This year we have continued our focus on obtaining a broader genetic base for resistance to virulent pathotypes of root-knot (Meloidogyne spp.) and dagger nematodes (Xiphinema index), broadening our search for resistance to ring nematode (Criconemoides xenoplax) and to sources of resistance to pin nematode (Paratylenchus sp.). We have also monitored the performance regarding nematode susceptibility of new and older rootstocks that are being compared in trials at various locations. We have continued our development of sterile dual-cultures of nematodes which will allow us to understand the mechanisms of resistance provided by various genetic sources. We continue to expand and to improve accessibility to the database for plant resistance to nematodes and for selection of rotation and cover crops. Further, we have disseminated results of our research to end users at grower meetings and in scientific media.
Our research has identified Muscadinia rotundifolia as an important source of resistance to dagger and ring nematodes, and possibly to others; it is also a source of genes that provide salt tolerance. Salt tolerance in grape rootstocks is an issue in areas with saline soils and where irrigation water is of lower quality. Consequently, crosses have been made in the Rootstock Breeding Program between fertile individuals of UCD-GRN1 and other sources of salt tolerance. There are 400 progeny of these crosses that need to be screened for resistance to various nematode species. Among the 400 progeny of crosses developed by the Rootstock Breeding Program were several different series, including those designated as the D series, the E series and Wild Source series. Also available were additional 07107 selections not previously screened for ring nematode resistance. Other than the 07107 selections, all were screened at least to the virulent root-knot nematode pathotypes, some also to dagger (X. index) and to ring nematode.
In greenhouse pot experiments, rootings from as many selections as could be propagated coincident with the availability of nematode inoculum were inoculated with various target nematode species. Our basic protocol for all nematode screening is to establish rooted cuttings in 10-cm diam. pots and inoculate the soil with either 500 or 1,000 individuals of the test nematode. After 3 months, the aboveground portion of the vine is removed and weighed, nematodes are extracted from the soil (and roots if appropriate) and counted; the root system is examined for symptoms. Controls in all experiments include susceptible cultivars and cultivars thought to be resistant. After nematode extraction and examination for symptoms, roots are dried for 24 hours at 85ºC and weighed. That enables us to assess impact of the nematodes on root vigor and also to express the nematode populations on a per g root basis. Ring nematode populations throughout the state differ enormously in vigor and impact on plant hosts although they are currently characterized as the same species (Criconemoides xenoplax). The Fresno population is particularly virulent so we test selections against it. Fortunately, crosses between Vitis spp. and M. rotundifolia continue to provide promising material. Table 1 indicates a range of responses of 07107 series selections to ring nematode. Clearly there are promising selections among these that have resistance equal to or better than that of O3916, a Muscdinia rotundifolia derivative used as a standard in this study. Those selections will continue to be used in the rootstock breeding program.