Identification of Fusarium species Associated with Decline of Cold-hardy Interspecific Grapevines in the Upper Midwest

Cold-hardy interspecific grapevines (CHIGs), bred for resilience in harsh winter climates, have enabled grape production in Northern regions. However, CHIGs have recently shown decline symptoms in the upper Midwest, including skipped spurs and stunted shoots – symptoms similar to grapevine trunk disease (GTD) symptoms in European grapes (Vitis vinifera). To determine the causal agents of CHIG decline, we conducted a statewide survey of symptomatic grapevines in Iowa and several counties in Upper Midwest states starting in 2023. From over 17 vineyards, we collected more than 1,500 fungal isolates from 30+ cultivars, with over 25% identified as Fusarium species. Species-level confirmation using TEF1-α, TUB2, and RPB2 sequences revealed four Fusarium species complexes. Pathogenicity tests following Koch’s Postulates confirmed that four Fusarium spp. caused symptoms on both CHIG and V. vinifera cultivars, with more severe symptoms observed in CHIG. This result, coupled with the lack of known GTD fungi isolated from our survey, suggests that Fusarium spp. are a unique causal agent of CHIG decline. Current studies include in vitro fungicide trials, mycotoxin analysis, and cross-host inoculations into other economically important crops. Overall, our findings indicate that Fusarium spp., rather than traditional GTD fungi, are a significant and unique driver of vine decline in CHIGs in the Upper Midwest, with important implications for vineyard disease management.