Just as an update from my last blog post, we are now finding Pierce’s disease (PD) symptomatic Crimson Cabernet vines in our Horticulture Farm trial in Watkinsville, GA — just outside of Athens. Shane Breeden, a Ph.D. student in the Plant Pathology Department identified symptoms last week, and Kendall Johnson, another Ph.D. student from the Plant Pathology Department, confirmed via a test kit that Xylella fastidiosa is present in symptomatic leaves (see below for symptoms). This is concerning, but we do not know whether the disease will advance in this cultivar to kill the vines. Crimson Cab has been anecdotally reported as being tolerant to PD, but we will observe the vines over time to determine the true impact. If you have this cultivar in your vineyard, we do not have a firm recommendation as to how to address symptomatic vines — leave them or destroy them as you would a symptomatic Vinifera grape. The same is true of Lomanto, though as reported last week, it may die in the southern part of the state anyway. Muscadines can sometimes show symptoms, especially during droughts, but most muscadine cultivars are highly tolerant of the disease, and symptomatic vines are never removed. With the Lomanto and Crimson Cabernet vines, I simply do not have an answer at this time. I would tend to gravitate towards the “wait and see” approach for now, as I would hate to destroy vines that would naturally recover over the winter. If we really want to determine whether these cultivars are going to work in Georgia, watching them over time would be the best approach in my opinion.
As an additional update on the winter temperatures observed over the last few years, please see below for rough graphs of the average low temperatures (Fahrenheit) and days below 15.1 F at Tiger, GA. As with much of the data associated with weather and climate, the relationships are not as solid as one might expect, but the trend lines are definitely concerning. The bottom line is that we need several days of cold temperatures below 15.1 F to cure PD from vines, and we have not had this for years. If we are not obtaining these curing temperatures at Tiger, you can understand why PD is so prevalent at lower elevations — even where we have successfully grown Vinifera grapes in past years. If we have a few colder winters, the disease will retreat again.
I will leave you with a recent drone shot of one of our northern Georgia vineyards provided by Clark MacAllister. You can clearly see the holes left in the vineyard from destruction of PD-infected vines. We will discuss PD more thoroughly in our meetings this coming year. We need to utilize all means at our disposal to reduce the spread of this disease, but we also need to consider what our other options might be for the long haul — especially PD-resistant or tolerant vines.
I am also providing a copy of an article from 2008 relative warming trends and PD development in the Southeast. If you have not seen this before, I use this for the cardinal winter curing temperatures for Pierce’s disease. Also, I am attaching our UGA fact sheet on Pierce’s disease. I am sure that most of you are busy with harvest at this point, but save this reading material for your Bahama vacations after harvest.