Managing summer apple scab epidemics using leaf scab incidence threshold values for fungicide sprays

In several apple production areas, most fungicide sprays applied to orchards target apple scab management. Recently, action thresholds were established to aid in decision making as to whether fungicide sprays are required to manage summer scab. To facilitate grower adoption of these thresholds, a sequential sampling for classification procedure (SSCP) was developed. The SSCP with an action threshold of 0.5% foliar scab was evaluated from 2005 to 2008 in seven commercial orchards as a tool to time fungicide sprays. At each site, at the end of the primary infection period, the orchard was divided into two sections: in one, summer scab was managed using the grower’s standard practices and in the other, summer scab was managed based on scan incidence. Starting in mid-June, each orchard section managed based on scab incidence was classified using the SSCP as below or above the action threshold or as in the “no decision” zone. When an orchard section was classified as above the action threshold, a fixed-interval spray program was initiated and generally continued until harvest. When an orchard section was classified as below the action threshold, initiation of the fungicide spray program was postponed until the threshold was reached. When an orchard was in the “no decision” zone, sampling was repeated one or two weeks later depending on the frequency of rain events. Similar incidences of foliar scab of 0.27% and 0.22% and of fruit scab of 1.11% and 1.27% were observed in orchard sections managed based on scab incidence and standard grower’s practices, respectively. However, fruit scab at harvest was above the economic threshold of 2.0% at three and six occasions in the sections managed using foliar scab incidence and using the growers’ standard practices, respectively. For the four years of the study, an average of 6.4 and 9.1 fungicide sprays were used to control summer scab in the orchard sections managed based on scab incidence and in those managed according to the growers’ standard practices, respectively. Considering that the same fungicide was used in all orchards, we concluded that the reduced number of fungicide sprays (30%) resulted from the elimination of sprays when scab incidence was below the action threshold of 0.5% leaves scabbed.

► Apple scab (Venturia inaequalis) is one of the most important diseases of apple.
► Studies on apple scab management has been concentrated mostly on primary infections.
► Little attention has been on management of secondary infections.
► The key element in making management decisions is the level of disease present in the orchard.
► The results of this study show that it possible to reduce usage of fungicides during summer.