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Effects of Shredding or Treating Apple Leaf Litter with Urea on Ascospore Dose of Venturia inaequalis and Disease Buildup

Ascospores produced on diseased leaves in the leaf litter constitute the primary inoculum causing scab in commercial apple orchards in the northeastern United States. Two sanitation practices, shredding the leaf litter with a flail mower and urea application, were evaluated for their potential to reduce the risk of primary scab. Three measures of a treatment's potential were made: leaf litter density was evaluated to determine reduction of the source or primary inoculum; trapped ascospores were counted to measure the reduction of primary inoculum; and scab lesions on trees and seedlings were counted to determine reduction in disease incidence and severity on leaves and fruit. The results show that in the northeastern United States, shredding the leaf litter in November or April will reduce the risk of scab by 80 to 90% if all of the leaf litter is shredded. If 10 to 35% of the leaf litter cannot be shredded because of the limited offset of the flail mower and spread of the tree canopy, then the risk of scab is reduced by 50 to 65%. Urea applied to the leaf litter in November (when approximately 95% of the leaves had fallen) reduced the number of ascospores trapped by 50%. Urea applied to the leaf litter in April (before bud break) reduced the number of ascospores trapped by 66%. The reductions are discussed in relation to a sanitation action threshold that links sanitation to a strategy that reduces the fungicide dose to control primary scab in an orchard assessed with a moderate amount of foliar scab the previous autumn.