Much of the following is a post from 2017 but timely for our situation at the moment:
Pecan scab is enjoying ideal weather in much of Georgia at this time. This has kept most growers busy trying to keep the trees sprayed between rain showers. A common question we get during such conditions is “how long does my fungicide need to be on before the rain arrives?”. This is a question pathologists have been trying to answer for quite some time but there are so many variables involved it can be quite difficult to generate data to provide a hard and fast answer. UGA Plant Pathologist Dr. Tim Brenneman recently offered the following suggestions based on his studies with pecan and also on similar studies he has conducted in peanut:
Highly systemic materials like phosphite must be absorbed into the plant. It may require as much as 1/2 a day for this process to take place so that the material can be highly effective. While DMIs and strobilurins or combination products like Absolute, Top Gard, Quadris Top, and Quilt have some systemic activity they are not as systemic as the phosphites. Still, they would need a little time to be effective, so several hours to half a day would be ideal. A surfactant (80/20 or other) would increase the uptake speed of these materials and would likely provide some benefit in the conditions we’ve had over the last week or so. However, do not include a surfactant with Phosphite.
Miravis Top contains a Group 3 (DMI) + Group 7. With regard to the topic of rainfastness, until we learn differently, treat it as you do the Group 3 + Group 11 materials.
While the DMIs and Strobis have some systemic activity they still function largely as protectants. Other materials like Dodine (ELast) or Tin are pure protectants. These materials (Dodine and Tin) would be the most prone to wash off when rain arrives shortly after spraying. Dodine does adhere tightly to the plant cuticle, which likely helps it. Rainfall that occurs less than one hour after spraying makes the fungicide pretty well ineffective. Any rainfall within 24 hrs after spraying with a pure protectant will likely reduce the effectiveness of control to some extent. For each additional hour between the spraying and rainfall you gain additional control.
Ideally, all fungicides should be applied prior to rain events. If circumstances prevent you from getting a spray on in time and you have to spray after a rain event, the DMI/Strobi combinations would be the best choice. If you get rain before the fungicide has been on the trees for at least 1 hour, re-spray those rows which were covered less than an hour before the rain.