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Patrick Conner

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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Patrick Conner

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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Patrick Conner

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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Patrick J. Conner

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Patrick J. Conner

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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Patrick J. Conner

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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Patrick J. Conner

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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Patrick J. Conner

A detached leaflet protocol was developed for the evaluation of resistance to Fusicladium effusum in a seedling pecan population segregating for resistance. Leaflets at half to full expansion were detached from seedling trees, sprayed with a conidial suspension (isolate De-Tif-3), placed in a polyethylene bag in a growth chamber, and evaluated microscopically 7 and 14 days after inoculation. The percentage germinated conidia producing subcuticular hyphae was the best determinant of susceptibility with genotypes producing more than 15% subcuticular hyphae considered susceptible. Leaflets at half expansion had higher percentages of subcuticular hyphae and gave a clearer separation between susceptible and resistant genotypes than leaflets at full expansion. An evaluation period of 14 days was preferable to 7 days to allow slower reacting genotypes to be better evaluated. The detached leaflet protocol was evaluated in contrasting environments and was found to be robust to differences in shading and leaflet wetness. Detached leaflet tests gave similar results to field inoculations but were superior in consistently detecting susceptible genotypes. This protocol will be useful in evaluating the inheritance of pecan leaf scab resistance in breeding progenies.