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Reflections on Resistance

Pierce’s Disease (PD) is an ever present threat in the minds of many grape-growers.  This disease, caused by the bacterium Xylella fastidiosa, is spread by many vectors, but most notably, the glassy winged sharpshooter (GWSS).  Though PD was first discovered in Anaheim, California and was spread by the blue-green sharpshooter, the presence of such an effective vector as the GWSS in the Southeast causes great concern for Georgia winegrape producers. PD is arguably the greatest limiting factor of bunch grape production for wine throughout the southeast.  While fungal diseases certainly make bunch grape production challenging (especially in years like this one) in the southeast, PD is different as it kills vines in short order.

The search for PD resistance in grapevines began about 20 years ago at UC Davis with Dr. Andy Walker leading the charge. After a large scale assessment of species, Vitis arizonica was found to have a single PD resistance-causing gene. Unfortunately, with resistance you often sacrifice fruit quality, and V. arizonica is no exception.  However, when it was crossed with Vinifera varieties, the offspring were completely resistant. Though the wine made from this cross maintained V. arizonica’s foxy, blue characters, traditional cross breeding eventually led to vines that were 88%, 94% and 97% V. vinifera.  The fruit from these high percentage V. vinifera varieties have little to no characteristics of V. arizonica, producing high quality wines and a promising future for PD resistance. All of this hybridization has been conducted using classical breeding methods, eliminating the concern of introducing GMO varieties.

Of course, when it comes to resistance, it is always preferred to have multiple genes to ensure the resistance is sustainable, and Dr. Walker and his team are continuously researching for additional sources of resistance. Recently, they have delved into sources of powdery mildew (PM) resistance.  While PM can be effectively managed with sprays, these rotations are costly and time consuming.  Walker’s lab has discovered several PM resistant varieties from Asia and is working to hybridize these with the muscadine M. rotundiflora which has single-gene resistance to PM.

Walker’s PD resistant variety 7355-75 will be the first to be available in nurseries; it will likely appear in small numbers in spring 2018/2019, and will hopefully become increasingly more available thereafter. However, the PM resistant varieties require more testing yet. Through classical crossbreeding assisted by modern genetic mapping advances, Walker and his team hope to strengthen PD resistance beyond just a single gene, further understand PM resistance, and make disease resistant varieties available to producers in the coming years.

On a final note, we were privileged to have tasted three wines made from Walker’s varieties – two 97% selections (a red and white) and one 94% selection (a red).  These wines are literally priceless, and are certainly some of the most rare wines in the world right now given their very low production up to this point.  Our UGA Extension Viticulture Team and others in attendance agreed that these were very fine wines, and were more representative of traditional wines compared to those produced from other current PD-tolerant bunch grape varieties.  This is great news, as Cain Hickey is working with industry collaborators to plant some of these varieties throughout north, central, and south Georgia in spring 2018 to eventually evaluate their growth, disease susceptibility (to other diseases than PD), and fruit composition in each region.