The earliest parts of the province are seeing green tissue in most apple varieties. With the rain expected this weekend, we will likely see a primary scab infection period. Make sure to keep this tissue covered!
Apple scab overwinters in the leaf litter and the ascospores (primary infection) begin maturing early spring, sometimes still under the snow cover. While the number of mature spores ready to be released is low in those early infection periods, the risk is still there. Keeping up a good protectant, or pre-infection fungicide program during these infection periods will keep you from chasing scab as the season progresses.
Based on the Mills Table, which can be found on Ontario AppleIPM, primary infection requires a minimum of 11 hours of leaf wetness at 10 degree Celsius. The minimum number of hours decreases with increasing temperature. This requirement will be met in many regions of the province this weekend if rain continues into Sunday.
Apply protectant fungicides such as metiram, mancozeb or captan now if not already covered. If heavy rains greater than 2″ occur, residues will have likely been washed off. Reapply at any break in the rain over the next few days if that is the case. Be prepared to cover again once things dry up after Sunday.
With heavy or prolonged rain during the cool, early spring, a protectant program may not be enough to stay covered. Consider including a post-infection product such as Syllit, Scala, Inspire Super, Luna Tranquility or Buran. For more information on apple scab fungicide characteristics, refer to Table 2-7, Characteristics of Apple Scab Fungicides in the 2018-2019 Publication 360, Fruit Crop Protection Guide.