In January 2008, the University of California released five new UC-patented grape rootstock cultivars developed in Dr. Andy Walker’s grape rootstock breeding program in the UC Davis Department of Viticulture and Enology, with the participation of Professor Howard Ferris in the Department of Nematology.

The rootstocks were selected for broad resistance to nematodes. Each was also screened to ensure that it was resistant to phylloxera.

Brief descriptions of the new rootstocks are included below. Additional information on the characteristics, trial data, and performance of the new rootstocks can be requested by contacting Dr. Walker by e-mail at [email protected] or by phone at 530-752-0382.

All five cultivars were put through complete disease testing at FPS to qualify them for provisionally-registered status in the CDFA Registration & Certification Program for Grapevines (R&C Program), and pre-release production of mist-propagated plants (MPPs) at FPS ensured that a modest number of plants were ready for delivery to nurseries in spring 2008. The initial release of MPPs to UC-licensed CDFA R&C Program nursery participants occurred at a press conference held at FPS on March 31, 2008, attended by UC dignitaries and representatives of nurseries, industry funding organizations, UCD Viticulture & Enology faculty and the press.

FPS is continuing to produce MPPs on a custom order basis for delivery to official UC licensees. UC Davis Technology Transfer Services (TTS) is currently accepting requests for licensing for the new rootstocks only from current nursery participants in the CDFA R&C Program. For more details or to request licensing, please contact Clint Neagley at TTS by email at [email protected] or by phone at (530) 754-8720.

— UCD GRN-1 (8909-05)

The most resistant of the five, GRN-1 is a Vitis rupestris x Muscadinia rotundifolia hybrid. One of the rare sources of resistance to ring nematode, this cultivar has extremely strong and broad nematode resistance, and is a member of a group of Vitis species x M. rotundifolia selections currently being tested for their ability to induce fanleaf tolerance.

Mother vines have sterile flowers, moderately-long shoots with shorter internodes and more laterals than the other rootstocks. Though GRN-1 was originally thought to be “too rotundifolialike” to allow it to root well, it has rooted and grafted at 80 percent success from dormant cuttings. However, its one-year-old canes were damaged by a recent year’s 20 degree winter temperatures, affecting its rooting ability.

— UCD GRN-2 (9363-16)

GRN-2 acquires its nematode resistance from V. rufotomentosa (highly resistant to Xiphenema index) and V. champinii ‘Dog Ridge’ (strong resistance to rootknot and dagger nematodes), and roots and grafts easily because of its V. riparia parentage. GRN-2 is a good mother vine with staminate flowers, long shoots and internodes, and few laterals. It is susceptible to citrus and ring nematodes, but has excellent resistance to root-knot and dagger nematodes.

UCD GRN-3 (9365-43)

A sibling to GRN-4, its strong nematode resistance is derived from V. rufotomentosa, V. champinii ‘Dog Ridge’ and from c9038, a form of V. champinii that appears to intergrade with V. monticola. Vitis monticola is an unusual species that grows on very dry, gravelly or rocky limestone sites. Vitis riparia was used in the cross to impart good rooting and grafting abilities. GRN-3 has excellent nematode resistance, resists citrus and lesion nematodes, and is moderately susceptible to ring nematodes. Pistillate-flowered, its mother vines have moderate vigor, long straight canes with moderately long internodes and a moderate number of lateral shoots.

— UCD GRN-4 (9365-85)

A sibling to GRN-3, it shares resistance and rooting characteristics with GRN-3. With very good resistance to root-knot and dagger nematodes, it also resists citrus and lesion nematodes, but is susceptible to ring nematodes. GRN-4’s resistance to Meloidogyne arenaria HarmA was the most severely impacted by higher temperatures when compared to the other four rootstocks. It is an excellent mother vine with long canes, good internode lengths and few lateral shoots.

— UCD GRN-5 (9407-14)

This selection derives its resistance from V. champinii ‘Ramsey’ and from c9021, a form of V. champinii that appears to intergrade with V. berlandieri. Vitis riparia was used to improve the rooting and grafting of cuttings.

GRN-5 has excellent root-knot and dagger nematode resistance, resists citrus and lesion nematodes, and supports low numbers of ring nematodes. This rootstock supported the highest level of nodosity-based phylloxera (similar to 101-14 Mgt), but research to date has concluded that high nodosity level feeding does not cause vine damage. Mother plants are staminate-flowered with moderate growth, long canes, good internode lengths and few laterals.