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.