By Michelle Moyer, Viticulture Extension Specialist; and Gary Grove, Plant Pathologist
Powdery Mildew, Botrytis Bunch Rot, Sour Rot
Powdery Mildew (PM): While weather conditions have remained conducive for powdery mildew control, we have passed the period of cluster susceptibility. Mildew management should be focused on controlling canopy outbreaks if/where vegetative growth continues, keeping in mind pre-harvest intervals on some products. For fungicide resistance management, the use of high-risk fungicides at the end of the growing season is not recommended, due to likely high prevalence of the pathogen. (See our previous recommendations on fungicide resistance management.) Make sure you are still practicing fungicide rotation.
Botrytis Bunch Rot (BBR): As sugar accumulation advances, the likelihood of Botrytis bunch rot also increases. Monitor weather closely, as heavy dew or precipitation events between now and harvest can result in an outbreak. Elevate, Scala, Endura, Rovral, and Vangard are all appropriate products for véraison to harvest Botrytis control. Copper also has limited efficacy for those practicing organic management.
Also remember, that “extended hang time” also extends the possibility of BBR infections. Be vigilant in BBR management if you are hanging fruit on the vine longer than what is “normal” for your vineyard.
For those who are not aware, a new Extension Factsheet on Botrytis Bunch Rot has recently been published.
Sour Rot: Sour rot, called as such due to the “sour” smell of the rotting clusters (often caused by Acetobacter and other bacteria, in addition to endogenous yeasts), is differentiated from BBR by its wet appearance. However, BBR and high levels of PM on clusters can lead to sour rot development. Managing sour rot once it is present in the vineyard is challenging, but often compounds applied to control/prevent BBR have limited activity in managing sour rot (e.g. Rovral), especially when applied in combination with copper-based compounds.
Updates and educational blurbs are also available on the Viticulture and Enology Extension Facebook site. The Facebook medium also provides a more interactive approach to information transfer.
Growing Degree Day Update
Warm temperatures at the end of August and in the beginning of September have caught up GDD accumulation in 2011 with that of 2010. While still behind our historical average, the recent accumulation has been “high quality”, meaning that high temperatures haven’t exceeded the upper threshold at which vines begin to shut down, and low temperatures were not routinely dropping to temperatures that would also slow basic metabolic processes. As a result, many people are seeing véraison, and even harvest, occurring at the same time if not earlier, compared to last year. Reports around the valley are showing that grapes deemed for sparkling production are coming in, and the first lots for Pinot gris and Sauvignon blanc are being picked.