Hydrogen peroxide (H2O2) localization and roles of peroxidases, malondialdehyde and reduced glutathione\ud were compared in leaves of apricot (Prunus armeniaca) plants: asymptomatic, European Stone Fruits\ud Yellows (ESFY)-symptomatic and recovered. Nested PCR analysis revealed that ‘Candidatus Phytoplasma\ud prunorum’, is present in asymptomatic, symptomatic and recovered apricot trees, confirming previous\ud observations on this species, in which recovery does not seem to be related to the disappearance of\ud phytoplasma from the plant.\ud H2O2 was detected cytochemically by its reaction with cerium chloride, which produces electron-dense\ud deposits of cerium perhydroxides. H2O2 was present in the plasmalemma of the phloem cells of recovered\ud apricot plant leaves, but not in the asymptomatic or symptomatic material. Furthermore, by labelling\ud apricot leaf tissues with diaminobenzidine DAB, no differences were found in the localization of peroxidases.\ud Protein content in asymptomatic, symptomatic and recovered leaves was not significantly different from\ud one another. In contrast, guaiacol peroxidase activity had the following trend: symptomatic\ud > recovered > asymptomatic, whereas reduced glutathione content followed the opposite trend:\ud asymptomatic > recovered > symptomatic. Moreover, no differences were observed in malondialdehyde\ud concentrations between asymptomatic, symptomatic and recovered leaves. The overall results suggest that\ud H2O2 and related metabolites and enzymes appear to be involved in lessening both pathogen virulence and\ud disease symptom expression in ESFY-infected apricot plants

A 3-year study was carried out in north-east Italy, the site of recent elm yellows epidemics, to identify vectors for the elm yellows phytoplasma. Using PCR analysis, Ulmus minor and Ulmus pumila , each with and without symptoms, were positive for the elm yellows phytoplasma. Macropsis mendax , a univoltine and monophagous leafhopper, was shown to be the vector of the elm yellows-associated disease agent. PCR analyses demonstrated that the insect was infected both in natural conditions and in the screenhouse after acquisition-feeding on infected elm plants. Groups of M. mendax , collected from naturally infected elm trees, transmitted elm yellows phytoplasma to elm test plants. In nature, Alnus glutinosa trees affected by alder yellows were found in the surroundings of yellows-affected elm trees; the associated disease agent of alder yellows was transmitted under controlled conditions from alder to elm test plants by grafting.

Pseudomonas syringae pv. actinidiae (Psa) biovar 3 caused pandemic bacterial canker of Actinidia chinensis and Actinidia deliciosa since 2008. In Europe, the disease spread rapidly in the kiwifruit cultivation areas from a single introduction. In this study, we investigated the genomic diversity of Psa biovar 3 strains during the primary clonal expansion in Europe using single molecule real-time (SMRT), Illumina and Sanger sequencing technologies. We recorded evidences of frequent mobilization and loss of transposon Tn6212, large chromosome inversions, and ectopic integration of IS sequences (remarkably ISPsy31, ISPsy36, and ISPsy37). While no phenotype change associated with Tn6212 mobilization could be detected, strains CRAFRU 12.29 and CRAFRU 12.50 did not elicit the hypersensitivity response (HR) on tobacco and eggplant leaves and were limited in their growth in kiwifruit leaves due to insertion of ISPsy31 and ISPsy36 in the hrpS and hrpR genes, respectively, interrupting the hrp cluster. Both strains had been isolated from symptomatic plants, suggesting coexistence of variant strains with reduced virulence together with virulent strains in mixed populations. The structural differences caused by rearrangements of self-genetic elements within European and New Zealand strains were comparable in number and type to those occurring among the European strains, in contrast with the significant difference in terms of nucleotide polymorphisms. We hypothesize a relaxation, during clonal expansion, of the selection limiting the accumulation of deleterious mutations associated with genome structural variation due to transposition of mobile elements. This consideration may be relevant when evaluating strategies to be adopted for epidemics management.

SummaryEpidemiology of European stone fruit yellows was studied by focussing on the life cycle and transmission characteristics of the vector Cacopsyllapruni. The proportion of both phytoplasma positive and inoculative insects was determined for the first C. pruni adults back colonising the stone fruit trees in spring and for the new generations of the vector, hatched at the beginning of summer. We showed that in spring, as soon as the insects moved to stone fruit trees from shelter plants, they were infective. After the vector fed on infected stone fruit trees, the proportion of phytoplasma positive insects increased. The new generation colonising Prunus species also acquired the phytoplasma from their hosts although several of these insects completed the latency period on secondary hosts. Results showed that the risk of natural transmission of European stone fruit yellows‐phytoplasma by C. pruni within orchards is high when the vector is present. These results have implications for the control of European stone fruit yellows.

Several uncultivated trees of the species Prunus spinosa , P. cerasifera and P. domestica , sampled both adjacent to European stone fruit yellows (ESFY)-infected orchards and in isolation from cultivated stone fruit plants, were found to be infected by ESFY phytoplasma. These species were also colonized by Cacopsylla pruni , vector of the ESFY agent. In contrast, uncultivated species of Prunus avium , P. cerasus and P. mahaleb hosted neither the pathogen nor the vector. Insect-and graft-transmission trials of ESFY phytoplasma conducted under controlled conditions confirmed the data obtained in the field. The role played by the wild Prunus species is discussed and appears to be fundamental in the epidemic cycle of the disease.