Karl Lund*
*UC Cooperative Extension, 145 Tozer Street Suite 103, Madera, CA 93638 (ktlund@ucanr.edu)

Plant parasitic nematodes can extensively damage grapevine roots that are not resistant to them, especially in vineyards with sandy soils, as is common in the San Joaquin Valley of California (CA). Fumigation is an effective control measure; however, regulations have restricted the use of fumigants in CA, heightening the importance of nematode-resistant rootstocks. The development of better nematode-resistant root- stocks is an ongoing effort. Documenting the viticultural performance of scions graft- ed to rootstock selections is critically important for proper selection and commercial acceptance of rootstocks and should be part of the process of selecting new material for potential release. Therefore, the performance of Petit Verdot winegrapes on newer nematode-resistant rootstocks (RS-3, RS-9, GRN1, GRN2, GRN3, GRN4, and GRN5), with 1103P and Freedom as controls, were planted in a replicated trial in a commercial vineyard in Madera county. Data collected over the past two years have shown differences in canopy growth and water stress based on rootstock. Shoot count shows that the entire GRN series along with Freedom start off with more shoots at the start of the season. GRN2, GRN4, and GRN5 maintained the largest canopies throughout the season, while GRN3 stayed in the middle and GRN1 became one of the smallest. Water relations measured as midday leaf water potential showed that despite having one of the largest canopies, GRN2 stays the least stressed of all the rootstocks, while RS-3 and RS-9 are the most stressed. Interestingly, GRN3 normally maintained a midrange water stress, except when the vineyard adds in water stress postveraison. During this dry-down, GRN3 becomes one of the most water-stressed rootstocks. This indicates that GRN3 is much more susceptible to drought stress. This ongoing trial will provide useful data for growers needing nematode-resistant rootstocks.

Funding Support: American Vineyard Foundation California Grape Rootstock Research Foundation