Department of Plant Biology, University of New Hampshire, 246 Spaulding Hall, Durham, NH 03824, USA

Background and objectives
Ascospores produced by Venturia inaequalis on overwintered scabbed leaves comprise the primary inoculum that infects developing leaves and fruit after the buds open in spring. A common assumption in regions where scab is a perennial problem is that the supply of ascosporic inoculum is always great enough to warrant initiating a fungicide program to control scab shortly after bud-break. It is incorrect, however, to assume that this rationale applies to all orchards. A 6-year study in several north-eastern US states demonstrated that scab can be managed effectively in an orchard with a low predicted potential ascospore dose with two-to-five fewer early-season fungicide applications than were recommended [1]. Another study showed that shredding the leaf litter or treating the leaf litter with urea in autumn or early spring before bud-break can, conservatively, be expected to reduce the ascospore dose 50-70% [2]. Is this reduction sufficient to make sanitation economically feasible, and when would sanitation be justified? The objectives of the research program were to (i) establish a 'scab-risk' action threshold and (ii) establish a 'sanitation' action threshold based on an autumn prediction of scab-risk.

Results and conclusions
In 1996, two new strategies for scheduling fungicides to control scab were added to the New England Apple Pest Management Guide: (i) a scab-risk action threshold that identifies when the first fungicide is needed to control scab; and (ii) a sanitation action threshold that identifies when sanitation can be employed to reduce the ascospore dose to a level that will allow fewer early-season fungicide applications. The action thresholds improve fungicide efficiency by (i) eliminating unnecessary early-season fungicide applications in a 'low-risk' orchard; (ii) employing sanitation practices to economic advantage in a 'moderate-risk' orchard; and (iii) alerting the grower to prepare for a full-season fungicide schedule in a 'high-risk' orchard.

The scab-risk action threshold states that if the number of scabbed leaves on 600 extension shoots assessed in autumn is <50 (a low-risk orchard), the first fungicide can be delayed until the pink stage of fruit bud development or until after three scab infection periods, but before the fourth infection period, whichever comes first. The sanitation action threshold states that if the number of scabbed leaves on 600 extension shoots assessed in autumn is 50-100 (a moderate-risk orchard), select a sanitation treatment (leaf-shredding or urea application), perform the treatment, and delay the first fungicide spray according to criteria recommended for the scab-risk action threshold strategy.

Not all orchards can benefit from the action thresholds. The autumn scab assessment may result in more than 100 scabbed leaves on the 600 shoots examined. In this instance, the grower is advised that the risk of scab is high and the recommended protective fungicide schedule should be followed.

The thresholds and related management strategies should be applicable to other regions, but similar studies must be conducted in each region to determine their feasibility and to adjust the thresholds, if necessary, due to differences in cultivar susceptibility, planting densities, tree height and shape, overwintering conditions, and weather patterns.

1. MacHardy WE, Gadoury DM, Rosenberger DA, 1993. Plant Dis. 77, 372-375.
2. MacHardy WE, 1994. Norw. J. Agric. Sci. Suppl. 17, 75-82.