Development of Organic Production Practices for Pawpaw on Selected Rootstocks
Pawpaw is a unique native tree fruit that is resistant to many diseases and insect pests, making this crop attractive to organic farmers. However, little information concerning organic production of pawpaw is available. Pawpaw cultivars with excellent fruit quality are usually propagated commercially by grafting cultivar buds (scions) onto common seedling rootstock of diverse genetic origin. In regional variety trials, the pawpaw cultivars PA-Golden and Sunflower have produced fruit earlier than other cultivars. Rootstocks produced from open pollinated seed from these cultivars could promote early bearing of grafted scions (cultivars) and result in early fruit production for farmers. The goal of this project is to develop organic horticultural practices with selected pawpaw rootstocks in an effort to promote earlier bearing and consistent tree performance, and longer tree life for organic and limited resource farmers.
Objective 1. To determine the optimal application rate of organic nitrogen (fish emulsion) that enhances tree establishment, growth, early flowering, and fruit production in the orchard.
Objective 2. To determine if flame cultivation can be used effectively compared to glyphosate (RoundUp) application for weed control to promote pawpaw tree establishment and growth in orchards.
Objective 3. To determine if seedling rootstocks derived from two pawpaw cultivars (‘PA-Golden’ and ‘Sunflower’) will enhance tree survival, growth, flowering, fruit set, and fruit size of four pawpaw cultivars (PA-Golden, Sunflower, Shenandoah, and K8-2) compared to rootstock produced from commercially available mixed seed. Plantings for this objective will be established at six sites in three states (KY, VA and NC), including sites at KSU, UK, and four farms. All sites will serve as demonstration orchards in the future for pawpaw production for limited resource and organic farmers.
A planting was established at KSU on April 18, 2005 to determine the optimal application rate of organic nitrogen that would enhance tree establishment, growth, early flowering, and fruit production in the orchard. Pawpaw seedlings were planted and 5 levels of organic N were applied to determine the optimal application rate. Survival of this planting was evaluated in late September, 2005. An average of 50% of this planting survived, possibly due to the small size of the trees and drought conditions. The greatest survival was in the trees treated with 4 oz and 0 oz of N, at 58% survival, with only 33% of trees treated with 2 oz of nitrogen surviving. In 2006, less than 10% of the trees survived and the experiment was terminated. The poor tree survival was likely the result of the ineffectiveness of organic weed control measures (wood chip mulch), high 2006 rainfall levels that promoted weed and grass growth, and a preexisting Johnson grass infestation in the field prior to planting in 2005.
Weed control is extremely important to promote pawpaw tree establishment and growth in orchards. Flame cultivation offers an organic alternative to herbicide application for the control of grass and perennial weeds and uses a torch-directed flame to kill weeds by causing the plant cells to rupture. A study was conducted in 2006 with the objectives to determine if 1) flame cultivation with a backpack flamer would control grass/weed coverage around pawpaw trees without damage to trunks and 2) flame cultivation is economically viable. There were four replicate trees in each treatment; each tree was 5 years old. On July 25, and August 2 and 18, 2006, a three foot area around treatment trees was either subjected to the flaming treatments or weed eating (to a height of 2 inches). On August 25, and September 8 and 15, 2006, re-growth in plots was rated from 1 to 10, with 1 having no grass/weed coverage and 10 having total grass/weed coverage. By August 25, all flame plots had significantly less grass/weed coverage (about 2.25 rating) than control plots (7.75). On September 15, flame treatment plots had increased grass/weed coverage (about 4.75), but less coverage than control plots (9.5). Additionally, trees in either flaming treatment did not display noticeable trunk damage or wilting. Trunk damage will be evaluated again in 2007. Flame cultivation was found to be 2.5 times less expensive than straw mulch (organic) but 15 times more expensive than glyphosate (conventional) for grass/weed control.
A study was conducted to determine if seedling rootstocks derived from two pawpaw cultivars will enhance tree survival, growth, flowering, fruit set, and fruit size of four pawpaw cultivars compared to rootstock produced from commercially available mixed seed. Two plantings were established in April and May, 2005. At KSU, grafted trees of two rootstocks and four different scions were planted to determine the effect of rootstock seedling source on scion growth. The planting was mulched with wood chips and new irrigation was installed. Mortality of this planting was evaluated in late September, 2005. An average of 35% of these trees survived. By 2006, 90% of the plants died and the experiment was terminated at KSU. The high tree mortality was again likely the result of the ineffectiveness of organic weed control measures (wood chip mulch), high 2006 rainfall levels promoting weed and grass growth, and a preexisting Johnson grass infestation in the field prior to planting. However, a rootstock trial at the University of Kentucky research farm has been more successful, with 55 trees in the planting; some are replacement trees established in 2006. The highest survival has been with the scion Sunflower on PA-Golden rootstock (88%) and poorest survival with the scion Shenandoah on K8-2 rootstock (56%).
As part of an effort to propagate trees for trial at two grower sites, an experiment to optimize production of budded trees was conducted. Pawpaw cultivars are propagated by chip-budding the desired variety onto seedling rootstock. It has been suggested that when chip-budding pawpaw, it is beneficial to leave 6-8 leaves on the rootstock seedling. The photosynthetic activity of these leaves provides energy for the scion bud, until the bud has broken and initiated leaves, at which time the rootstock’s leaves would be removed. To test this hypothesis, a 2x2x2 factorial experiment was implemented, with 2 scions (Sunflower and Susquehanna), 2 seedling rootstocks (Sunflower and K8-2), and 2 leaf treatments (removing leaves vs. leaving 6-8 leaves). Trees were chip-budded in late June 2006. Leaf number was counted weekly. The remaining rootstock leaves were removed after 6 weeks. Retaining leaves on the rootstock seedling increased scion bud break, with 75% of buds breaking on rootstocks with leaves remaining, and 53% bud-take on rootstocks with all leaves removed, before leaf removal. Removing the remaining rootstock leaves after 6 weeks had a positive effect, with an additional 13% of buds on rootstocks that had previously retained their leaves breaking by 4 weeks after leaf removal, vs. only 2% more buds breaking on rootstocks that had leaves removed from the beginning. Removing the rootstock’s leaves upon budding had a positive effect on scion leaf number, with scions budded onto rootstocks whose leaves had been removed having an average of 9 leaves, and scions budded onto rootstocks with leaves remaining having 5 leaves. Approximately 200 of the budded trees are being over-wintered at the KSU farm and will be planted at two grower sites (Mr. McIntosh and Mr. Powell) in May, 2007. These growers have agreed to record survival, growth, and flowering data in the coming years. These sites will serve as demonstration orchards in the future for pawpaw production for limited resource and organic farmers.
Impacts and Contributions/Outcomes
The long-term economic viability of small family farmers, who currently rely heavily on tobacco in the southeastern United States, may be achieved by crop diversification. Pawpaw has great potential for farmers in this region as an alternative high-value fruit crop for processing and fresh market sales. Pawpaw fruit have a unique flavor and were sold in 2006 at the Farmer’s Market in Lexington, Kentucky, for $3.00 per pound. Little information concerning organic production of pawpaw is currently available. Development of organic production recommendations would allow growers to facilitate fruit production and assist in the development of pawpaw as a niche organic crop. Many pawpaw cultivars on seedling rootstock of diverse origin will not produce harvestable crops for 4 to 5 years after planting, this compared to apple where production can begin 3 years after planting. Development of organic orchard management methods and rootstocks which promote early pawpaw fruit production will allow farmers to adopt these economically viable, environmentally sound and socially responsible practices. A web site describing the project has been constructed and can be found at: http://www.pawpaw.kysu.edu/sare.htm
The pawpaw rootstock and flame cultivation experimental plots at the Kentucky State University Research Farm were viewed by participants at monthly “Third Thursday” Sustainable Agriculture Workshops and at the KSU Farm Field Day. Results from the project were disseminated to growers and scientists by the following presentations:
Pomper*, Kirk W. 2006. Growing Pawpaw in Kentucky and the Southeastern United States. Southern Sustainable Agriculture Working Group (SAWG) meeting held at the KSU farm on January 20, 2006. (approximately 75 people attended)
Pomper*, Kirk W. 2006. Pawpaw Growing in Kentucky. Spring meeting of the Kentucky Nut Growers Association on April 22, 2006 in Elizabethtown, KY. (approximately 125 people attended)
Pomper*, Kirk W. 2006. Gooseberry, Pawpaw, and Blackberries Trials at the KSU farm. Third Thursday Workshop, June 15, 2006. (approximately 75 people attended)
Pomper*, Kirk W. 2006. Pawpaw, Currant and Gooseberry Varieties for Kentucky and the Southeastern United States. The North American Fruit Explorers Annual Meeting in Lexington, Kentucky, August 31, 2006. (approximately 125 people attended)
Pomper*, Kirk W. 2006. Pawpaw 101. Ohio Annual Pawpaw Festival in Albany, OH, 16-17 September. (approximately 2000 people attended)
Pomper*, Kirk W. 2006. Pawpaw Production in Kentucky. Pawpaw Field Day at the Kentucky State University Research Farm on September 21, 2006. (approximately 75 attended)
Pomper*, Kirk W. and Sheri B. Crabtree. 2006. Is Flame Cultivation a Viable Method for Organic Weed Control in Pawpaw Orchards? Oral presentation at the Kentucky Academy of Science annual meeting held at Morehead State University, Morehead, KY, on November 10, 2006.
Crabtree*, Sheri B. and Kirk W. Pomper. 2006. Rootstock Leaf Retention Aids Bud Break in Chip-Budded Pawpaw (Asimina triloba). Poster presentation at the Kentucky Academy of Science annual meeting held at Morehead State University, Morehead, KY, on November 10, 2006.
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