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

Plant parasitic nematodes can damage grapevine roots that are not resistant to them extensively, 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 rootstocks is an ongoing effort. Documenting the viticultural performance of sci­ons grafted 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 newer nematode resistant rootstocks (RS-3, RS-9, GRN1, GRN2, GRN3, GRN4, and GRN5), along with 1103P and Freedom as controls, were planted in a replicated trial in Madera, CA. Data collected over the past three years show differences in canopy size and water stress. GRN2, GRN4, and GRN5 maintain the largest canopies throughout the season. GRN3 and RS-3 are intermediate and GRN1 and RS-9 have smaller canopies. Larger canopies are linked to greater water need. However, larger canopies also show less water-stress, measured as midday leaf water potential. GRN2 stays the least stressed throughout the season, while RS-3 and RS-9 are the most stressed. Interestingly, GRN3 normally maintains 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 may indicate that this rootstock is more susceptible to drought stress.

Funding Support: American Vineyard Foundation California Grape Rootstock Improvement Commission