Apple Scab Situation in Ontario

Apple scab is present on the leaves and fruit in many Ontario orchards this year. A similar situation has been reported in Michigan and New York orchards. It is easy to blame the higher incidence of scab this year on the development of fungicides resistant scab, but there could be many reasons why scab is more prevalent this year than in past years. Resistance to both the stobilurin and Sterol inhibitor fungicides has been documented in Michigan, and to the sterol inhibitor fungicides in New York. Initial survey a few years ago indicates that some orchards in Ontario have tolerance to Sterol Inhibitor type fungicides. The higher incidence of apple scab observed in Ontario this year does not necessarily mean that resistances has developed to these fungicides, however, this can not be over looked at this point either. Unfortunately there is no fungicide resistance testing service in Ontario to confirm if resistance is the reason for the high incidence of scab being observed this year. OMAFRA is looking into finding a lab to conduct resistance testing.

So what happened this year? The inoculum that build up in orchards last year and the environmental conditions conducive for ascospore release for primary scab infection in late April and early May this past spring resulted in high disease pressure. Growers who did not apply a fungicide when green tissue first appeared earlier in the spring may be seeing more scab than growers who did apply a fungicide early. It is possible that some growers did not spray before infection took place due to rain. Further more, some growers may have underestimated the number of hours of leaf wetness required for scab infection and presumed an infection period did not occur when in fact it did. In some orchards, heavy rain immediately after a fungicide application may have washed off enough of the fungicides resulting in less protection. Growers who sprayed alternate rows probably did not get the coverage required to protect their trees and when followed by misty rains over a period of several days, scab became established on new leaves.

So what can apple growers do about the scab situation in their orchards this year? Growers should focus on protecting the apple crop from being infected with scab for the remainder of the season. This will require the application of protective fungicides such as captan or mancozeb on a 7 day schedule, especially before it rains. Always read and follow fungicide labels carefully paying particular attention to the pre-harvest interval. It is not recommended to use a strobilurin or Sterol Inhibitor type fungicide if they were used already this past spring. This will only encourage resistance developing if scab is present in the orchard. If the weather remains warm and dry, infection is less likely and the interval between applications may be stretched. As the fruit become more mature, they also become more resistant to infection as well. However, orchards that have scab should receive an application of captan one or two week prior to harvest to reduce the possibility of pinpoint scab developing after harvest.

It will be very important in the fall or early next spring to reduce the inoculum that is building up on leaves that will become the source of primary infection for next year. Research has shown that chopping or removing infected leaves in the fall or encouraging their decomposition with an application of urea in November or early next spring significantly reduces scab infections the following year.