Our Chardonnay vines are getting close to full bloom, and of course, we are expecting more rain over the weekend. Just an another reminder that from bloom to 4-5 weeks after bloom is the critical period for downy mildew, black rot, and powdery mildew infection on clusters, and at bloom application is very important one for Botrytis management. Please see recent post about the at bloom fungicide application consideration as well as recent updates on fungicides. This would be the timing where you will thrown in some good materials into your tank mix! It is much easier to protect the vines than try to play a catch up game after disease outbreak,
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?
I understand this is a long winded question(s) but any advice would be much appreciated. Please and thank you.
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), so, I cannot give you a quick answer.
Typically, after drying, these fungicides stick pretty well with leaves and rates are often time much higher that what you need to control diseases. Thus, even after considerable amount of rain, you can still achieve a good disease control (http://msue.anr.msu.edu/news/fungicide_properties_and_weather_conditions)
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 might recommend to spray at 7-day interval, rather than extending it to a longer period.