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

Delayed ripening is one of the most-reported fruit symptoms in grapevine red blotch virus-infected (GRBV+) grapevines. Potassium (K) is closely linked with berry ripening and early studies on K in GRBV+ grapevines indicated low K status in GRBV+ leaves. Thus, it was hypothesized that foliar K application would improve berry ripening through improved leaf K status and sugar translocation. A split-plot field experiment was conducted in 2020 and 2021 to test two formulations of foliar K fertilizers (0-0- 24) against a water control on previously identified healthy and infected grapevines. Treatments were applied weekly for four weeks beginning at veraison. At each appli­cation, the equivalent of 2.0 kg/ha of K was applied with a backpack sprayer to eight single-vine replicates. Leaf blades and petioles were sampled for nutrient analyses both before and after treatment application in 2020 and after treatment only in 2021. In both years, there was little to no effect of K application on K status in leaf blades, with all values in the normal range. In vines sprayed with K, berry fresh weight was reduced by 10% and 23% in 2020 and 2021, respectively (p < 0.0001 in both years), independent of GRBV status. Collaterally, berry total soluble solids (TSS) at harvest were increased by 1.6 and 4.1 Brix in 2020 and 2021, respectively (p < 0.0001 in both years) by both K fertilizers independent of GRBV status. However, there were no significant effects of K application on berry phenolic composition in either year. The lack of treatment effects on berry phenolic composition suggests that while foliar K application has limited effects on fruit quality in GRBV+ grapevines, the increased TSS may help reach an otherwise unachievable TSS target or advance the harvest date in heavily-infected blocks.

Funding Support: USDA-NIFA-SCRI grant number 2019-51181-30020