Giant Knotweed Extract Increases Grapevine Fruit Yields Despite Insignificant Effects on Red Blotch Symptom Severity

Grapevine red blotch disease (GRBD), caused by grapevine red blotch virus (GRBV), is characterized by interveinal foliar blotching and reductions in photosynthesis, leading to lower sugar and anthocyanin levels in fruit. Enhanced plant defense responses were hypothesized to mitigate these effects via products eliciting systemic acquired resistance (giant knotweed extract [GNE]) or induced systemic resistance (ISR; acibenzolar-S-methyl [ASM]). This study evaluated these products on Cabernet franc vines at bloom, berry set, and veraison in 2023 and 2024. Significant treatment effects on GRBD symptoms at harvest (P = 0.039) were observed: lower expression in GNE-treated vines (65%), higher in ASM-treated vines (75%) vs. the control (70%). The overall yield effect was significant (P = 0.015), driven by lower yield with ASM (1.5 kg) and increased yield with GNE (2.2 kg) compared to control (1.7 kg). Based on improved yields with GNE, earlier applications at bud break and two weeks later were hypothesized to aid berry set. In 2024, GNE, ASM, and an additional elicitor of ISR, chitosan, were evaluated at this new application frequency (five times; budbreak through veraison). No effects were observed from the five-time application treatments on GRBD symptom expression (P = 0.46). Five-time GNE applications increased yield by 41% (P = 0.10) but was statistically insignificant. These findings suggest GNE modestly lowered symptom expression and significantly increased cluster numbers, but overall, treatments offered minimal benefit in mitigating broader fruit quality impacts of GRBD.