What to do when you receive a lots of rain after fungicide application?

At Winchester area, we ended up having a pretty good weekend. There were several periods with rain, but it was not as extensive as the forecast. We received about an inch total over 2-3 days. However, it looks like the central VA received more rain than we did.

I would like to share a question I received from a grower today.

“With all of the rain events we have had lately and bloom being an important stage for fungicide application Should we spray after a significant rain event before the recommended 7-14 day interval? Specifically, I applied fungicide June 3rd and received quick but heavy rainfall on the 4th and 5th. Riding through the vines it looks as though there is still chemical residue on leaves and clusters. Can I base a decision off of this observation?”

It really depends on what you have sprayed, what we are expecting in the next few days, how the vines are maintained (shoot thinning, etc), cultivars, etc, so, I cannot give you a quick answer.

Typically, after drying, these fungicides stick pretty well with leaves and application rates are often time much higher that what you need to control diseases. Thus, even after considerable amount of rain and removal of spray residues, you can still achieve a good disease control. Please see this excellent article from MSU (http://msue.anr.msu.edu/news/fungicide_properties_and_weather_conditions) for more information.

However, since this is the critical time, I would be on a safer side. I don’t think you have to spray today or tomorrow, but I may recommend to spray at 7-day interval, rather than extending it to a longer period. Depends on how the weather goes and target disease to be, but if you have a concern on downy mildew or black rot, this may be the time you want to use something with a kick-back activity (a phosphite or Ridomil product for DM, DMI for black rot). Powdery mildew fungus prefers dry weather, but if you have been struggling with PM in the past, and you are expecting a dry weather after this rain event, you may want to consider adding a PM specific material such as Quintec, Vivando, Torino, etc.

Also, as I noted in a previous post, some of you may have a rather long period of bloom due to damages caused by the April frost events. (For example, some of our Chardonnay vines are in full bloom today yet others are still lagging behind, which are probably on shoots from secondary buds) Depending on the situation, you may have to consider two applications for at bloom disease management especially to target Botrytis (e.g., Rovral, Elevate, Switch, Luna Experience, etc), ripe rot (QoI, mancozeb, zirum, captan, copper, etc), and bitter rot (same as ripe rot). It will get tricky, but please make sure to rotate FRAC codes!

Finally, I am not an entomologist, but this is also the time I start to see the incoming of grape berry moth in our vineyards. Please keep eye on them so that they won’t cause damages on later in the season.

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