify those plots. All symptomatic vines sampled until September 2020 proved to be infected by BN or, occasionally, by PGY. Eight vines with typical symptoms were sampled in vineyards adjacent to alder stands in the winegrowing region of Rheinhessen in September 2020. Symptoms comprised leaf rolling and discoloration, incomplete lignification, black pustules on shoots, dried inflorescences and shriveled berries. Diseased shoots were black and necrotic in December. Leaf midribs were sampled for total DNA extraction. The phytoplasma 16S rRNA gene was amplified by generic primers R16F2/R2-mod followed by a nested PCR using 16Sr(V) group-specific primers R16(V)F1/R1, and primers R16(I)F1/R1 (Lee et al. 1995) to detect 'Candidatus Phytoplasma solani', associated with BN. While BN was detected in seven vines, one sample tested positive for 16SrV phytoplasma. This result was confirmed by triplex real-time Taq-Man assay based on rpl14 gene sequences (IPADLAB), by multiplex real-time PCR of map locus as well as by Loop-mediated isothermal amplification (LAMP) according to the EPPO diagnostic standard PM 7/079(2) (EPPO 2016). PCR-products of the map- and the vmpA-Gene (Malembic-Maher et al., 2020) were sequenced and compared to reference sequences to distinguish between FD- and non-FD genotypes. The isolate from the diseased vine exhibited 100% identity with map-M38 (Accession No. LT221933), a genotype of the map-FD2 cluster. The same genotype was detected in A. glutinosa and Allygus spp. sampled at the infested site. A 234 bp sequence of the first repeat of the vmpA-gene showed 100% identity with the S. titanus transmitted isolate FD-92 (Accession No. LN680870) of the vmpA-II cluster. It can be concluded, that the 16SrV-isolate detected in a symptomatic grapevine is infected by FD and not PGY. This is the first report of FD in a vineyard in Germany. The infected vine of cv. Silvaner was 25 years old. While infected planting material is an unlikely source of the infection, a transmission of FDp from alder is highly probable. Finding a single FD-infection after several years of testing implies a low risk originating from the wild compartment, but the approach of the vector S. titanus justifies further monitoring activities. The infected vine was eradicated.

Pentastiridius leporinus (Hemiptera: Fulgoromorpha: Cixiidae) is considered the main vector of three phloem-restricted pathogens, γ-proteobacteria ‘Candidatus Arsenophonus phytopathogenicus’, ‘Candidatus Phytoplasma solani’ (Stolbur phytoplasma), and 16SrXII-P phytoplasma, which infect two economically important crops, sugar beet and potato. In this study, four pop-up tents (1 m²) were set up in a carrot (Daucus carota L.) field located in Bingen on the Rhine to determine whether egg laying, development of nymphs, and infection is possible on this vegetable. We further show that the planthopper P. leporinus can complete significant parts of its life cycle on carrots in the field, suggesting that it has expanded its host range to this crop in addition to sugar beet and potato. In addition, this study shows that P. leporinus is able to transmit phloem-restricted pathogens to carrots.

Epidemiological traits of “flavescence dorée” (FD)-related phytoplasmas were studied in Palatinate, a FD-free viticultural area in southwestern Germany. Grapevines with yellows symptoms, alder trees and leafhoppers on alders were sampled and tested for 16SrV-group phytoplasma presence. Genotyping of positive samples was based on the gene map. A high proportion of alder trees was infected by various genotypes including the FD-related genotype M38. About 22% of 330 sampled grapevines were infected with 16SrV-group phytoplasmas, but the map genotyping disclosed only the PGY-type of the 16SrV-C subgroup. The most abundant Deltocephalinae leafhoppers were Orientus ishidae, Allygus mixtus and Allygus modestus. Infection rates by group 16SrV phytoplasmas were 44%, 38% and 41%, respectively, with the predominant map genotype being M38. Allygus spp. and O. ishidae were able to transmit it to either Vicia faba, Alnus glutinosa, or both. A survey in vineyards revealed no evidence for the presence of the FD vector Scaphoideus titanus in spite of its local occurrence in the adjacent Alsace region. Thus, the Palatinate can still be considered free from FD and its vector.