MARKER-ASSISTED SELECTION FOR RESISTANCE TO XYLELLA FASTIDIOSA: ACCELERATED BREEDING OF PIERCE’S DISEASE RESISTANT GRAPES Project Leaders:

Efforts at identifying genetic markers tightly linked to Xylella fastidiosa (Xf) resistance are continuing. These efforts are primarily focused on the 9621 mapping population (D8909-15 x F8909-17) and are in cooperation with fine-scale mapping efforts discussed in our report “Map based identification and positional cloning of Xylella fastidiosa resistance genes from known sources of Pierce’s disease resistance in grape.” This project is adding 200 SSR markers to the 9621 map and has positioned Xf resistance from the male parent F8909-17 on the lower arm of linkage group 14, where Xf resistance is flanked by multiple markers. Resistance from the female parent D8909-15 maps as a quantitative trait locus. The addition of additional SSR marker is expected to reduce the distance between SSR markers and Xf resistance to about 2 cM and lead to the development and utilization of very specific sequence characterized amplified region (SCAR) primers. These markers will be applied to populations in the breeding program derived from D8909-15 and F8909-17 and tested for effectiveness in Xf resistance backgrounds derived from other species. INTRODUCTION Marker Assisted Selection (MAS) is the process whereby indirect selection on a trait of interest (such as disease resistance) is made by screening for the presence of a DNA marker allele tightly linked to the trait. MAS for disease resistance can be used to eliminate susceptible genotypes in a breeding population early in the selection process, which allows for evaluation of much larger effective populations. Larger effective population sizes increase the opportunity to identify genotypes with high disease resistance and high horticultural qualities (such as good fruit size, color, texture etc.). Other key aspects of the MAS process is that confounding environmental effects on the trait phenotype can be avoided and progress in breeding programs can be accelerated while saving space and time, allowing for more efficient use of resources (Paterson et al. 1991, Kelly 1995). Rapid screening time is particularly valuable when applied to perennial crops such as grape with relatively long generation times (Alleweldt 1988, Striem et al. 1994). Markers linked to grape resistance genes of other diseases have been published. AFLP and RAPD markers tightly linked to powdery mildew resistance (Dalbo et al. 2001, Pauquet et al. 2001) and downy mildew resistance (Luo et al. 2001) are some examples. To effectively use such linked markers in MAS only requires that the markers be highly reproducible, linked in coupling phase i.e. on the same homologous chromosome, and within 5 cM (cM = centimorgan, a mapping unit representative of the distance between two loci or genes) of the resistance locus (Kelly 1995). Conversion of AFLP and RAPD markers to SCAR primers allows for a more reproducible marker system and identifying tightly linked markers is a direct function of numbers of markers screened. In the case of powdery mildew resistance MAS has already been successfully utilized for screening a grape breeding population (Dalbo et al. 2001) and it is expected that this project will have a high chance of success for developing a functional MAS system for screening PD resistant genotypes. Markers tightly linked to PD resistance should have immediate benefits toward accelerating the breeding of PD resistant wine, table and raisin cultivars. OBJECTIVES 1. Refine localization of primary QTL’s associated with PD resistance derived from Vitis arizonica. 2. Saturate regions of primary QTL’s with AFLP markers via Bulk Segregant Analysis (BSA). 3. Identify tightly linked flanking markers around PD resistance QTL’s and convert to SCAR primers. 4. Confirm candidate marker linkage to resistance within a (8909 x V. vinifera table grape) family. 5. Utilize resistance markers to eliminate susceptible progeny within a (8909 x V. vinifera) x V. vinifera table grape backcross generation and future generations of the continuing UCD/USDA collaborative PD resistance breeding program. RESULTS AND CONCLUSIONS Objective 1. This proposal expands upon a portion of a project funded by the AVF and last year by the CDFA entitled “The Genetics of Resistance to PD”. That project developed a genetic map in a Vitis rupestris x V. arizonica population (9621 = D8909-15 x F8909-17; see Walker, Tenscher, Ramming Progress Report in this proceedings for more detail on this population) segregating for Xylella fastidiosa (Xf) resistance and was based on about 500 DNA markers. The parents of this cross were half siblings sharing a common V. rupestris parent which is susceptible to PD. The progeny D8909-15 is a female