The sooty blotch and flyspeck model*:

infection potential starts after app. 300 hours of leaf wetness are counted from 10 days after apple petal fall (leaf wetness duration of 3 hours or less not included); after the 300 hours threshold is reached, any 3 hour leaf wetness period signals a possible infection

Example:

Leaf wetness hours are counted beginning 10 days after apple petal fall (May 18 below). In this computer model, only leaf wetness hours that last 3 hours or longer are counted. The threshold is reached after 250 hours (for southern states) or 300 hours (for northern states), July 19 and 26 respectively. From this point any leaf wetness interval of 3 hours or longer could result in sooty blotch/flyspeck infection, hence fungicide protection should be in place. Note that duration of fungicide protection depends on two factors: fungicide used, and amount of rain since last application.

*model based on 'Disease of Tree Fruits in the East,' Alan Jones and Turner Sutton, 1996; and SpecWare 7 Professional Disease and Insect Guide, Spectrum Technologies, Inc.

For more information and an alternate discussion, see: Flyspeck fungicide respray estimates on Orchard Radar.

Also see Summer Diseases in Scaffolds Fruit Journal.