We continue to see quite a bit of Pierce’s disease (PD) in our vineyards this year, to include our northern Georgia sites. I am convinced that bacterial titer drives this disease, and bacterial levels in the plant are determined by the vectors (type and number of sharpshooters involved), winter temperatures, and possible alternative hosts. I visited a Lomanto site last week near the Florida line, and though this grape cultivar is reported as being tolerant of PD, most vines were showing significant symptoms; of five samples taken, all were positive for PD. I believe tolerance is relative to the location for some cultivars, and though we have never seen PD in west Georgia or the Athens/Atlanta area with Lomanto, the vector pressure farther south may just be too great. This has been reported in southern Mississippi as well. We continue to learn, but the school of hard knocks is not fun.
Vinifera grapes can grow in the northern part of the state, but with warmer winters, the disease increases. We are having warmer winters. In Tiger, GA (~2000 ft or higher elevation), we had 8 days below 15.1 F in the winter of 2008-2009 and 11 days below 15.1 F in the winter of 2009-2010. Over the last three winters, we have had 1 day at Tiger (cumulative across all three winters) below 15.1 F, which is a critical temperature for PD curing in Vinifera vines. This year, we have observed significant PD in Tiger vineyards.
Where PD is observed, the immediate destruction of infected plants and control of vectors is critical. We really need to consider using applications of imidacloprid insecticides applied to the soil through irrigation. Foliar sprays are helpful, but the irrigation route is best for efficacy. I am attaching a great resource from Texas. Not all of this will apply to Georgia, but you can learn quite a bit by reviewing this resource. I hope it will be helpful. If you have questions concerning PD or its identification or management, please contact your local county agent. They are more than willing to help.