Rains and fungicide sprays

It looks like we will see a series of rain events in the next few days. As noted in the previous virtual viticulture meetings, we had several black rot and downy mildew infection events in the past two weeks, and some people start to see the development of these diseases.

Some growers asked me how long does it take for fungicide materials to be dry enough to be rainfast. It depends on your sprayer, droplet size, weather conditions, etc., but typically, I hear 1.5 hours to 2 hours are required. Some fungicide labels show the expected dry time.
The other common question is about how much rain is enough to wash off fungicide materials from the plant surface. Please check the excellent articles from Dr. Annemiek Shielder (link 1, link 2). She talks about the rain fastness of some products using a rain simulator. It looks like there is a sharp drop only after 0.04 inches of rain, but after that, the material tends to stay on the surface, even if there are more precipitations. In the article in the link #2,  she discusses the efficacy of the product (= disease development after wash off). She indicated that even after the wash off, the products she tested were still effective since the rate we are applying is often time much higher than the minimum effective dose. Thus, the old rule of thumb still applies, which is “2 inches of rain or 2 weeks, whichever comes first”.  The rule won’t apply to early-season when shoots are proliferating.

If you start to see downy mildew in your vineyards, here’s what I would do.
First, scout your vineyards and get an idea of disease incidence (yes/no). Check 10 random leaves at a spot and record how many leaves have downy mildew, repeat at 5-10 random locations, and get a percentage.
– 1-10% (sporadic here and there): mancozeb, captan, copper PLUS either a Phos acid or Ridomil. (There is a product called Gavel which has mancozeb plus another DM material too.)  Of these three, I have been impressed with copper against downy mildew, but one drawback is that you cannot mix a copper product with a Phos acid.
– 10-25% (notable outbreak): mancozeb, captan, or copper. There is a product called Zonix, that claims it kills zoospores. It may be a good mixing partner, but I don’t have any hands-on experience. Some growers told me that it worked well for them. After things settled down, i.e., after seeing hot and dry weather to slow downy mildew, I may start using a Phos acid, mixed with either mancozeb or captan. Also, there is a new product called OxiPhos, which is a combination of Oxidate and a Phos acid, which may come in handy to have an extra kick to a Phos acid. We have not seen resistance with Phos acid, yet, but there are cases on hops downy mildew, so, please use it with caution. I would not spray Ridomil when there is an outbreak (i.e., above 10-15%). It is too risky.
– more than 25%: I would stick with mancozeb, captan, or copper in a tight schedule to protect healthy tissues.

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