BURLINGTON, VT--A major limitation to organic apple production is the available fungicide options for apple scab, a disease of apples grown in humid regions. Apple scab can be devastating to production, decreasing both fruit quality and yield. Fruit infection causes deformities, cracks, and fruit drop, while foliar infection leads to premature defoliation and reduced tree health. Under organic certification standards sulfur or lime sulfur is used for scab management, however, these fungicides are caustic and can be injurious to trees and ecosystems.
A study published in HortScience reported on three alternatives to sulfur tested on 'Empire' apple trees over a 2-year period. Researchers compared potassium bicarbonate (PB), neem oil (NO), and Bacillus subtilis (Bs) to standard sulfur/lime-sulfur (SLS) application and a non-treated control to evaluate potential non-target impacts on pests and beneficial arthropod populations.The five treatments were applied to five single-tree replications arranged in a random design at the University of Vermont Horticultural Research Center in South Burlington. The fungicides were applied with a handgun on a weekly schedule from April to June, and then applied every 2 weeks through July.
The study found that the SLS treatment had the most effective results for scab control for both years. In 2007, scab lesions were evident on 1.1% of the leaves in August and 0.4% of the fruit at harvest. The alternative treatments were found to have less scab in August than the non-treated control trees, but significantly more scab than the SLS treatment. On the PB- and NO-treated trees, the percentages of fruit with scab at harvest were 11.2% and 11.6%, respectively. Both the SLS and NO treatments were shown to have disadvantages including phytotoxic burning on the fruit and/or more russeting on the fruit at harvest, resulting in reduced fruit quality. In both years one or more of the alternative treatments, especially the Bs, resulted in higher insect damage than the non-treated control trees.
The researchers concluded that potassium bicarbonate, Bacillus subtilis, and neem oil do not offer advantages over the standard sulfur/lime/sulfur application in apple production, and sometimes have distinct disadvantages in terms of non-target impacts.
The complete study and abstract are available on the ASHS HortScience electronic journal web site: http://hortsci.ashspublications.org/content/46/9/1254.abstract
Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at ashs.org
Alternative Organic Fungicides for Apple Scab Management and Their Non-target Effects
Morgan L. Cromwell, Lorraine P. Berkett, Heather M. Darby, and Takamaru Ashikaga
HortScience 46:1254–1259. [Abstract][Full Text][PDF]