Necrosis and canker lengths after <i>Neofusicoccum parvum</i> inoculation on cuttings from sensitive or resistant powdery and downy mildew genotypes or cultivars

Currently, there is no qualitative resistance to grapevine trunk diseases (GTDs), just vulnerability (possibly quantitative or partial resistance). Similar findings apply to Botryosphaeria dieback disease, which involves Neofusicoccum parvum. The goals of this study were to compare the susceptibility to N. parvum of (i) known susceptible grapevine cultivars (Merlot, Cabernet-Sauvignon, Ugni blanc), (ii) downy and powdery resistant cultivars (Solaris, Voltis, Regent, Artaban, Prior), and (iii) additional grapevine genotypes (BC1-54, BC1-56, BC1-60, BC1-01, BC4-AB and BC5-AB), based on canker and necrosis lengths measured after controlled inoculations repeated over a minimum of two vintages. Cuttings were inoculated with a particularly aggressive N. parvum isolate (PER20), known to be aggressive. After four months, the cuttings were sampled and analysed. Necrosis and cankers were measured. The climate was the characteristic that regulated the measured symptoms across multiple vintages in our pathosystem, regardless of cultivar. This demonstrated that cultivar susceptibility varied depending on environmental factors (PCA analysis explaining 81.8 % of the variability). Furthermore, comparing the symptoms in wood of susceptible and resistant cultivars to downy and powdery mildew (Plasmopara viticola and Erysiphe necator, respectively) revealed that the vintage substantially affected symptom expression, regardless of the genetic background. For example, the Artaban grape cultivar includes both susceptible (Merlot) and resistant (Regent) cultivars in its pedigree. It showed similar symptom intensities to N. parvum, as did these two cultivars. However, the mildew resistance QTLs present in the Regent cultivar (Ren3, Ren9, and Rpv3.1) and the Artaban cultivar (Run1, Ren3, Ren9, Rpv1, and Rpv3) had no impact on N. parvum resistance levels. We also found that canker and necrosis symptoms were not always significantly positively correlated. This depended on the year and grape cultivar. The responses observed in the wood of the downy and powdery mildew-resistant genotypes or cultivar plants under investigation demonstrated that neither offered a significant benefit in terms of reducing symptoms in our experimental conditions. This system could provide an interesting trade-off between studies on dormant wood (not very variable) and studies in the vineyard (highly variable), enabling us to assess the effectiveness of treatments or to gain a better understanding of the role of the environment.