Joseph DeShields, Achala KC, and Alexander Levin*
*Oregon State University, 569 Hanley Rd, Central Point, Central Point, OR, 97502 (alexander.levin@oregonstate.edu)

Grapevine red blotch disease (GRBD), caused by the grapevine red blotch virus (GRBV), manifests foliar symptoms consisting of interveinal blotching on the leaves and is associated with reduced photosynthesis and carbon translocation, leading to lower sugar and anthocyanin levels in the fruit. Previous research established the mechanistic role that potassium (K) plays in facilitating sugar transport within the phloem. It has also been reported that GRBV-infected grapevines that are asymptomatic do not exhibit the same fruit quality reductions as symptomatic grapevines, suggesting that increased plant defense response could mitigate some of the effects caused by GRBD. Therefore, it was hypothesized that foliar- and/or soil-based applications of K and products that elicit systemic acquired resistance (SAR) or induced systemic resistance (ISR) may mitigate the fruit quality effects of GRBD. In 2023, two rates of soil-applied K and a single rate of foliar-applied K were evaluated using a split-plot design on GRBV-infected Cabernet franc and Merlot. In both cultivars, there was a significant effect of foliar K on total soluble solids (TSS; p = 0.046) and a significant effect of soil K on juice pH (p = 0.001), where increased foliar and soil K rates resulted in higher juice TSS and pH, respectively. In addition, a significant increase in sugar per berry was observed in response to increased soil-applied K in Merlot only. Two SAR/ISR products, Regalia and Actigard, were applied in a separate randomized complete block design on separate Cabernet franc vines. These products had a significant effect on yield (p = 0.032), with a 70% increase in Regalia-treated grapevines and an 18% decrease in Actigard-treated vines. Although some effects of K and plant resistance inducers were evidenced in this study, repeated evaluation over multiple years is necessary to understand the long-term effects of these treatments.

Funding Support: Oregon State University, Agriculture Research Foundation (OSU ARF)