Weather equipment that monitors temperature and hours of surface wetness must be used to determine if conditions were favorable for infection. When an infection period is determined to have occurred, a fungicide with postinfection activity is applied if no fungicide was applied within the previous seven days. A postinfection strategy can reduce the number of spray trips and the amount of fungicide used, especially during a spring with few infection periods spaced widely apart. Prolonged rainy or windy weather, orchards that are too wet for travel, breakdowns, or other unexpected occurrences can also prevent timely application of a protectant. This could result in extensive infection, so it may be desirable to have an SI available for an emergency backup.
Fungicide resistance is another factor to consider.
Unfortunately, scab can develop resistance to these materials, so repeated use is not recommended, especially as postinfection sprays. The low prediction that qualifies an orchard for the protectant delayed spray strategy beginning at the pink fruit bud stage also qualifies the orchard for the 4 spray SI schedule. Spray coverage is adequate and spray calibration is accurate . Effective control with an SI requires that the leaves and fruits receive adequate fungicide. Factors such as cultivar or weather during spray application do not increase scab risk significantly .