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Hongwen Huang and Desmond R. Layne

The pawpaw is the largest tree fruit native to the United States and the only temperate member of the tropical Custard Apple family (Annonaceae). In 1995, Kentucky State Univ. was established as the USDA-ARS-National Clonal Germplasm Repository for Asimina spp. Seedling trees from 400 pawpaw accessions representing 70 distinct geographic regions from 17 states are currently being grown at our research farm. In a preliminary study, 18 pawpaw cultivars were assayed in 30 enzyme systems using an isoelectric focusing polyacrylamide slab gel system of pH 4-9. Twelve enzymes produced high resolution without tissue specificity and were further used for evaluation of allozyme diversity of geographic populations. Degree of genetic diversity within populations and differentiation between populations as evaluated by the expected heterozygosity (He), the proportion of polymorphic loci (P), the average number of alleles per locus (A), chi-squared analysis of allele frequency heterogeneity, Nei's standard genetic distance (D), and identity (I) will be discussed. Dendrograms were generated by cluster analysis using the unweighted pair group method to demonstrate the relationships of geographic populations in the 17 states evaluated. The strategy for germplasm conservation and cultivar development through breeding will also be discussed.

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Desmond R. Layne and Guido Schnabel

In 2003, a replicated long-term research trial was established on a commercial peach replant site with a history of Armillaria root rot and other soilborne diseases. The objectives of the trial were to determine the short- and long-term effects of preplant fumigation, rootstock, and preplant root dipping with mycorrhizal fungi and beneficial bacteria on tree growth, productivity, and survival. Preplant fumigants included none (control), methyl bromide, Telone II, or Enzone. Rootstocks tested included Guardian, Lovell, and Halford. Root dipping (or not) was with MycorTree. The scion cultivar was Big Red. There were a total of 24 experimental treatment combinations and the trial site comprised more than 1500 trees on 11.5 acres. By 2 years after planting, fumigation with Enzone was disadvantageous when compared with no treatment at all. Enzone-treated blocks had higher tree mortality or were significantly reduced in growth compared to other treatments. Preplant fumigation with Telone II or methyl bromide, however, resulted in reduced tree stunting and phytotoxicity and increased tree growth when compared to the untreated control. After 2 years, 10% of the total trees planted were dead. Half of these were from the Enzone treatment. Enzone does not appear to be a viable preplant fumigation product for South Carolina peach growers, based on this preliminary data. Both Guardian and Halford rootstocks had performance superior to Lovell during the first 2 years. Although Guardian trees were smaller than Halford at the time of planting, by the end of the second growing season, their TCA was not significantly different. There was no benefit to preplant root dipping with MycorTree. Experimental results were not influenced by the location of trees on the site.

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W.R. Okie and Desmond R. Layne

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W.R. Okie and Desmond R. Layne

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Desmond R. Layne and R. Neal Peterson

In 1993, PPF and KSU embarked on a joint venture to test within pawpaw's native range many of the commercially available named pawpaw cultivars and PPF's advanced selections. Orchards for RVT were planted in 17 locations from Fall 1995 through Fall 1996 (possibly into 1997) consisting of 300 trees each. At each RVT site, eight replicate trees of each of the 28 grafted scion varieties will be tested in a randomized complete-block design. Named varieties that are secured for testing include Middletown, Mitchell, NC-1, Overleese, PA-Golden, Sunflower, Taylor, Taytwo, Wells, and Wilson. The other 18 clones to be evaluated originated in PPF orchards at the Univ. of Maryland Experiment Stations at Wye and Keedysville. Seedling trees from local native sources were planted around the perimeter as a buffer against edge effects and to allow comparisons with local germplasm. Identical orchards of the RVT are located in the following states: Arizona, Indiana, Iowa, Kentucky (two sites), Louisiana, Maryland, Michigan, Nebraska, New York, North Carolina, Ohio, Oregon, South Carolina, Tennessee (two sites), and the Chinese Academy of Forestry, Beijing, China. An orchard of nonidentical design is located in Florida. Additional sites in Connecticut and Chile are contemplated. Variables being studied in the trial include climate, culture, pests, growth, flowering, yield, and fruit characteristics. Trees will be evaluated for several years for yield, year-to-year consistency, regional suitability, etc. At the end of the trial period, regional recommendations will be made. Scion–rootstock compatibility based on percent scion take, scion growth (scion height and cross-sectional area), and first year field data are presented and discussed.

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Desmond R. Layne and W.R. Okie

White-fleshed peaches and nectarines are delicacies that have been enjoyed for centuries around the world. They are native to China and were introduced to the United States in the 1800s. Some white-fleshed peaches and nectarines are highly perishable and bruise easily, but are of very high eating quality. These are perhaps best suited for the local roadside market, where they can be sold and consumed more quickly. Others are much firmer at harvest, have a longer shelf life. and are suitable for long-distance transport to wholesale markets. White-fleshed peaches and nectarines may have some acidity or they may be very low acid with high sugar content (°Brix). Some novel flat (peento or donut) types also exist. Proximity to an urban market with a substantial Asian population is advantageous because Asians, in particular, often prefer the low-acid flavor and are willing to pay premium prices for high quality fruits. In our peach and nectarine cultivar evaluation program at Clemson University, we are currently evaluating 70 cultivars and advanced selections at four different locations in South Carolina. Several of these have been evaluated since 2000 and the “top performers” over the last six seasons by ripening date (earliest to latest) include the following: `Sugar May', `Scarletpearl', `Snowbrite', `Southernpearl', `White Lady', `Sugar Lady', `Summer Sweet', `Sugar Giant', `Stark's Summer Pearl', `Snow King', and `Snow Giant'. In general, most of the white nectarines and the flat/donut peaches and nectarines have serious problems with insect damage and brown rot. Complete details of our peach and nectarine (yellow- and white-flesh) evaluation work in South Carolina since 2000 will be noted by referring to my peach website (http://www.clemson.edu/hort/Peach/index.php).

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Desmond R. Layne and J.A. Flore

A series of experiments were conducted with one-year-old potted sour cherry trees to evaluate the effects of source reduction (removal of 70% of the expanded leaves = Defol.) or source enhancement (continuous illumination = C.L.) on source leaf gas exchange. There was a significant increase in net CO2 assimilation (A) and stomatal conductance (gs) of Defol. within one day in contrast to the non-defoliated control (Cont.). Defol. had lower daily dark respiration rates (Rd) and higher A values throughout the 14 h diurnal photoperiod than Cont. Defol. had daily assimilation rates 50% higher than Cont. in as early as 3 days. One month later, specific leaf weight, leaf chlorophyll and A was higher in Defol. Non-defoliated plants were also placed under either a 14 h photoperiod (Cont.) or a 24 h photoperiod (24h). A of 24h was reduced from Cont. by 50% after one day. The diurnal response of A in Cont. was removed when plants were put in C.L. Following 7 days in C.L., 70% defoliation of 24h plants resulted in a complete recovery from photosynthetic inhibition within 48 hours. The short-term effects of source manipulation on photochemical and carboxylation efficiencies, photorespiration and stomatal limitations will also be addressed.

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Desmond R. Layne and Michael G. Kwantes

A two-factor factorial, randomized block design was used to study the influence of root-zone temperature (RZT) and fertilization regime (Fert) on early growth and development of pawpaw seedlings in the greenhouse. Stratified seeds were sown in 45-inch3 Rootrainer cells in Promix BX potting medium. Plants were watered/fertilized twice weekly to runoff. RZT was at two levels—ambient (75F–Amb) or bottom-heated (90F–Heat), and it commenced at sowing date. Fert was at four levels—125, 250, 500, or 1000 ppm Peter's 20N–20P–20K plus soluble trace elements, and it commenced at emergence date. Bench position in the greenhouse was used as the blocking variable (three blocks). The 20 most uniform seedlings were selected from each treatment/block for weekly growth measurements. At the termination of the experiment (equivalent to time for transplanting to a larger container), 10 plants per treatment/block were harvested destructively. RZT significantly influenced time to seedling emergence where Heat seedlings emerged 10 days earlier than Amb seedlings. Heat plants were significantly taller, had greater trunk diameter, leaf number, and leaf area than Amb plants. Fert at 125 to 250 ppm resulted in the most robust seedlings, with 1000 ppm Fert often inducing early terminal bud set. Impact of RZT and Fert on seedling development over time will be discussed.

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Desmond R. Layne and Eric J. Hitzler

In 1999, a trial was established at the Clemson University Musser Fruit Research Farm to investigate the effects of training system/tree density, rootstock, and irrigation/fertilization on tree growth, productivity and profitability. The replicated trial comprised three training systems/tree densities (open center–332 trees/ha; quad V–664 trees/ha; and perpendicular V–996 trees/ha). The two rootstocks used were Lovell and Guardian. Three irrigation/fertilization treatments included: natural rainfall only plus granular program; supplemental irrigation plus granular; and supplemental irrigation plus fertigation at a reduced nitrogen rate from the granular program. The scion was the popular midseason cultivar Redglobe. There were a total of 18 treatment combinations replicated 4 times with 5 trees per treatment plot. Soil moisture was determined by TDR and pan evaporation monitored by weather station. Seventy-two minirhizotron tubes were installed in 2002 to monitor fine root growth. During 1999–2001, drought conditions limited rainfall to 35% below the annual average. Spring freeze events in 2001 and 2004 reduced crop load disproportionately in shorter, open-center trees. As tree density within the tree row increased, trunk cross sectional area (TCA) decreased. Trees on Guardian rootstock had significantly greater TCA than Lovell each year. Trees that received supplemental irrigation had greater TCA than nonirrigated trees each year. By reducing N rate through fertigation, TCA was reduced. Cumulative yield was not affected by training system/tree density. Cumulative yield was not affected by rootstock. Cumulative yield was 7% greater by supplemental irrigation. Pruning, thinning and harvesting was easier in V-systems than for open center trees.