Optimizing the Breeding of Pierce's Disease Resistant Winegrapes with Marker-Assisted Selection
Pierce's disease (PD) greatly limits the production of Vitis vinifera based grapevines across the southern United States. The disease is caused by a xylem-limited bacterium, Xylella fastidiosa, and is spread by the sharpshooter family of leafhoppers (Cicadellide). Many Vitis species in the southern United States are resistant to X. fastidiosa, however their resistance is multi-genic and complex, which has limited breeding progress. However, resistance from several forms of V. arizonica (b43-17 and b40-14) from northern Mexico is inherited as a single dominant gene; all F-1 generation offspring are resistant and F-2 populations segregate 1:1 resistant: susceptible. This resistance has been genetically mapped to chromosome 14 where the locus, PdR1, is flanked by two closely linked SSR markers. Physical mapping identified a 201Kb region that encompasses PdR1, which is currently under study. The tightly linked flanking markers have been used for marker-assisted selection (MAS) for PdR1. MAS in conjunction with aggressive training in the field has brought the seed-to-seed cycle down to two years. In 2010, a total of 4,360 seedlings from 34 crosses of modified BC4 seedling populations (97% V. vinifera) were marker tested and over 2,000 resistant seedlings were planted in the field. These are destined for fruit and wine evaluations in 2012 prior to commercialization tests. Small-scale wines have been made from BC3 94% V. vinifera PD resistant selections with very promising results. PD resistant selections from the BC1, BC2 and BC3 level have also been planted in a severely infected area in Napa Valley. These plants are hand inoculated each year and their symptoms and bacterial titers are evaluated and compared to known resistant and susceptible controls.