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. This year, 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 Vitis sources. 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. We also continued to test the durability of nematode resistance of the rootstock candidates when they are exposed to combinations of nematode species by determining the durability of resistance at different temperatures. Resistance of the parents of the rootstock candidates to several root-knot nematode variants was compromised at soil temperatures of 30°C and above but not below 27°C. However, some of the rootstock parents maintained resistance to even the virulent Meloidogyne arenaria strain A at high temperatures, indicating that there is durability to temperature among the parentage.
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.