The recent rain and current hot, humid conditions are PERFECT for the development of fire blight (bacteria, Erwinia amylovora) on Rosaceous trees such as apple, pear and mountain ash. The bacteria enters through stomates or mild wound sites and can cause stem and leaf dieback in a short period of time. OMAF and MRA staff have been watching the weather a issuing prediction models for Ontario based on the weather forecast (Cougar Blight model). Check it out:
http://www.omafra.gov.on.ca/english/crops/facts/fireblight-2.htm
Nursery stock and landscape plants are still at risk. Registered fungicides include Blossom Protect (biological), copper, Kasumin, Streptomycin.
ONnurserycrops Blog 