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Field Evaluation of a New Sequential Sampling Technique for Determining Apple Scab “Risk”

Most fungicide sprays applied to apple orchards in the New England states are targeted at the management of apple scab. Researchers have developed action thresholds that aid in decision-making on whether early spring fungicide applications could be eliminated without a significant increase in the incidence of fruit scab at harvest. To facilitate grower adoption of these thresholds, a simplified, sequential sampling technique in autumn to determine the “scab risk” of an orchard for the following spring was proposed in the scientific literature. However, this technique had not been evaluated in the field. In autumn 1999, 2000, and 2001, orchards were evaluated using the new sequential sampling technique to determine scab risk. Risk ratings were compared with those obtained by the original, nonsequential procedure in each orchard. Data also were examined using a simulation sequential sampling computer program to determine whether or not risk ratings would change if different trees or shoots were used. In two of the assessed orchards, “delayed-spray” experiments involving two treatments (a delayed-spray and full-spray treatment) were conducted in 2000 and 2001. Delayed-spray replicates were to receive no fungicide sprays until after the third primary infection period (but before the fourth) or until the pink stage of bud development, whichever came first; full-spray replicates received fungicide sprays starting at the green-tip stage of bud development. The sequential sampling technique provided scab-risk ratings consistent with the original, nonsequential procedure, at potentially significant time savings. Also, following the delayed-spray strategy in low-risk orchards did not result in significant differences in fruit scab at harvest compared with initiating spraying at the green-tip phenological bud stage.