Rainfall beginning yesterday May 1 which will extend through today has caused an apple scab infection period is virtually all Vermont orchards. Rainfall amounts range in the 0.1 – 0.5 inch range, which would not be enough to wash off a well-applied fungicide spray made just before the wetting period. If there is any question about coverage for both the current infection period as well as the expected extension of it with more rain expected Tuesday night through Thursday, tonight and tomorrow May 3 are your best window to reapply. There are a lot of mature ascospores out there, don’t be lulled into thinking that you’re okay to slide on this one because we’re still early in the season. Best products would be a protectant (mancozebs, captan; sulfur for organics growers) plus I would consider a material with post-infection activity such as an SDHI or (if they still work in your orchard) DMI. Alternately, the post-infection material could be applied Friday after the infection period, but if you’re relying on that solely to cover newly emerged tissue with no fungicide residue or to make up for poor coverage from your last spray, you’ll be using the materials at the very edge of their efficacy and encouraging resistance development.
For organic growers, if a full application (10 lb/ acre) of sulfur was made prior to the rains, I would consider making another touch up application at 5-7 lb/acre to cover new growth and compensate for washoff. If you had poor or questionable coverage, lime sulfur may be used and will provide kick-back activity and protection through the end of the week’s rains, but it’s a material best used sparingly so plan to keep covered with sulfur for future infection events.
Foliar nutrients can start any time you have sufficient foliage to provide for uptake, I would recommend applying at tight or open cluster through pink. Zinc and boron are important minerals which will help trees recover from cold damage, and nitrogen is needed at this time of year when rapid growth taxes tree reserves. Refer to Mary Concklin’s quick summary here for more information: http://blog.uvm.edu/fruit/files/2016/04/Tree-Fruit-Foliar-Nutrient-Applications-and-Nutrients-PGRs-for-Frost-Freeze-Situations.pdf