"Whole Farm" Apple Scab Integrated Pest Management (IPM) Project

A joint project between the University of Vermont and the University of New Hampshire scales up the most recent research in apple scab IPM for demonstration and implementation on whole farms in New England.

In autumn 1995, seven apple orchards in New Hampshire and Vermont were selected to participate in this pilot project. Orchardists participating in the project agreed to follow an apple scab IPM program developed in New Hampshire that had demonstrated over several years that the first fungicide application in an orchard with a predicted low level of ascosporic inoculum of Venturia inaequalis, the pathogen causing scab, could be delayed until the pink fruit bud stage or until after three scab infection periods had occurred. The project utilizes a determination of potential ascospore dose (PAD) in autumn to identify orchard blocks expected to have a low amount of inoculum. To further insure that there would be a low level of inoculum in each orchard, each orchardist agreed to shred the leaf letter and apply urea to the leaf letter in autumn and again in spring before bud break. These two sanitation practices had been show in a New Hampshire study to reduce the ascosporic inoculum of the scab pathogen by 60 to 90%.

It was a challenging year for apple scab management in the Northeast; the cool, moist weather was ideal for the growth and spread of the apple scab fungus. This unusual season helped to make our project a truly collaborative learning experience between researchers and growers. Project participants were protected against potential crop loss due to apple scab damage through crop insurance provided by the Federal Crop Insurance Protection Program. This relieved the risk to the growers in adopting orchard-wide trials of new and innovative IPM apple scab management techniques before scaling up these methods for industry-wide use.

Due to the extremely late leaf fall in 1995 and a very wet early spring in 1996, the recommended sanitation practices were fully implemented only at one site, partially implemented at three sites, and not implemented at all at three sites.

Six of the seven orchards had at least one "low" PAD (<600) block in which the initiation of fungicide applications was delayed. The remaining orchard had a block in which the PAD exceeded the criterion of low PAD, but was planted with Empire, a less susceptible cultivar. Of the six "low" PAD sites, two reduced the number of fungicide application by 20-30%, compared to 1995 spray schedules, and kept fruit scab below 1%. Two sites had the same number of applications both years and maintained fruit scab below 1%. One site was dropped from the project because of severe hail damage. The remaining "low" PAD site applied one more fungicide spray in 1996 than in 1995. Primary scab and subsequent scab buildup was severe at this site, and the PAD predicted for 1997 is well above the 600 threshold. The one orchard that had predicted PAD above the <600 required for the delayed-fungicide strategy maintained fruit scab below the 1% acceptable level of scabbed fruit with a spray schedule that applied two more fungicide sprays than in 1995. The PAD predicted for 1997, however, is still above threshold.

Two participating orchards in Vermont provided us with an opportunity to discover a new potential area for future research. Both of these orchards had previously used a sterol-inhibitor (SI) program to manage apple scab. An SI does not kill the fungus; it inhibits it. We do not know what happens to the inhibited fungus the following season. When we assessed foliar scab in these two sites in the fall of 1995 they had foliar scab levels well below the threshold for recommending use of a delayed fungicide program. Due to late leaf fall in the autumn and excessively wet orchard conditions the following spring, mowing of overwintering leaves and application of urea was not possible at either site. The first application of fungicide was delayed until after four infection periods at one orchard and until after five infection periods at the second orchard.. In mid June, when we visited the orchards for the initial foliar scab infection assessment we found much higher levels of infection than we had anticipated. With previous apple scab management with SI fungicides at both of these sites, it is hypothesized that when we assessed foliar scab in the fall of 1995 the fungus was inhibited by the SI. If that inhibition was removed in the autumn after the leaves fell and died, it is possible that renewed growth of the fungus from the previously inhibited infections could have accounted for a much greater than expected scab inoculum and infection rates the next spring. Control of apple scab to protect the fruit was achieved at both orchards because the infections were detected before the fungus was able to spread too far. This experience points to an important area of research: modified fall foliar assessment for orchards moving from SI management to delayed fungicide strategy.