Development of Sustainable Practices for Integrated Management of Apple Diseases

Project Title: Development of Sustainable Practices for Integrated Management of Apple Diseases

Patricia McManus, Assistant Professor, Dept. Plant Pathology, 1630 Linden Drive, UW-Madison, 608-265-2047, psm@plantpath.wisc.edu

John Andrews, Professor, Dept. Plant Pathology, 1630 Linden Drive, UW-Madison, 608-262-1410, jha@plantpath.wisc.edu

Kenneth Albrecht, Professor, Dept. Agronomy, 1575 Linden Drive, UW-Madison, 608-262-2314, kaalbrec@facstaff.wisc.edu

Matt Stasiak, Associate Researcher, Peninsular Agric. Research Station, UW-Madison, Sturgeon Bay, WI, 920-743-5406, rweidman@facstaff.wisc.edu

Jessica O’Mara, Research Specialist, Dept. Plant Pathology, UW-Madison, 608-262-2093, jko@plantpath.wisc.edu

Nick Voichick and John Edmunds, former Research Specialists, Dept. Plant Pathology, UW-Madison

Bob Johnson, Turkey Ridge Organic Orchard, Route 1, Box 131A, Gays Mills, WI 54631, 608-735-4660

USDA Sustainable Agriculture Research and Education

July 15, 1997-present

June 1, 1999-May 31, 2000

Our main objective is to develop a biologically based program to control the major fungal diseases of apple fruit and foliage in the northern USA. If successful, this would break the continuing reliance on toxic fungicides used by conventional growers and provide a reliable, non-pesticide, low input, and low cost framework for organic growers.

The specific objectives of our current research are to:

1. Assess the ability of an orchard floor cover crop, Kura clover, to break the life cycle of the apple scab pathogen.

2. Test the efficacy of environmentally benign compounds used as sprays during the growing season to control scab and other important foliar and fruit diseases such as cedar apple rust and the sooty blotch/flyspeck complex.

Fungicides account for about 8% of all pesticides used in the U.S.; however, on apple, fungicides account for about 51% of pesticides used. Captan and benomyl, two of the most commonly used fungicides on apple in Wisconsin, are suspected carcinogens and are in Priority Group 1 for tolerance reassessment by EPA. Mancozeb, maneb, and myclobutanil are other commonly used apple fungicides in Group 1. Apple is receiving extra scrutiny because FQPA requires special consideration and more stringent limits on pesticides applied to food consumed in large quantities by infants and children. Apple is second only to milk in the diets of infants in the U.S.; per body weight infants consume 7 times as much apple and 17 times as much apple juice as adults.

Wisconsin has about 325 apple orchards covering about 6500 acres. The most common varieties (McIntosh, Red Delicious, Cortland) are highly susceptible to scab. Orchardists in Wisconsin routinely apply 8-15 fungicide sprays per year, primarily to combat apple scab, at a cost of about $15-30 per acre. Although there are only a few organic apple orchards in Wisconsin, one of them (Turkey Ridge) is the largest in the Midwest. Organic growers depend, at least in part, on scab-resistant apple varieties for scab control, but need more strategies to manage flyspeck/sooty blotch (FS/SB). In warm, humid years, FS/SB blemishes reduce the price at which fruit can be sold.

In 1998, Kura clover significantly reduced aerial ascospore concentration compared to the existing ground cover control at Sturgeon Bay but not at Madison. It did not reduce apple scab disease at either location. In 1999, Kura clover did not significantly reduce the aerial ascospore concentration compared to the existing ground cover at either location (Fig. 1), nor did it significantly reduce apple scab symptoms on the leaves of the mature or potted bioassay trees at either location (Figs. 2-5). There were significantly more earthworm castings and holes in the Kura plots at Madison but not at Sturgeon Bay (Fig. 6).

In 1998 all of the experimental sprays reduced apple scab infection on leaves and fruit, under mild – moderate disease pressure. In 1999, under moderate – severe disease pressure, M-R and BTH reduced apple scab on both leaves and fruit (Figs. 7 and 8). M-R and KHCO₃ significantly reduced sooty blotch and flyspeck infection on fruit in both years (Figs 9 and 10). Cedar apple rust infection on leaves or fruit was not evaluated in 1999, because results from 1998 showed little impact of the compounds on this disease.

Kura clover takes at least three years to become well developed. At Madison where the clover is better established, earthworm activity is greater and there is a trend toward lower scab incidence on potted trees. A decrease in aerial ascospores and scab is expected when a better stand of the clover is established. Identifying environmentally benign alternatives to synthetic fungicides will especially benefit organic growers but will also relieve traditional orchardists from dependence on more toxic compounds. Combining the clover as a ground cover with the use of the most effective of the experimental sprays should result in effective, non-toxic, and sustainable disease control.

The experimental sprays used in this study are currently more expensive than traditional fungicides ($0.25 to $0.30 per gallon vs. $0.08 per gallon for Captan 50 WP). However, if the chemicals prove to be effective alternatives to fungicides, presumably they could be purchased in bulk at substantial savings. For our research, chemicals were purchased in small quantities from scientific suppliers.

Reducing apple scab or other fungal diseases will result in economic gains through higher quality fruit. Apples are typically graded by quality; a #1 or #2 apple is sold as fresh fruit whereas inferior apples are usually made into cider or vinegar. The grade is dependent somewhat on size and shape but primarily on surface blemishes such as disease lesions. At Turkey Ridge Organic Orchard, value decreases by 60% from a #1 apple to a cider apple ($30/box vs. $12/box). The experimental sprays tested in this study were shown to reduce the number of diseased apples by more than 50% which represents a potential 30% increase in profit for an apple grower.

In the past year, two formulations of potassium bicarbonate have become available and are legal for use on apple. Will these prove effective against flyspeck and sooty blotch as potassium bicarbonate did in our studies?

Despite relatively low numbers of ascopsores trapped on Rotorods, apple trees in our plots got severe scab. Could scab be overwintering on the trees as has been previously described? What is the epidemiological significance of inoculum that overwinters on trees?

Results of this project have been presented/discussed at several grower meetings and schools including: Apple IPM Field Day (1997 in Mequon; 1998 in Sturgeon Bay; 1999 in Hixton); UW-Madison Apple Short Course (January 1998); UW Institute for Pest and Pathogen Management (May 1998). Recently Cris Carusi of the PURR staff interviewed Bob Johnson and Patty McManus and is producing an article for state agriculture newspapers.

This project continues to be funded by USDA-SARE. We are pursing the original objectives, the significance of scab inoculum that overwinters on trees rather than on the orchard floor, and the application of potassium bicarbonate and methionine-riboflavin at three commercial orchards. McManus will be a PI on a proposal joint with several other states aimed at finding strategies to reduce fungicide use in managing flyspeck/sooty blotch.