Evaluating nematode resistance of grapevine rootstocks based on Xiphinema index reproduction rates in a fast screening assay

By feeding on grapevine roots, the ectoparasite Xiphinema index transmits grapevine fanleaf virus (GFLV), causing fanleaf degeneration in the plant. Therefore, rootstock breeding programs are considered a key to resistance to both threats. Previous screening depends on long-lasting inoculation experiments and mainly focuses on evaluating gall formation on the parasitized roots to identify potential resistance in grapevines. Since the roots of grapevine rootstocks resistant to X. index and even those of susceptible cultivars showed no reliable gall formation after parasitism, and further, root thickening was regularly found on non-inoculated control plants, nematode reproduction rates were used to validate rootstock phenotypes regarding susceptibility or resistance to X. index . For our approach, a glass vial assay was performed, and 35 days after inoculation the nematode population was determined by counting juveniles after the extraction. Reproduction rates were high on susceptible rootstock cultivars and decreased the more resistant a cultivar is considered. Besides focusing solely on root gall formation, the nematode reproduction rate should be included as a reliable indicator for resistance screening. The described assay setup enables the evaluation of resistance levels by fast screening to select promising rootstock candidates.