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Funded Project

Funding Program: Regional IPM Competitive Grants - Northeastern

Project Title: A Sanitation Procedure to Manage Scab, Leafminers, and Voles in Apple Orchards

Project Directors (PDs): William MacHardy [1] Alan Eaton [2] William Lord [3]

Lead State: NH Lead Organization: University of New Hampshire

Research Funding: $89,231

Start Date: Sep-01-2000 End Date: Aug-31-2003

Pests Involved: scab, leafminers, voles, mammals

Site/Commodity: apples

Area of Emphasis: cultural controls

Summary: Removing leaf litter, other debris, and drops from the orchard floor has the potential to drastically reduce or eliminate the pesticides applied to manage four important apple pests: Venturia inaequalis (the causal agent of apple scab) and the apple blotch and spotted tentiform leafminers that overwinter in apple leaf litter and the meadow vole that utilizes leaf litter and other debris for nesting material and establishing trails that help protect it from predators and eats drops as an alternative food source. The focus of all scab management programs, including IPM programs, is to prevent the leaves and fruit from becoming infected by keeping them protected with fungicide. This is a defensive tactic, and the main goal of nearly all scab IPM research, in orchards planted with susceptible cultivars, has been to improve the efficiency in scheduling fungicides rather than to replace fungicide with nonchemical practices. The IPM programs have increased the efficiency of fungicide usage, but they have also reached a plateau with respect to reducing the seasonal fungicide dose to control scab. The Principal PI has presented talks to numerous grower organizations throughout the northeastern US and eastern Canada that included a discussion of our research designed to reduce the seasonal fungicide dose to control scab through sanitation (shredding or treating the leaf litter with urea). This is an offensive tactic aimed at reducing the source of the inoculum (ascospores) that cause the initial infections in spring. After every talk, two questions were always asked. The first question was. . . "why not just remove the leaf litter?" This is the obvious approach to manage scab (and also leafminers), but it has always been assumed that removing the leaf litter would be too daunting a task. However, we have found that equipment used on golf courses, parks, highways, and with other cropping systems can, when used in the proper combination, remove >90% of the leaves, debris, and drops from the orchard floor in a process that is practical for large apple acreage. The "blow-sweep-shred-vac" treatment we will test requires two trips through an orchard. In the first trip, a blower attached to the tractor's PTO will blow leaf litter, debris, and drops from under the tree canopies in one row to the row-alley downwind, and this will be repeated for all rows. In the second trip, duel leaf sweepers attached to the front of the tractor will sweep each row alley to form a windrow of leaves, other debris, and drops that will be shredded and vacuumed into an enclosed bin by a 'Versa-Vac' (Goosen Industries) unit attached to the tractor's PTO. The second question growers asked expressed concern that ascospores blown from a source outside an orchard that had received a sanitation treatment would not allow infection periods to go unprotected, regardless of how effectively sanitation reduced the ascosporic inoculum. Addressing that concern will be critical to the adoption of the proposed sanitation procedure, and, thus, is an important component of the proposal. We now need to demonstrate the economic, environmental, and human health benefits that can be achieved through reduced pesticide usage in a 'sanitized' orchard. Thus, our objectives are to demonstrate that by removing >90% of the leaf litter, other debris, and drops, it will be possible to (1) eliminate the four-to-six fungicide sprays applied to control scab during the primary scab season and the three-to-five fungicide sprays applied to control scab during the remainder of the growing season, (2) eliminate the one or two costly insecticides applied to control leafminers, and (3) eliminate broadcasted rodenticide bait to control orchard voles that is also attractive to domestic pets, birds, and other non-target wildlife. There are several reasons in addition to environmental and human health concerns that justify increased efforts to reduce the seasonal pesticide dose through sanitation: growers can more easily (a) follow pesticide resistance-management guidelines, (b) omit or replace fungicides harmful to beneficials with fungicides compatible with beneficials in arthropod management programs that are becoming increasingly more "biologically based," and (c) improve pest and crop management programs to comply with Integrated Fruit Production (IFP) and Integrated Crop Production (ICP) guidelines or the guidelines of groups such as Core Values Northeast that are based on IFP or ICP guidelines. Problem, background, and justification Core Values Northeast is a project of Mothers and Others, and in 1999 was comprised of 43 leading apple growers from throughout the Northeast committed to producing high quality apples using biointensive IPM production methods based, in part, on Guidelines for Integrated Production of Pome Fruits in Europe but adapted to reflect the growing conditions and best farm practices of the Northeast. The Guidelines encourage safer, more ecologically sound ways to manage pests. This proposal specifically addresses one of the stated objectives of Core Values Northeast: use a diversity of production methods to manage pests through safer, more ecologically sound methods. One method encouraged by Core Values Northeast, disrupting a pest's life cycle, is used effectively to control some pests, e.g., pheromone to control codling moth through mating disruption, but this method is not used effectively to manage several other important pests even though there is considerable potential to reduce or eliminate the pesticides applied to manage them through sanitation aimed at disrupting or interrupting their life cycle. Four of these pests are Venturia inaequalis (the causal agent of apple scab), the apple blotch leafminer (Phyllonorycter cratuegella), and the spotted tentiform leafminer (P. blancardella) that overwinter in apple leaf litter and the meadow vole (Microtus pennsylvanicus) that utilizes the leaf litter and other debris on the orchard floor for nesting material and for establishing trails that help protect it from predators. Apple scab. Apple scab is the most important disease of apple worldwide, and the repeated application of fungicides to control scab is a main reason why apple ranks near the top among crops with respect to pesticide use per hectare. In the northeastern United States and in most apple growing regions, ascospores produced in overwintered scabbed leaves on the orchard floor in early spring cause the initial scab lesions, and conidia produced on lesions cause scab to build up on foliage and fruit during the remainder of the growing season. The main focus of all scab management programs, including IPM programs, is to protect the tree against infections by ascospores. These programs are centered on a warning system that predicts scab infection periods. This is a defensive management approach designed to provide timely applications of fungicide to protect the leaves and fruit from becoming infected. Its purpose is not to replace fungicides with other control practices; rather, it is to improve the scheduling of fungicides. This defensive approach has increased fungicide effciency in controlling scab, but it has also reached a plateau with respect to reducing the seasonal dose of fungicides to control scab. Offensive approaches to control scab include any practice that reduces the ascosporic inoculum at the source or makes use of apple's natural resistance to scab: sanitation, biological control, scab resistant cultivars, and plantings of cultivar mixtures with partial resistance. In orchards planted with susceptible cultivars, an offensive approach is seen as the key to successful minimal fungicide or non fungicide scab management, because it reduces or eliminates the amount of inoculum that can infect the leaves and fruit. The sanitation procedure proposed here may not prevent ascospore production completely, but it does have the potential to reduce the ascosporic inoculum to a density that will require less, or perhaps no, fungicide the next growing season applied specifically to control scab. However, before growers are likely to embrace a broadened approach to scab management that integrates offensive and defensive tactics in a way that reduces the amount of fungicide used to control scab, a study is needed that will determine the minimum fungicide dose needed to control scab in an orchard that had received a sanitation treatment. Why the proposed sanitation procedure has never been tested. In the 1930s and early 1940s, there were numerous reports of chemicals applied to leaves before or after leaf fall in autumn or to the leaf litter in early spring prior to budbreak that reduced the ascosporic inoculum >90%, and in many instances there was a comparable reduction in the number of scab lesions. Although such percentage reductions in inoculum are impressive, the treatments did not allow all fungicides to be eliminated, which was the ultimate objective of most of these studies. Consequently, sanitation was abandoned in the 1940's in favor of the more efficacious, less phytotoxic organic fungicides that were becoming available. Unfortunately, the sanitation studies were conducted in high-inoculum research orchards rather than in low-inoculum commercial orchards that had been reasonably well controlled for scab. Reducing the ascosporic inoculum >90% is impressive, but in an orchard with a predicted potential ascospore dose (PAD) estimated at 1,000,000 ascospores per meter of orchard floor (a low-inoculum density for a research orchard), the PAD would be >10,000 after sanitation that reduced the inoculum >90%, and studies conducted in several northeastern states have shown that an orchard with >10,000 PAD would require a full season fungicide program. However, reducing inoculum >90% in a commercial orchard with a predicted PAD <500 (a common prediction for commercial orchards) would lower the PAD to <50, and several studies have shown that an orchard with a predicted PAD <500 may not require any fungicide intervention during the primary scab season. With this understanding of expected inoculum levels, it is not unreasonable to hypothesize that removing >90% of the leaf litter in an orchard with a predicted moderate (500 3000 PAD) or low (<500 PAD) "scab risk," and perhaps even orchards with a greater predicted "scab risk," will eliminate the need for fungicide intervention. However, we are not aware of any studies that investigated the potential of leaf removal to reduce the fungicide dose to control scab, perhaps because it appeared that removing >90% of the leaf litter from several acres or more of apples was too daunting a task, even though the justification for doing it is obvious. What we have discovered is that equipment now used on golf courses, parks, highways, or with other cropping systems to remove leaves and debris can, when used in the proper combination, remove >90% of the leaves, other debris from the orchard floor in a process that is practical for large apple acreage. We now need to demonstrate the economic, environmental, and human health benefits that can be achieved with leaf removal through reduced pesticide usage. Are there situations in northeastern US apple orchards in which the proposed sanitationlreduced fungicide procedure may fail? An unacceptable incidence of scabbed fruit (>1.0%) in unprotected subplots in a "sanitized" orchard could be due to primary conidial inoculum within the subplots, insufficient removal of the leaf litter, or ascospores dispersed to the subplots from an external source. Conidia of V. inaequalis have been found on budscale lesions, and it has been reported that conidia can overwinter internally in apple buds, but this was found only in four orchards that had been poorly managed for scab. Generally, the fungus does not survive winter (freezing conditions) when it is exposed on the surface of an apple tree, and even when mycelium does survive the winter, it apparently does not produce conidia. When the viability of conidia obtained from lesions on fruit and leaves in western NY was tested each month from October through May to determine when conidia on scab lesions die during winter, no viable conidia were detected in lesions examined during March or in subsequent examinations. In the study proposed here, an incidence of scabbed fruit <1.0% at harvest in "sanitized" plots that had not been protected with fungicide will be evidence that there was no significant supply of primary conidial inoculum within the block. The Principal PI has presented talks to numerous grower groups throughout the northeastern US and in several foreign countries that included a discussion of studies on orchard sanitation. After every talk there was the inevitable question expressing concern that ascosporic inoculum from a source outside an orchard would not allow infection periods to go unprotected regardless of how effectively sanitation reduced the ascosporic inoculum within an orchard. Addressing that concern will be critical to the adoption of the proposed sanitation/reduced fungicide procedure, and, thus, is an important component of the proposal. Apple blotch and tentiform leafminers. Apple blotch and spotted tentiform leafminer pupae overwinter in the apple leaf litter, and the adult leafminers that emerge early in the season mate and lay eggs on the underside of leaves. The larvae that hatch cause the initial "sapfeeding" mines. Thus, removing >90% of the leaf litter should reduce the initial leafminer population >90%, and it is expected that this will eliminate the need to apply the one or two costly insecticide applications growers now rely on to protect trees against these two insect pests. Many growers schedule insecticide applications to control leafminers based on action thresholds in the New England Apple Pest Management Guide. It is hypothesized that the number of leafminer adults trapped on red sticky traps during early season and the number of first generation and second generation sapfeeding mines detected during scouting later in the season will not reach the respective action thresholds for insecticide intervention in orchard blocks that had received the sanitation treatment. If the number of mines in the sanitation block is above the action threshold, scouting data from arent blocks will be analyzed to evaluate the possibility that the flight of leafminer adults from those blocks was responsible for the action threshold being exceeded in the "sanitized" block. Meadow voles. Surface vegetation creates a sheltering habitat and nesting material for meadow voles. Maintaining in-row strips free of leaf litter and debris or free of surface vegetation (in herbicide strips) and row alleys free of leaf litter and debris will limit the supply of nesting material and, more importantly, greatly reduce or eliminate the network of surface trails through the groundcover, thus greatly increasing the exposure of voles to natural predators such as hawks, owls, and foxes. The sanitation procedure will also remove dropped apples, thus eliminating an important alternative food source for voles. It is hypothesized that the proposed sanitation program will reduce the meadow vole population in young orchards to a density that will not require broadcast rodenticide baits attractive to domestic pets, birds, and other non target wildlife. Weed management. The proposed sanitation program can be considered part of a ground cover management system, and weed management should improve as a positive side effect of the efforts to remove the leaf litter and debris. However, there will be no attempt to evaluate improvements in weed control even though it is expected that increased attention to mowing and the sanitation procedure itself will likely result in improved weed management. Reasons in addition to environmental and human health concerns that justify increased efforts to reduce the seasonal pesticide dose in an orchard through sanitation: Managing pesticide resistance in a pest population has become an important and integral component of IPM programs, but selecting and scheduling appropriate pesticides in accordance with pesticide resistance strategies has become more complicated and difficult as insect and disease IPM programs have become more complex. The buildup of fungicide-resistant strains of the apple scab pathogen to several fungicides and the buildup of insecticide-resistant strains of apple blotch and tentiforrn leafminer populations have occurred. Concern for resistance buildup to new fungicides is evidenced by two current projects: a Hatch project in New York (Fungal resistance of Venturia inaequalis to antifungal strobilurins, W. Koeller) and a State funded project in Michigan (Diseases of tree fruit crops and their control, Jones et al.) that is evaluating methods for detecting and counteracting V. inaequalis resistant strains. The options available for selecting fungicides and reducing the number of fungicide applications to control scab in an orchard in which sanitation has drastically reduced the ascosporic inoculum of V. inaequalis will enable a grower to more easily follow a fungicide resistance management program. The proposed sanitation procedure conforms fully with the following guidelines for managing pesticide resistance presented to growers in Ontario (Publication 310): rotate between pesticide groups, do not exceed the recommended number of applications per season for plant diseases, reduce initial inoculum levels through good cultural practices, and apply pesticides only when necessary, i.e., when threshold levels are reached. Eight to ten, or even more, applications of fungicide throughout the season to control scab are not unusual, even in programs labeled "IPM," and several popular fungicides are harmful to beneficials introduced to manage important insect and mite pests. The numerous applications of fungicides and the use of fungicides harmful to beneficials are counteractive to arthropod management tactics that are becoming increasingly more "biologically based," and it is likely that scab management prograrns will continue to counteract arthropod tactics that utilize beneficials unless nonfungicidal tactics, e.g., the sanitation procedure proposed here, are employed that enable the grower to control scab with a fungicide program compatible with arthropod IPM tactics. The application of tank-mixed fungicides has become increasingly more common, and this has increased the seasonal fungicide dose and pest management costs. It should not be necessary to apply a tank mix of fungicides to control scab in a "sanitized" orchard. Finally, fungicide is accounting for a much greater proportion of the yearly pesticide dose in many apple IPM programs in recent years. This is due to the success of entomologists in developing economic thresholds for making spray decisions and replacing insecticides and miticides with beneficials, and this is placing a greater burden on plant pathologists to improve disease management programs for compliance with Integrated Fruit Production (IFP) and Integrated Crop Production (ICP) guidelines or the guidelines of groups such as Core Values Northeast that are based on IFP or ICP guidelines. Objectives: Through removal of >90% of the leaf litter, demonstrate that it will be possible to: 1. eliminate fungicides applied to control apple scab during the primary scab season or for the entire growing season. Apple growers typically apply four-to-six fungicide sprays during the primary scab season and another three-to-five fungicide sprays during the remainder of the growing season. It is hypothesized that removing >90% of the apple leaf litter will eliminate the need to apply fungicide specifically to control scab during the primary season or for the entire season in orchards that have been identified as "low-scab-risk" and "moderate-scab-risk." 2. eliminate insecticides applied to control leaf miners that overwinter in the leaf litter. Apple growers typically apply one or two costly insecticide sprays each year to control apple blotch and spotted tentiform leafminers that overwinter in apple leaf litter. It is hypothesized that by removing >90% of the apple leaf litter, the population of leafminers will not exceed the action thresholds for insecticide intervention. 3. eliminate rodenticide baits to control orchard voles (meadow vole and possibly the pine vole).

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