BackgroundGrapevine downy mildew, caused by Plasmopara viticola, is a very serious disease affecting mainly Vitis vinifera cultivated varieties around the world. Breeding for resistance through the crossing with less susceptible species is one of the possible means to reduce the disease incidence and the application of fungicides. The hybrid Bianca and some of its siblings are considered very promising but their resistance level can vary depending on the pathogen strain. Moreover, virulent strains characterized by high fitness can represent a potential threat to the hybrid cultivation.ResultsThe host response and the pathogen virulence were quantitatively assessed by artificially inoculating cv Chardonnay, cv Bianca and their siblings with P. viticola isolates derived from single germinating oospores collected in various Italian viticultural areas. The host phenotypes were classified as susceptible, intermediate and resistant, according to the Area Under the Disease Progress Curve caused by the inoculated strain. Host responses in cv Bianca and its siblings significantly varied depending on the P. viticola isolates, which in turn differed in their virulence levels. The fitness of the most virulent strain did not significantly vary on the different hybrids including Bianca in comparison with the susceptible cv Chardonnay, suggesting that no costs are associated with virulence. Among the individual fitness components, only sporangia production was significantly reduced in cv Bianca and in some hybrids. Comparative histological analysis revealed differences between susceptible and resistant plants in the pathogen diffusion and cytology from 48 h after inoculation onwards. Defence mechanisms included callose depositions in the infected stomata, increase in peroxidase activity, synthesis of phenolic compounds and flavonoids and the necrosis of stomata and cells immediately surrounding the point of invasion and determined alterations in the size of the infected areas and in the number of sporangia differentiated.ConclusionsSome hybrids were able to maintain an intermediate-resistant behaviour even when inoculated with the most virulent strain. Such hybrids should be considered for further field trials.

The development of new sustainable containment strategies of plant diseases is very important to guarantee food security while reducing the environmental impact of agriculture. Research of new biocontrol agents is a long and difficult process that involves several steps that start from the identification of possible candidates which, for example, show antibiotic activities, and ends with in field, large scale trials. In this study, the plant growth promoting potential and antifungal effect exerted by a novel, putative candidate biocontrol agent, strain R16, identified as Paenibacillus pasadenensis by sequence analysis of 16S rRNA and rpoB genes, against three important plant pathogenic fungi (Botrytis cinerea, Fusarium verticillioides, and Phomopsis viticola), were assessed. Biochemical assays to determine plant growth promoting potential gave negative results for siderophore production and phosphate solubilization, and positive results for ACC-deamination and IAA production. Further biochemical assays for endophytic lifestyle and antifungal activity gave positive results for catalase and chitinase activity, respectively. In vitro antagonism assays showed that strain R16 is effective against B. cinerea, reducing mycelial growth both in dual-culture and through volatile substances, characterized to be mostly composed by farnesol, and inhibiting conidia germination. Good antagonistic potential was also observed in vitro towards P. viticola, but not towards F. verticillioides. Moreover, in vivo assays confirmed the strain R16 activity reduced the infection rate on B. cinerea-inoculated berries. The obtained results firstly proved that P. pasadenesis strain R16 is a putative plant growth promoter and effective against phytopathogenic fungi. Further studies will be needed to investigate the possible application of this strain as a biocontrol agent.

Evidence from a preliminary survey highlighted that ‘Candidatus Phytoplasma solani’, the etiological agent of bois noir (BN) disease of grapevine, infects grapevine varieties in Georgia, a country of the South Caucasus. In this study, field surveys were carried out to investigate the BN symptom severity in international and Georgian native varieties. ‘Ca. P. solani’ was detected and identified by polymerase chain reaction-based amplification and restriction fragment length polymorphism analysis of 16S ribosomal DNA, and further characterized by multiple gene typing analysis (vmp1 and stamp genes). Obtained data highlighted that the majority of Georgian grapevine varieties showed moderate and mild symptoms, whereas international cultivars exhibited severe symptoms. Molecular characterization of ‘Ca. P. solani’ from grapevine revealed the presence of 11 distinct phytoplasma types. Only one type (VmGe12/StGe7) was identical to a strain previously reported in periwinkle from Lebanon; the other ‘Ca. P. solani’ types are described here for the first time. Phylogenetic analyses of vmp1 and stamp gene concatenated nucleotide sequences showed that ‘Ca. P. solani’ strains in Georgia are associated mainly with the bindweed-related BN host system. Moreover, the fact that ‘Ca. P. solani’ strains are distributed in grapevine cultivars showing a range of symptom intensity suggests a different susceptibility of such local cultivars to BN.

Grey mould, caused by Botrytis cinerea, is a disease severely affecting grape production in northern Italy. However, little information is available on the variability of B. cinerea populations associated with grapevine. The mode of reproduction, sensitivity to fungicides, and for the first time in Italy, the genetic structure of B. cinerea populations isolated from grapevine in a northern Italian region are reported. Botrytis cinerea isolates (317) were completely genotyped for six microsatellite loci and characterized for the presence of the transposable elements Boty and Flipper, for the mating type and for resistance to cyprodinil, fludioxonil, boscalid and fenhexamid. All the isolates were found to belong to B. cinerea Group II, indicating the absence of B. pseudocinerea in the investigated areas. The populations possess a high genotypic diversity, different frequencies of transposable elements and a mixed mode of reproduction. At a regional level, B. cinerea populations belong to a large and interconnected pathogen population that includes the major grape-growing districts. The populations were generally sensitive to fungicides, with a low proportion (8%) of isolates resistant to cyprodinil, fludioxonil and boscalid. A small genetic distance was found between B. cinerea populations. However, the populations geographically isolated from the others by a mountain range showed a small but statistically significant genetic differentiation and a different pattern of fungicide resistance. The results show that northern Italian B. cinerea populations possess a high evolutionary potential and adaptive capacity.