Biology and Genetics of Rootstock Resistance to Grape Phylloxera

Phylloxera resistant grape rootstocks which also resist nematodes, viruses and fungi and tolerate salt and drought are needed in California. To breed such stocks we will need to know biological and genetic mechanisms of phylloxera resistance in parental grape species to be used for the breeding of the rootstocks. We proposed to initiate work on determining the biological and genetic mechanisms of V. rupestris resistance by studying a population from the genetic cross V. rupestris x V. vinifera hybrid vines. These vines have been growing in the U.C. Davis vineyard for several years. We started by doing bioassays leading to a knowledge of the genetics. We used a modification of our 25-day biotyping bioassay for assessment of the resistance of 235 vines to biotype A grape phylloxera. 132 vines were hybrids of V. vinifera cv. ‘Aramon’ x V. rupestris, 50 were selfed ‘Aramon’ x ‘Aramon’ seedlings, and 53 were selections or crosses of selections of V. rupestris. We estimated survival and developmental rates (a developmental index) at day 25 and reproductive rates from day 25 to day 29, then calculated a combined estimate of the growth rate of the colony, the ‘vulnerability index’. All Aramon vines had high (1.49 to 7.33) vulnerability indexes, and except for cultivar ‘Geant’ (vulnerability = 1.36), all V. rupestris vines had very low vulnerability indexes (0.00 to 0.37). With the average vulnerability index of the hybrids at 0.47, most appeared to have inherited resistance to biotype A from the V. rupestris parent, but resistance was compromised to varying degrees by inheritance of susceptibility from the Aramon parent. Eleven of the hybrids had vulnerability indexes from 1.46 to 2.69 at or near the lower end of the Aramon range, and another at 6.39 appeared to be highly susceptible to biotype A. These results suggest that inheritance of resistance is multigenic with some genes suppressing survival or fecundity and others retarding development. The next step is to begin characterizations of a sample of these vines that span the vulnerability spectrum. Characters which are quantitatively, negatively correlated with the level of vulnerability, may in fact be related to the resistance mechanism. We have chosen 15 accessions with relatively high survivorship for this based on variation in the developmental index. These criteria will allow us to focus on mechanisms which affect nutrition rather than cause decreased survival. We have also tested a method for evaluating the link of fungal and phylloxera resistance in these stocks. The preliminary results suggest that fungal and phylloxera resistance is not linked in a number of rootstocks.