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Downy mildew reminder



Of course, once I mentioned the lack of rainfall, we (i.e., northern VA) were hit by a series of rain. To make things more complicated, we had at least three consecutive nights of warm and humid (RH > 90%) condition prior to the rains. Humid nights encourage downy mildew pathogen to produce spores, and rains will splash spores to healthy tissues. Thus, the risk of downy mildew during recent rain events was very high. (Note: Central VA also had two humid nights in 7/5-7/6 at the Charlottesville airport, but the chance of rain is not high until this Thursday.)

There are a number of materials can be used for protection: captan (FRAC = M4), mancozeb, ziram (FRAC = M3), Revus, Forum (FRAC = 40), Zampro (FRAC = 40 plus 45), Ranman (FRAC = 21)). Mancozeb products have the 66-day PHI, but ziram's PHI is 21 days. Another good option is a copper (FRAC = M1) material, which is more economical than other materials. Copper has good efficacy against downy mildew, and tends to do well under frequent rain condition. There are several newer copper materials that cause fewer phytotoxicity issues even on relatively copper sensitive cultivars (e.g., Cueva, Champ, etc). If you have already having downy mildew issue, use copper, ziram, or captan as the backbone of your spray program.

Both a phosphite material (FRAC = P07) such as Prophyt and Phostrol, as well as Ridomil products (FRAC = 4 plus M3 (MZ) or + M1(Copper)) have a kickback activity against downy mildew (i.e., it can stop the ongoing infection process). Ridomil MZ contains mancozeb, so, we cannot use it at this time of the season due to its 66-day PHI. Ridomil Gold Copper has a 42-day PHI, so, you may able to use it, depending on the cultivar. Please note that Ridomil is known to have resistance issues, thus, let's limit the use of Ridomil to twice a season at the most. We also have seen multiple Revus-resistant isolates in VA.

So what to do, if you are not sure your previous fungicide application was sufficient (e.g., applied more than 10-14 days ago and/or received more than 2 inches of rain)? 

Options are:
A) a phosphite or Ridomil Gold Copper plus captan or ziram
or
B) a phosphite or Ridomil Gold Copper plus Revus/Forum or Zampro or Ranman. 

These mixes provide both protective and kick-back activities. 

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Downy mildew gallery

At Winchester, we had light rain events during the night of 6/12/09, but it was short events and the relative humidity was low (80% or so), thus it probably did not promote any infections. However, we are experiencing continuing favorable nights for downy mildew sporulation (average T>55F, high RH (80-100%)) for 10 days now. Yesterday, we conducted a formal disease assessment, and observed first incidence of powdery mildew for this season. We had plenty of infection events in last two months, so it was not surprising. At this point, it is a trace level of infection on untreated vines. Downy mildew was the major disease so far. We had up to 40% incidence on untreated vines. Next runner-up was black rot. It varies vine to vine, but some of vine had 10-15% incidence. Phomopsis was omnipresent as I expected from early May rain falls, but severity was low overall. We will examine diseases again in the near future, and I will update as the season goes. Here is downy mildew ga