Development, Testing and Introduction of Grape Rootstocks with Broad & Durable Nematode Resistance

We continue toward our goal of developing and releasing grape rootstocks with broad and durable resistance to nematode species that are important in California vineyards. In previous years, we have screened rootstock candidates against the root-knot nematode (Meloidogyne incognita race 3), two strains of root-knot nematode that overcome the resistance of Harmony rootstock (Meloidogyne arenaria strain A and Meloidogyne incognitastrain C), and the dagger nematode (Xiphinema index). Fourteen rootstock candidates exhibit broad resistance to those nematodes. Those fourteen have been narrowed to a group of six with desirable horticultural characteristics and broad resistance across all the nematodes tested. During 2006, we continued to test the breadth of that resistance beyond the range of the primary screen species by evaluating the resistance of the 14 candidates to the ring nematode, Mesocriconema xenoplax, in the presence of other nematode species. We also evaluated ring nematode resistance in the parents of the current rootstock candidates and in some other Vitissources. Only two of the rootstock candidates exhibit any resistance to the ring nematode and that may not be durable when other nematodes are present. We continue to seek new sources of resistance. Field testing of the rootstock candidates continues in fields that were heavily infested with root-knot nematodes. Nematode population levels are declining in the root-zones of all rootstock candidates, indicating that reproduction of root-knot nematodes is not occurring. However, population levels of ring nematodes at the field site are high on most of the selections, underscoring the need for obtaining new sources of resistance to that nematode. Rootstock candidates have also been tested in soils from three vineyards with populations of root-knot and ring nematodes. We initiated a new series of tests to screen for sources of resistance to ring nematode. Those tests will continue in 2007.