Cindy Kron, Emily Bick,* and Frank Zalom
*University of Copenhagen , Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark, Denmark (enbick@ucdavis.edu)

The three-cornered alfalfa hopper (TCAH) was shown to transmit Grapevine red blotch virus (GRBV), the causative agent for Grapevine red blotch disease, in a greenhouse study on grapes. GRBV is a major concern of winegrape growers due to its economic impact on wine quality. Plants in the family Fabaceae are preferred hosts of TCAH and are commonly planted as cover crops or present in a vineyard’s native vegetation. In late winter, during grapevine dormancy, TCAH migrate into vineyards to feed and reproduce on these cover crop and weed hosts. Tilling vineyard floor vegetation provides growers an opportunity to disrupt the life cycle of early imma­ture stages that are relatively immobile, reducing TCAHs first-generation population. Nymphal presence is difficult to detect in the field due to their small size and light coloration. First through third immature stages were not detected by sweep net in a two-year weekly sampling study, while fourth and fifth immature stages were first found on the same sample date as emerging first-generation adults. Once TCAH emerge as an adult, they can disperse and thereby circumvent control measures. A degree-day model was developed that predicted the ideal time frame for tillage at between 1310 and 1565°C accumulated degree days, when ~25 to 80% of early imma­ture stages are present. Vineyards were sampled to further validate the model in six wine regions of Napa County (Oakville, Calistoga, Mt. Veeder) and Sonoma County (Healdsburg, Geyserville, Glen Ellen). Growers and viticulturists can access the UCIPM website and use a free degree day calculator that uses weather station data closest to their vineyard to calculate degree days accumulated. Using this model to exploit the time frame at which TCAH is most susceptible to cultural control measures can benefit vineyards that already include tilling in their seasonal operations.

Funding Support: CDFA Specialty Crops Block Grant Program, CDFA PD/GWSS Grant, the California Grapevine Rootstock Improvement Commission, American Scandinavian Foundation Fellowship