Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant and Soil Science

First Advisor

Josef H. Gorres


Despite substantial consumer demand and willingness to pay premium prices for organically grown fruit, apple growers in Vermont and other New England states have been slow to adopt certified organic practices. Barriers cited in the past to increased adoption of organic apple production in the region include susceptibility of traditionally grown cultivars to apple scab, lack of effective insect pest management materials, and few available effective options for fruit thinning. Recent changes in apple cultivar plantings in the region, introduction of new insect pest management materials, and advances in crop thinning justified an evaluation of organic apple production systems containing cultivars identified as important to the future of the apple industry. In 2006, two apple orchards were established at the University of Vermont Horticulture Research and Education Center in South Burlington, VT to comprehensively evaluate the five commercially-important apple cultivars of `Ginger Gold', `Honeycrisp', `Liberty', `Macoun', and `Zestar!' over eight growing seasons in two organically-managed orchard production systems, including a newly-planted high-density orchard (Orchard 1) and in an existing, medium-density orchard which was top-grafted to the new cultivars (Orchard 2). Parameters for tree growth and survival, crop yield, disease and arthropod pest incidence on foliage and fruit, and long-term economic return, including a twenty-year projection of net present value (NPV) of each cultivar in the two systems were evaluated in this study.

`Ginger Gold', despite high incidence of some diseases on foliage and fruit, performed the best in both orchard systems overall. The cultivar was among the cultivars with the highest measurements of tree growth. `Ginger Gold', along with `Honeycrisp', had the highest cumulative net crop yield per tree in Orchard 1 and the highest in Orchard 2. Notably, apple scab on `Honeycrisp' foliage and fruit and `Zestar!' fruit in both orchards was at a level that was not significantly different from `Liberty', a scab-resistant cultivar on which no scab was observed. However, `Honeycrisp' had the highest incidence of fruit rots in both orchards, but it was not significantly different than `Zestar!' in Orchard 1. Management of lepidopteran pests of fruit was a major challenge on all cultivars over the years of the study. For most of the tree growth parameters and cumulative net crop yield, `Liberty' was among the lowest group of cultivars in both orchards. Cumulative net crop yield of both `Macoun' and `Zestar!' were also among the lowest in both orchards with the top-grafted `Macoun' and `Zestar!' trees having significant tree death compared to the other cultivars in Orchard 2.

Harvested fruit were graded to commercial standards and cumulative gross and net income calculated from grade distribution, crop yield, and fruit price data. In Orchard 1, `Ginger Gold' and `Liberty' had greater cumulative gross income per hectare from 2006-2013, in excess of US$40,000, compared to `Liberty'. However, after management costs were deducted, all cultivars in Orchard 1 had negative cumulative net income of $-77,892 or less. In Orchard 2, all cultivars had positive cumulative net income for 2006-2013, and `Ginger Gold' had the highest at $109,717/ha. The twenty-year projected NPV was negative for all cultivars in Orchard 1, but in Orchard 2, all cultivars had positive NPV with `Ginger Gold' having the highest among the cultivars.



Number of Pages

223 p.