The three-cornered alfalfa hopper, Spissistilus festinus, is an epidemiologically important vector of grapevine red blotch virus in northern California vineyards

Grapevine red blotch virus (GRBV, genus Grablovirus, family Geminiviridae) is the causative agent of red blotch disease. Infected grapevines experience delayed fruit ripening and reduced fruit quality, ultimately negatively impacting wine quality. Spissistilus festinus (Hemiptera: Membracidae)transmits GRBV in the greenhouse; yet its role as a vector of GRBV in the vineyard is poorly understood. We hypothesized S. festinus can acquire GRBV from infected, asymptomatic vines and transmit GRBV to healthy vines in the vineyard. To test our hypotheses, S. festinus were exposed for two weeks to infected, asymptomatic vines in northern California in June and were then allowed a 48-h gut clearing on alfalfa. Approximately half (45%, 46/102) of the released insects tested positive for GRBV by PCR, including in dissected salivary glands (11%, 3/27), indicating virus acquisition in the vineyard. Next, following controlled exposures of viruliferous S. festinus for two- to six-weeks on GRBV-negative vines in a vineyard in northern California and in New York in June, transmission of GRBV was detected when two S. festinus were restricted to a single leaf (3%, 2/62 in California; and 10%, 5/50 in New York), revealing virus transmission, but not with cohorts of 10-20 specimens on entire or half shoots. This work was consistent with greenhouse assays in which virus transmission was facilitated by feeding of fewer S. festinus on a restricted area of grapevine tissue. Together, this study illustrates S. festinus as a GRBV vector of epidemiological importance in the vineyard.