Hi Frances, 

Thanks for reaching out. This week is critical for apple scab control. Apple scab is a fungal disease that overwinters in the leaf litter and fallen infected fruit. As green tissue emerges in the spring, apple scab spores will launch out of the fallen leaves during rain events. It is very important to cover the new leaves with fungicides prior to these rain events. The products are protective, but not curative. 

If you really want to control apple scab, I would treat trees with fungicides prior to rain events from now until probably mid-June. From there, treatments about every two weeks will contain it. 

Look for products with the following active ingredients: "Captan, mancozeb, chlorothalonil, myclobutanil, propiconazole, copper (organic), sulfur (organic), Bacillis subtilus (organic), or Bacillis amyloliquifaciens (organic)" https://extension.psu.edu/apple-scab-in-the-home-fruit-garden

MSU Enviroweather has a model that predicts when apple scab infections will happen and how severe they will be. You can enter your nearest weather station to get information specific to your area. https://enviroweather.msu.edu/crops/apple/applescab

Good luck with scab!