Breeding Grapevine Rootstocks for Resistance to Soil-Borne Pests
Crosses – About 1,300 seedlings from the 54 crosses made in 1994 were planted in the vineyard. Most of these crosses were designed to produce rootstocks resistant to root knot and/or dagger nematodes. We now have about 11,500 seedlings in the campus vineyard. Most of the 1995 crosses were designed to study the nematode resistance and taxonomy of Vitis champinii. Champinii is one of the best sources of nematode resistance within Vitis, but it is a poorly described species. Some of the taxonomists consider champinii to be a true species and others describe it as a hybrid complex between rupestris and candicans. We made crosses among all of the species whose ranges overlap in central Texas where champinii is found. Many of these should produce offspring with strong resistance to root knot and dagger nematodes. Peter Cousins (a PhD student working on the champinii project) is also examining our previous crosses within champinii (9207, 9208, 9209, 9210 and F9301) to genetically characterize the heritability of root knot nematode resistance within champinii, and investigate molecular markers (DNA and protein-based) linked to nematode resistance. These crosses and the ones completed this year will identify other sources of nematode resistance within champinii and may also identify less vigorous selections ofchampinii. Lower vigor forms of champinii with equivalent or greater nematode resistance would be very valuable. 1989 Seedlings – Two of the 1989 rupestris x rotundifolia seedlings, excelled in Mike McKenry’s field tanks with aggressive root knot nematode strains – 8913-02 and 8913-21. About 250 of the 1989 seedlings have been tested for Xiphinema index resistance. 8913-02 and 8913-21 were highly resistant, as were many of the 8916 seedlings. The best rooting and most resistant of the seedlings have been bench-grafted (successfully this year!) and are destined for field trials in Rutherford, Ripon and Paso Robles. The 8925 (berlandieri x rufotomentosa) population also had excellent resistance to X. index, but are very difficult to root. We also identified X. index resistance in champinii collections I made in Texas. Mapping X. index resistance – Yimin Jin (a MS student of mine) has identified RAPD (randomly amplified polymorphic DNA) bands in the rupestris x rotundifolia populations that are linked (associated with) to X. index resistance. The two primers that give the tightly linked bands are OP A 12 and OPU-1. He is also mapping X. index resistance with RAPDs. This will give us the bench marks for future genetic studies with this group, and allow us to pre-screen seedlings for X. index resistance, accelerating the evaluation process. 34 In vitro Phylloxera Resistance Screen – Evaluation of a tissue culture-based screen for phylloxera resistance is completed and the data is now being analyzed. We tested 4 selections from each of the following species: aestivalis, berlandieri, californica, champinii, cinerea, girdiana, labrusca, riparia, rotundifolia, rufotomentosa and rupestris, and 1 selection of amurensis and vinifera. Feeding success, reproduction, total phylloxera numbers and root and leaf damage were scored. Among the most interesting results were moderate susceptibility in the tested champinii; extreme resistance in cinerea, while berlandieri was more susceptible with one selection very susceptible; feeding and damage on rufotomentosa, but no reproduction; some susceptibility in aestivalis; high susceptibility on the leaves, but not roots of californica; high susceptibility in girdiana; and although St. George was a relatively good host, two other selections were highly resistant. Pest Diversity – Our DNA based studies of Californian phylloxera which found relatively high levels of diversity, were published. Jeffrey Granett and I are investigating native phylloxera populations in Arizona and New York where we found surprisingly low levels of diversity. We also continue to study the basis by which rootstocks and Vitis species alter phylloxera, and how phylloxera adapts to grape roots. A project to develop microsatellite (SSR) primers for phylloxera DNA was recently funded by the Viticulture Consortium and will allow Hong Lin (a post-doc in the lab) to produce a much more reliable and precise means of examining differences in phylloxera populations, both in California and the US. We have almost completed a project to examine the genetic diversity (based on RAPDs) in dagger nematode (Xiphinema index) populations across the state. We are analyzing data from populations we collected in Kern, Madera, Mendocino, Monterey, Napa, Santa Clara, San Joaquin and Sonoma counties. This study will help determine if dagger nematodes are different and will lead to studies on their ability to vector fanleaf virus and damage grape roots. Rootstock Identification – My lab adapted Carole Meredith’s microsatellite (SSR) primers to use in testing rootstock DNA and has produced a very precise means of identifying rootstocks. Like our previous isozyme-based test this test uses cambial tissue enabling the identification of rootstocks without leaves, or after they been grafted however this DNA technique is much more precise. We now have a unique profile for 58 rootstocks.