L.P.Berkett, W.E.MacHardy, D.K.Sutton, G.Neff, T.Bradshaw.

University of Vermont, Department of Plant and Soil Science, Burlington, VT

University of New Hampshire, Department of Plant Biology, Durham, NH

This demonstration project, conducted in 1996, was designed to explore and learn how a relatively new disease management strategy, i.e., a delayed-spray strategy for initiating a scab fungicide program, would fit into all the other decisions and factors involved in apple production over a growing season. This strategy is based on an action threshold, determined mainly by an autumn assessment of foliar scab, from which is predicted the potential ascospore dose (PAD). Federal Crop Insurance was made available as a safety net to attract growers who otherwise would be hesitant to consider testing a new IPM strategy for possible incorporation into their pest management program. Valuable insights were gained by this collaborative learning experience between apple growers, researchers, and extension personnel. We sincerely thank the seven orchardists who participated in this project.

Venturia inaequalis (Cke.) Wint., the fungal pathogen causing apple scab, overwinters on infected apple leaves. Ascospores produced in pseudothecia on these leaves in early spring comprise the primary inoculum in the northeastern US. Research has established a procedure to predict the potential ascospore dose (PAD), which is defined as the density of ascospores produced per m² orchard floor. A PAD prediction is based mainly on the amount of foliar scab assessed in autumn. In commercial orchards PAD has ranged from less than 10 to greater than 1 million.

In an orchard well-managed for scab, i.e., £ 600 PAD, an action threshold has been established that states that if £ 50 scabbed leaves on 600 terminal shoots (10 shoots on 60 trees) are detected in an autumn assessment, the first fungicide application can be delayed until the pink fruit bud stage or until after three scab infection periods, but before the fourth infection period, whichever occurs first. Basing the action threshold on the incidence of scabbed leaves (rather than a calculation of PAD) has the advantage to orchardists of being a relatively easy procedure. The delayed-spray strategy is important because it offers the potential of omitting several early-season sprays scheduled according to other strategies.

• Less than 1% scabbed fruit at harvest in 1996. Growers generally consider this acceptable control of fruit scab in their current programs.
• The 1996 autumn foliar scab assessment would be at a level that would allow for a delayed-spray strategy in 1997.

Seven commercial orchards across VT and NH participated. Five orchards were planted mainly with susceptible cultivars McIntosh and Cortland and had a predicted 1996 PAD below the action threshold. Two orchards (Orchards A and B) were planted mainly with Empire; Orchard A had a predicted 1996 PAD above the threshold, but was included in the study because Empire is a more resistant cultivar.

The PAD predicted for the 1996 and 1997 growing seasons was calculated according to the procedure reported by Gadoury and MacHardy (1).

Electronic weather monitoring equipment was set up in each orchard before the 1996 growing season to monitor rainfall, leaf wetness, and temperature needed to predict scab infection periods.

Project personnel worked with the orchards on sprayer calibration and tree-row-volume calculations before the 1996 growing season to ensure that the proper amount of fungicide would be applied.

• June, at the end of the primary scab season: all leaves on 25 fruit clusters and 25 terminal shoots on each of 20 trees/orchard block were assessed for scab. Based on that assessment, orchardists and project personnel discussed and developed a disease management plan for the remainder of the growing season, taking into account the potential for secondary scab development as well as two important summer diseases, sooty blotch and flyspeck.
• July: all leaves on 400 terminal shoots (20 shoots/tree, 20 trees/block) and 200 fruit (10 fruit/tree, 20 trees/block) were assessed for scab.
• September, just prior to harvest: 50 fruit/tree on 20-30 trees/block were assessed for scab.
• October/November, before leaf fall: an assessment of foliar scab was conducted to predict the PAD for 1997.

see Table 1

• Weather: The 1996 growing season was an excellent "test" year for apple scab management; weather conditions were very favorable for disease development. On average, there were 24 infection periods (8 primary infection periods and 16 secondary infection periods) in contrast to a total of 13 infection periods in 1995. In addition, wet weather in the spring prevented some of the orchardists from applying their first fungicide before the fourth infection period as recommended (Table 1, Orchards A, B, C, D).
• Sterol-inhibitor factor: Use of a sterol-inhibiting (SI) fungicide program in Orchards B and C during the 1995 growing season is thought to have influenced disease development. It is suspected that these fungicides inhibited foliar scab development but did not kill the fungus. There were few lesions at the end of the 1995 season, but it is suspected that these lesions did not fully represent the latent infections that were present. In effect, there was a greater amount of overwintering inoculum than was indicated by the predicted 1996 PADs of 108 and 111, respectively, in those two orchards.
• Cultivar: The impact of cultivar susceptibility was evident in Orchards A and B where the delayed-spray strategy was carried out primarily on Empire.

• "Less than 1% scabbed fruit at harvest in 1996" was the first goal for orchards using the delayed-spray strategy. Even with weather conditions that were very favorable for disease development, only Orchards B and C (orchards that used a SI program the previous year), exceeded that goal in September (see Table 1). In Orchard B, the delayed-spray strategy was carried out predominantly on Empire, although there were a few rows of McIntosh in the orchard block. The difference in cultivar susceptibility shows up in the % fruit scab. The Empire were well below the goal; the McIntosh were above.

• The impact of cultivar susceptibility was also evident in Orchard A. Since Empire has more resistance to scab than McIntosh, it was thought a higher PAD than that recommended for McIntosh (i.e., £ 600 PAD) would still allow a delayed-spray strategy. The predicted 1996 PAD in Orchard A was over 4,000 and even with four unprotected primary infection periods, fruit scab at harvest was under 1%. For foliar scab at the end of the growing season, the predicted 1997 PAD was comparable to the previous year.

Until we know more about how best to factor in cultivar resistance into a different PAD threshold for Empire, we will hold off recommending a delayed-spray strategy at this higher PAD level.

• In all other orchards the delayed-spray strategy was successful in effectively managing both foliar and fruit scab. Each of these orchards had a resulting PAD that placed them as candidates for the delayed-spray strategy in 1997.

We conducted an informal poll of the orchards surrounding four of the project participants to find out how many fungicide sprays they had applied during the growing season. Table 2 summarizes this brief survey:

• In orchards with a PAD below the action threshold, the delayed-spray strategy can be successfully implemented, even during a season with conditions very favorable for disease development.
• The project provided valuable insight into a potential impact of SI fungicides on overwintering inoculum and underscores the need for continued research in this area.
• This project is significant because it was the first time that the Federal Crop Insurance Program was available as a safety-net for growers otherwise hesitant to incorporate a new IPM strategy into their pest management program.

1. Gadoury, D.M. and MacHardy, W.E. 1986. Forecasting ascospore dose of Venturia inaequalis in commercial apple orchards. Phytopathology 76: 112-118.

This project would not have been possible without the support of a number of people whom we would like to acknowledge, including the Honorable Senator Patrick Leahy of Vermont, Mr. James Cubie, Mr. Ray W. Allen, Dr. Kate DeRemer, and Dr. Vondie W. O’Connor, Jr. We especially thank the following orchardists who participated in the project:

Vermont

Ray W. Allen, Allenholm Farm, South Hero

Geoffrey Demong, Belmont Orchard, Shoreham

Robin Price, Golden Apple Orchard, Charlotte

New Hampshire

Paul Franklin, Riverview Farm, West Lebanon

Ezekiel Goodband, Alyson’s Apple Orchard, Walpole

Chuck Souther, Apple Hill Farm, Concord

David Stowe, Stoneybrook Orchard, Gilford