First report of Vitis cryptic virus (VCV) infecting mildew-resistant grapevine interspecific hybrids in France.

Vitis cryptic virus (VCV), a deltapartitivirus identified in Japan in Vitis coignetiae (Nabeshima and Abe, 2021), is known from only two other countries. It was detected in China (Fan et al., 2022) and in Russia, including in a V. labrusca and the Saperavi Severnyi interspecific hybrid (Shvets et al., 2022). There is no information on VCV pathogenicity but deltapartitiviruses are generally not pathogenic. Fan et al. (2022) reported VCV graft transmission and chlorotic mottling symptoms developing on a graft-inoculated vine, in spite of the fact that cryptic viruses are not known to move cell-to-cell or be graft-transmissible. In fall 2022, a few plants of the Prior interspecific hybrid (https://www.vivc.de) showed unusual red blotch and leaf curl in Bordeaux (France), prompting the HTS analysis of two plants using total leaf RNA. Following host genome substraction, the ribodepleted RNASeq data was assembled de novo using CLC Genomics Workbench (Candresse et al., 2018) and contigs annotated by BlastX against the GenBank database. Rupestris stem pitting virus, grapevine pinot gris virus, hop stunt viroid and grapevine yellow speckle viroid 1 were identified. In addition, mycoviral contigs were identified, together with contigs for Rhopalosiphum padi virus and a divergent isolate of barley aphid RNA virus 10 (the later only in one plant), and the two genomic RNAs of VCV. The VCV RNA1 contigs were 1570 and 1574 nucleotides (nt) long, respectively, and 100% identical, showing 97.1% nt identity to a Japanese isolate (LC746759). They integrated 6480 and 4613 reads (0.2 and 0.4% of total substracted reads) for a coverage of 611 and 433x, respectively. The VCV RNA2 contigs were also 100% identical and shared 95.5% identity with a Japanese isolate (LC746761). They were 1518-1519 nt long, integrated 11338 and 9999 reads (0.4 and 0.9% of reads) for a coverage of 1109 and 972x, respectively. The Prior VCV RNAs were deposited in GenBank (OR474475-76). Specific RNA2 primers 5' TTACAGGTTTGATTGGAATCATG 3' and 5' ATAGTAGGTCCAATCACTAATC 3' (Tm 56°C) were used to confirm VCV presence in the original plants as well as in three other asymptomatic Prior vines. Amplicons 100% identical to the contigs were obtained from 4 of 5 plants. Two plants of Bronner, one of Prior parents, also tested positive. The rootstock (Fercal) of a VCV-infected Prior and two plants of another hybrid, Artaban, (sampled in the same plot as Prior) tested negative. BlastN datamining identified VCV reads in RNASeq data from a range of wild grapevines including V. acerifolia (SRX2885763), V. quinquangularis (SRX1496837), V. romanetii (SRR3938616), V. cinerea (SRR10135144), V. davidii (SRR3255926), V. amurensis (SRX13387918) and V. vinifera subsp. sylvestris (HAOE01029819, HAOE01001237). Although not experimentally verified, detection in wild Vitis, including V. amurensis, a Saperavi Severnyi, Bronner and Prior progenitor, suggests VCV might have been introduced in these hybrids through crosses aiming to develop powdery and downy mildew resistant varieties. To the best of our knowledge, this is the first report of VCV infection in grapevine in France. The symptoms that prompted this research have not recurred in 2023 and are not linked to VCV because the virus was also identified in symptomless Prior plants. The risk of introducing VCV in European grapevine through breeding efforts appears limited, but VCV may be present in fungal disease-resistant cultivars in a range of countries.

Field evaluation of an expertise-based formal decision system for fungicide management of grapevine downy and powdery mildews.

BACKGROUND: In France, viticulture accounts for 20% of the phytochemicals sprayed in agriculture, and 80% of grapevine pesticides target powdery and downy mildews. European policies promote pesticide use reduction, and new methods for low-input disease management are needed for viticulture. Here, we present the assessment, in France, of Mildium, a new decision support system for the management of grapevine mildews. RESULTS: A 4 year assessment trial of Mildium has been conducted in a network of 83 plots distributed across the French vineyards. In most vineyards, Mildium has proved to be successful at protecting the crop while reducing by 30-50% the number of treatments required when compared with grower practices. CONCLUSION: The design of Mildium results from the formalisation of a common management of both powdery and downy mildews and eventually leads to a significant fungicide reduction at the plot scale. It could encourage stakeholders to design customised farm-scale and low-chemical-input decision support methods.

Genetic structure and aggressiveness of Erysiphe necator populations during grapevine powdery mildew epidemics.

Isolates of the causal ascomycete of grapevine powdery mildew, Erysiphe necator, correspond to two genetically differentiated groups (A and B) that coexist on the same host. This coexistence was analyzed by investigating temporal changes in the genetic and phenotypic structures of E. necator populations during three epidemics. Group A was present only at the start of the growing season, whereas group B was present throughout all three epidemics. Group A was less aggressive in terms of germination and infection efficiency but was more aggressive than group B in terms of the latency period, lesion diameter, and spore production. Our results are consistent with a temporal differentiation of niches, preventing recombination, and suggest an association between the disease level and the frequencies of genetic groups.

Impact odorants contributing to the fungus type aroma from grape berries contaminated by powdery mildew (Uncinula necator); incidence of enzymatic activities of the yeast Saccharomyces cerevisiae.

Powdery mildew due to the fungus Uncinula necator is an important disease for the vineyard. The development of the fungus at the surface of the berries leads to the occurrence of a very characteristic and sometimes intense mushroom-type odor cited as an important default for grapes quality. Gas chromatography/olfactometry, gas chromatography, and multidimensional gas chromatogaphy/mass spectrometry techniques were used to investigate the most important odorants of grapes diseased by powdery mildew. Among 22 odorants detected, strongly odorant compounds were identified or tentatively identified in purified extracts obtained from grapes diseased by powdery mildew. Aroma extraction dilution analysis (AEDA) analysis revealed that 1-octen-3-one (mushroom odor), (Z)-1,5-octadien-3-one (geranium-leaf odor), and an unidentified odorous zone (fishy-mushroom like odor) were the most potent volatiles of the diseased grapes. In the presence of nonproliferating Saccharomyces cerevisiae yeast cells, and consequently during alcoholic fermentation, the enzymatic reduction of 1-octen-3-one and (Z)-1,5-octadien-3-one to much less odorant compounds, namely 3-octanone and (Z)-5-octen-3-one, was shown. Those results explain to some extent the disappearance of the fungal aroma specific to powdery mildew grapes during alcoholic fermentation.