Outcome of Organic Apple Thinning Strategies

Executive Summary of Findings:

Objective 1 Survey of Organic Apple Growers for Current and Best Practices

! A 25% response rate was achieved of a mailed survey of organic apple growers in 15 US states, Canada, and New Zealand.

! The survey represented approximately 7,000 acres of orchards and growers with conventional, transitional, and certified organic acreage.

! 40-45% of respondents indicated their organic acreage was steady to increasing while only 10% indicated a decrease in organic production

! The majority of respondents were using traditional apple cultivars on M.9 or M.26 rootstock and with tree densities in the 200-500 tree range.

! Motivation for organic apple production included economic, environmental and ethical reasons; growers were deemed to be “capitalists with conscience”

! A vast majority of respondents indicated fruit thinning as very important to their production systems.

! The most common fruit thinning strategy used by organic growers was post-bloom hand thinning.

! Approximately 17% used lime-sulfur sprays during bloom to regulate cropping but rated the success of the treatments as being less than acceptable.

! The survey indicated alternative thinning strategies are desperately needed and appropriate technologies limit their production systems.

Objective 2. Development and Testing of Thinning Techniques for Organic Apples

! Field tests were conducted in 2001 in Colorado and Arkansas to test current best practices indicated in the survey (Obj. 1).

! Field tests were conducted in 2002 in Colorado to retest best treatments of 2001 field studies and additional treatments reported in other regions.

! In 2001, 2% lime-sulfur plus 2% Crocker fish oil applied at full bloom resulted in the greatest reductions in fruit set, the most blank or resting spurs, and the greatest percentage of single fruits per spur, in most cases. " There was variation in attenuation of treatment results among the two cultivars tested " Concentrations of lime-sulfur above 2% did not appear to have significantly different affects

! In 2002, due to management and springs frosts, three of four trials had to be abandoned after treatments were established. However, one orchard trial was successful. Bloom was very heavy but fruit set relatively light due to heavy June-drop in 2002 on ‘Smoothee Golden Delicious’. " Treatments did not significantly reduce fruit set (fruits/100 flower clusters) but did result in reductions in fruits per limb, and reduced fruits set per flower cluster, especially the percentage of clusters with four fruits. " The percentage of flower clusters with only one fruit was increased by all treatments. " The most effect thinning treatments were 4% lime-sulfur plus fish oil applied at full bloom and petal fall, 4% vegetable oil emulsion applied at full bloom plus 4% lime sulfur at petal fall, 2% NaCl applied at first bloom and full bloom, and 2% NaCl applied at 30% bloom plus 2% lime-sulfur at full bloom plus 4% vegetable oil emulsion applied at 80% bloom. " High concentrations (10%) of lime-sulfur alone did not affect fruit set. " When all treatments are considered, it appears that multiple treatments or multiple applications of treatments throughout the bloom period were more affective than single treatments. 

Outcome PDF: