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  • Author or Editor: Patrick Conner x
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Pecan (Carya illinoinensis) seedling rootstocks require several years of growth in the nursery before they are large enough to graft. In this experiment, first-year pecan seedlings were fertigated with varying amounts of calcium nitrate in an attempt to stimulate growth rates. Pecan seedlings were fertigated every 2 weeks from May through October for a total of 10 applications. Total amounts of nitrogen (N) applied by fertigation were 0, 4, 10, 20, and 40 g of N per seedling. Leaf samples were taken after the fourth and tenth fertigations, and leaf elemental concentration was affected by fertigation rates. Seedling height and caliper were measured monthly. Seedling caliper continued to increase throughout the experiment, whereas height increase stopped in September. Seedling height and caliper were unaffected by N fertigation except for the N rate of 40 g, which suppressed seedling growth. These results suggest that the N needs of the seedlings were met by a preplant application of 50 lb/acre N applied as 10N–4.4P–8.3K.

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Pecan seedling rootstocks require several years of growth in the nursery before they are large enough to graft. In this experiment, first-year pecan seedlings were fertigated with varying amounts of calcium nitrate to stimulate growth rates. Pecan seedlings were fertigated every 2 weeks from May through October for a total of 10 applications. Total amounts of nitrogen (N) applied by fertigation were 0, 4, 10, 20, and 40 g of N per seedling. Leaf samples were taken after the fourth and 10th fertigation, and leaf elemental concentration was affected by fertigation rates. Seedling height and caliper were measured monthly. Seedling caliper continued to increase throughout the experiment, while height increase stopped in September. Seedling height and caliper were not affected by N fertigation except for the N rate of 40 g, which suppressed seedling growth. These results suggest that the nitrogen needs of the seedlings were met by a preplant application of 56 kg·ha-1 N applied as 10N–10P–10K.

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Pecan is a highly heterozygous outcrossing species that is normally propagated by grafting or budding onto seedling rootstocks. The four-flap or banana graft is commonly used by growers or researchers because of its high percentage of success, especially when employed by novice grafters. We removed scion buds before grafting in an attempt to delay budbreak, thus providing more time for vascular connections to form before leaf development and its associated demand for water takes place. Removal of buds from the scion wood was successful in delaying bud and leaf development, but did not increase graft success, and in one treatment actually lowered graft success.

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Nineteen pecan (Carya illinoinensis) clones were evaluated over a period of 18 years in a test orchard located in southern Georgia. Clones tested were primarily U.S. Department of Agriculture selections, but two grower-discovered cultivars, Jubilee and Surprize, were also trialed. Annual yields were measured for each tree in the test throughout the test period and the alternate bearing intensity of each cultivar was calculated. Average annual in-shell nut production in years 1–10 ranged from 12 lb in the precocious USDA 76-4-41 to 0 lb in the non-precocious USDA 72-8-4. Wide variation was seen in nut production of trees in years 11–18, averaging from 10 to 60 lb nut yield per year. A subsample of nuts was taken from each tree annually and percent kernel, nuts per pound, specific gravity, and nut volume were determined. Significant differences were found between clones for each of these traits. Differences were also found for the presence of damage from pecan scab [Fusicladium effusum (synonym Cladosporium caryigenum)] and black pecan aphid (Melanocallis caryaefoliae). Most clones were not acceptable for use in Georgia due to small nut size or poor kernel quality, but two clones merit further testing in this region. USDA 70-3-34 produced a large nut with good quality and scab resistance, but needs to be evaluated with mechanical crop thinning to improve kernel quality in high crop-set years. USDA 74-1-12 produced good yields of excellent quality, medium-sized pecans and should be trialed with greater tree numbers. Several other clones were found to have traits of interest to pecan breeding programs including: pecan scab resistance, large nut size, and high kernel percentage. Results of this trial suggest that cultivar development programs in Georgia should place greater selection emphasis on large nut size.

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A detached leaf screening technique was developed for studying specific interactions between pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars and isolates of the pecan scab fungus, Cladosporium caryigenum. Monoconidial isolates were obtained from leaf scab lesions on `Wichita', `Desirable', `Cape Fear', and `Elliot'. Each isolate was then inoculated onto detached leaves of each of the four cultivars and fungal growth was observed under the microscope after eight days. `Wichita', `Desirable', and `Cape Fear' isolates produced subcuticular hyphae at a much higher frequency when inoculated back onto the cultivar from which they were isolated in comparison to the other cultivars. The `Elliot' isolate was able to produce a high frequency of subcuticular hyphae when inoculated onto `Elliot' and `Cape Fear', but not when inoculated onto `Desirable' and `Wichita'. Field inoculations conducted with the `Wichita' and `Desirable' isolates validated the detached leaf protocol. The results obtained indicate that pecan scab is composed of multiple races with a high degree of specificity for host cultivars. A rapid whole-leaf staining system is presented which appears to have wide applicability to assessing fungal growth in leaves.

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Twenty-six muscadine grape (Vitis rotundifolia Michx.) cultivars and selections were evaluated for a range of skin and flesh texture attributes. Two Vitis vinifera L. and one Vitis labruscana Bailey table grape cultivars were included for comparison. Penetration tests using a flat cylindrical probe were used to assess whole berry texture. Ideal whole berry texture is firm and easily broken down during mastication, which was measured as small berry deformation at first peak and berry maximum force, respectively. Muscadine berry deformation at first peak ranged from 4.35 to 7.82 mm and berry maximum force ranged from 5.7 to 13.9 N. V. vinifera table grape berries were firmer (3.14 to 3.19 mm berry deformation at first peak) and more tender (4.0 to 4.9 N berry maximum force) than muscadine berries. Berry penetration work was strongly correlated with both berry deformation at first peak and berry maximum force and ranged from 13.0 to 54.1 mJ in the muscadine germplasm. Penetration tests of muscadine berry flesh revealed a range of flesh firmness from very soft (0.65 N) to firm (3.06 N) but none was as firm as the V. vinifera berry flesh (3.9 N). Penetration tests of muscadine berry skins revealed newer selections bred for table use had relatively tender skins with a skin break force of 12.1 N, which was not different from V. vinifera samples. Berry penetration work and flesh maximum force were determined to be the most useful characteristics for routine screening of breeding program material.

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Storage of pollen from 1 year to the next is often needed to enable desired crosses to be made in a pecan [Carya illinoinensis (Wangenh.) K. Koch] breeding program. Stored pollen is usually tested for viability through the use of in vitro germination tests. An in vitro germination testing system was developed for this purpose using cellophane booklets to provide a solid support for the pollen grains. Optimized germination media contained 5% sucrose, 20% polyethylene glycol 8000, 0.05% Ca(NO3)2, 0.025% H3BO3, and 10 mm 2-(N-morpholino)ethanesulfonic acid pH 6.0. Pollen should be rehydrated for 2 to 4 h in a humidified chamber before germination testing. A germination time of 4 to 24 h produces similar final germination percentages. Testing of pollen samples stored at –80 °C indicates that pecan pollen can be stored for at least 8 years without a decrease in viability. Chemical names used: polyethylene glycol (PEG); 2-(N-morpholino)ethanesulfonic acid (MES).

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Pecan, [Carya illinoinensis (Wangenh.) C. Koch], is a member of Juglandaceae family and is one of the most important nut crops produced in the United States. The objective of this study is to generate the first genetic linkage maps for pecan. Maps were constructed for the cultivars `Elliot' and `Pawnee' using the double pseudo-testcross mapping method whereby a separate linkage map is made for each parent using markers heterozygous in that parent. First generation maps consisted primarily of randomly amplified polymorphic DNA (RAPD) markers. We have now used fluorescently labeled amplified fragment length polymorphism (AFLP) markers to produce more complete maps. In the development of the AFLP markers, 64 primer combinations were originally screened to find the most informative combinations. Ten primer combinations were then chosen to produce markers for the maps. The maps currently consist of approximately 100 RAPD and 100 AFLP markers on each cultivar map. `Pawnee' is a high quality commercial pecan cultivar with a very early ripening date. `Elliot' possesses high levels of resistance to pecan scab, caused by the fungus Cladosporium caryigenum. The maps will be used to find markers linked to scab resistance genes and other traits of interest to the breeding program.

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Germination of muscadine seed has frequently been low and irregular in the University of Georgia breeding program. A systematic study was undertaken to determine the best seed treatments and germination conditions for muscadine seed. Open-pollinated seeds of ‘Fry’ muscadine were used for all treatments. Stratification of seeds was performed by placing dry seed in damp vermiculite at 4 °C for periods of 0, 30, 60, and 90 d. The 90-d stratification period gave the highest germination percentage, with successively lower germination in the shorter stratification treatments. Pretreatment of seeds before stratification with three rates (0.5, 1.0, and 2.0 M) of hydrogen peroxide (H2O2) and four rates (1, 2, 4, and 8 g·L−1) of gibberellic acid (GA3) were used in an attempt to promote germination. Low rates of H2O2 (0.5 M) and GA3 (1 g·L−1) were beneficial in some instances, whereas high rates of GA3 were detrimental. Nicking the seedcoats before stratification and soaking seeds in running water after stratification were ineffective in promoting germination. Germination temperatures of 32/22 °C (8 h/16 h) were superior to 22/22, 27/22, and 37/22 °C.

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