Bois noir (BN) associated with ‘Candidatus Phytoplasma solani’ (Stolbur) is regularly found in Austrian vine growing regions. Investigations between 2003 and 2008 indicated sporadic presence of the confirmed disease vector Hyalesthes obsoletus and frequent infections of bindweed and grapevine. Infections of nettles were rare. In contrast present investigations revealed a mass occurrence of H. obsoletus almost exclusively on stinging nettle. The high population densities of H. obsoletus on Urtica dioica were accompanied by frequent occurrence of ‘Ca. P. solani’ in nettles and planthoppers. Sequence analysis of the molecular markers secY, stamp, tuf and vmp1 of stolbur revealed a single genotype named CPsM4_At1 in stinging nettles and more than 64 and 90 % abundance in grapevine and H. obsoletus, respectively. Interestingly, this genotype showed tuf b type restriction pattern previously attributed to bindweed associated ‘Ca. P. solani’ strains, but a different sequence assigned as tuf b2 compared to reference tuf b strains. All other marker genes of CPsM4_At1 clustered with tuf a and nettle derived genotypes verifying distinct nettle phytoplasma genotypes. Transmission experiments with H. obsoletus and Anaceratagallia ribauti resulted in successful transmission of five different strains including the major genotype to Catharanthus roseus and in transmission of the major genotype to U. dioica. Altogether, five nettle and nine bindweed associated genotypes were described. Bindweed types were verified in 34 % of grapevine samples, in few positive Reptalus panzeri, rarely in bindweeds and occasionally in Catharanthus roseus infected by H. obsoletus or A. ribauti. ‘Candidatus Phytoplasma convolvuli’ (bindweed yellows) was ascertained in nettle and bindweed samples.

Oxidative stress in compatible virus‐host plant interactions was studied in cucumber mosaic virus and zucchini yellow mosaic virus‐infected Cucumis sativus and Cucurbita pepo plants. Both cucumber mosaic virus‐ and zucchini yellow mosaic virus‐infected plants showed an enhanced peroxidation of polyunsaturated fatty acids indicating an advanced disintegration of membranes. Radical intermediates formed during lipid peroxidation co‐oxidize pigment molecules and might thus account for virus‐induced yellowing symptoms. Furthermore in infected plants an induction of superoxide dismutases, catalases, total peroxidases and ascorbate peroxidases was observed. All the peroxidase isoforms detected in Cucumis sativus and Cucurbita pepo, however, not only functioned as radical scavengers but also catalysed the formation of H2O2. Thus it can be presumed that the enhancement of peroxidases contributes to the oxidative stress in systemic plant‐virus interactions. By way of oxidation of indole‐3‐acetic acid upregulated peroxidases might also be responsible for growth reductions and malformations in virus‐infected plants.

Seventeen Auchenorrhyncha species were studied for their ability to transmit stolbur phytoplasma. Adult insects were collected alive in Bois Noir infected vineyards, transferred to Vicia faba seedlings (two leaf stage) in cylindrical pot cages and kept in a growth chamber at 23°C (L16:D8). After 5 weeks, plants were inspected visually and sampled for PCR analysis. Anaceratagallia ribauti, Dryodurgades reticulatus and Euscelis incisus were also reared in the laboratory. Insects trapped in the vineyard were allowed to oviposit on V. faba seedlings. Hatching nymphs were transferred to cubical breeding cages containing potted V. faba seedlings and stolbur-infected Convolvulus arvensis collected in the vineyard. Plants in these cubical cages were routinely tested by PCR for stolbur infections. Adults raised in these cubical cages were subjected to further pot cage experiments with V. faba seedlings. In five of 40 experiments with field-trapped A. ribauti, stolbur transmission was observed. Laboratory-reared A. ribauti transmitted stolbur phytoplasma to seven of 23 broad beans in the cubical breeding cages. Laboratory-reared A. ribauti adults transmitted stolbur phytoplasma in one of 13 pot cage experiments. No transmission of stolbur phytoplasma with any other Auchenorrhyncha species was observed. Furthermore, we characterized Austrian stolbur isolates by Restriction Fragment Length Polymorphism analysis of the tuf gene. Plant samples were collected in 17 symptomatic vineyards all over eastern Austria. Vicia faba plants from transmission experiments were also analysed. All isolates belonged to tuf-type II.

The epidemiology of ‘Candidatus Phytoplasma prunorum’ was studied in Austria and Hungary from 2014 to 2018. Testing of root samples showed average infections rates of 61 and 40% of the Austrian Prunus spinosa and Prunus domestica spp. insititia samples, respectively. In Hungary, on average 21% of the P. spinosa and 13% of the feral Prunus cerasifera samples were infected. The pathogen was found in 18 out of 19 apricot orchards and PCR positive Cacopsylla pruni were observed at 11 out of 17 sampling locations in both countries. In cage experiments with C. pruni remigrants successful pathogen transmission to Prunus armeniaca, P. domestica and P. spinosa seedlings in budding and foliated developmental stages was recorded, an inoculation access period of 4 hr was sufficient for transmission. A field experiment with ungrafted apricot seedlings planted in 2012 and 2014 indicated a prominent role of the insect vectors for disease spread. In 2017, 40 and 28% of the trees planted in 2012 and 2014, respectively, were infected. Molecular characterisation based on the genes aceF and imp allowed the discrimination between 10 phytoplasma types in apricots. Around 70% of the phytoplasma types in apricots were also common in P. spinosa, in P. domestica spp. insititia and in remigrant C. pruni pointing to a possible pathogen exchange by insects between wild and cultivated Prunus spp. For disease control, vector management over the entire flight period of the remigrants seems necessary; when selecting active compounds, the short inoculation access period of not more than 4 hr should be considered.

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