Development of Sustainable Practices for Integrated Management of Apple Diseases
Our main objective is to develop a biologically based program to control the major fungal diseases of apple fruit and foliage in the northern U.S. 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.
Inoculum (ascospores) of the apple scab pathogen is released in the spring from previously infected apple leaves that have overwintered on the orchard floor. Kura clover is proposed to reduce ascospore dispersal by both providing a barrier to prevent the escape of ascospores from the clover canopy as well as promoting decomposition of the apple leaf litter.
For Objective 1, apple plots at two locations were seeded with kura clover in the spring of 1997. In the spring of 1998 and 1999, ascospore release was measured using spore samplers positioned above the kura clover plots and control plots (existing ground cover). Apple scab disease was also monitored on the leaves of the orchard trees and leaves of small potted trees placed in the orchard as bioassay plants. Earthworm activity was assessed by counting castings in the kura and control plots.
For Objective 2, three environmentally benign experimental sprays, benzothiadiazole (BTH, an inducer of disease resistance), a methionine (amino acid)-riboflavin (vitamin) mixture (M-R), and potassium bicarbonate (KHCO3), were applied to apple tree branches. Apple scab, cedar-apple rust and the flyspeck/sooty blotch disease complex on apple leaves and fruit were assessed throughout the season.
In 1998, kura clover significantly reduced aerial ascospore concentration compared to the existing ground cover control at one location but not at the other. 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, nor did it significantly reduce apple scab symptoms on the leaves of the mature or potted bioassay trees at either location. There were significantly more earthworm castings and holes at one location but not at the other.
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. M-R and KHCO3 significantly reduced sooty blotch and flyspeck infection on fruit in both years. 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 the site 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.