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Tommy E. Thompson

Variability in soluble solids concentration (SSC, °Brix) in liquid endosperm (LE) among individual pecan [Carya illinoinensis (Wangenh.) K. Koch] fruits and among fruits from different trees and cultivars using a sugar refractometer was determined at College Station, Texas, in 1997. Repeatability of readings from LE from the same fruit was excellent. Fruits from the same tree did not vary for SSC, but significant differences among clones were common. Soluble solids concentration appears to decrease as the fruit matures. The SSC values for two full-sib clones (one susceptible to water split and one resistant to water split) were similar. This information discounts the possibility that high osmotic water potential gradients alone induce the water split phenomenon. A wide range of SSC percents was recorded. A low of 0.5% was recorded for LE from a `Houma' fruit, while 6.1% was recorded for LE from a fruit from a drought-stressed `Burkett' tree.

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Tommy E. Thompson and L.J. Grauke

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Tommy E. Thompson and L.J. Grauke

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Tommy E. Thompson and L.J. Grauke

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Tommy E. Thompson and L.J. Grauke

Putative resistance to the yellow aphid complex (Monellia caryella (Fitch) and Monelliopsis pecanis Bissell) in the `Pawnee' pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivar was first noted in greenhouse tests by rating cultivars for relative amounts of honeydew on adaxial leaf surfaces. This resistance was confirmed in two field tests monitored from mid-June to mid-Oct. `Pawnee' supported significantly lower aphid populations during every rating period when relatively large numbers of these insects were present. `Navaho' also showed resistance, with `Desirable' having intermediate resistance and `Stuart' being very susceptible. Insect populations were also monitored on the four quadrants of each tree, with this quadrant effect being significant in only one test. This test had the highest populations on the West and lowest populations on the East.

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L. J. Grauke and Tommy E. Thompson

Thirteen cultivars of pecan [Carya illinoinensis (Wangenh.) K. Koch] were monitored for bud break, pollen shed and stigma receptivity for 4 years at LSU Pecan Station, Robson, LA. Cultivars were generally consistent in displaying clear patterns of protogyny or protandry, although patterns were uncertain for some cultivars in some years. Mean dates of cultivar phenology varied significantly by year. Years with warm winter and spring temperatures had earlier seasons of growth and flowering than years with cooler temperatures. The duration of pollen shed and stigma receptivity varied between years. Protogynous cultivars, as a group, had greater bloom overlap than protandrous cultivars, although overlap varied between years for both dichogamy classes. The sequence of cultivar flowering relative to other cultivars varied between years, resulting in variable amounts of bloom overlap between cultivars in different years.

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L.J. Grauke and Tommy E. Thompson

The commercial pecan [Carya illinoinensis (Wangenh.) K. Koch] nursery industry relies on open-pollinated seed for rootstock production. Current choice of seedstocks by commercial pecan nurserymen was surveyed by telephone. Nurseries were called if they appeared in the directory used for the 1994 release of `Navaho'. Factors influencing the choice of seedstock include seed availability, nut fill, nut size, nut shape, seedling vigor, stand uniformity, and root characteristics. Local availability is important in the choice of seedstock. Those who harvest from their own trees usually credit the seedstock with other valuable characteristics, such as improved germination or vigor. Those who purchase seed usually target a preferred seedstocks for particular reasons but plant available seed in its absence. Well-filled nuts are recognized as being important for good germination. Small nuts are often preferred, especially when seed is purchased because more nuts per pound increases potential production. Round nuts are generally preferred over long nuts due to improved performance in some mechanical planters. Distinct regional preferences are apparent in the choice of seedstocks. Regionally preferred seedstock selections are generally validated by a survey of the research literature. Patterns of selection are consistent with climatic and geographic constraints. Tree procurement patterns have changed: many small nurseries have gone out of business, many large nurseries transport trees far from the nursery for sales, and quarantine restrictions have altered procurement patterns in Arizona. Recommendations are made to nurserymen, pecan growers, and researchers concerning continued progress toward improving regionally adapted pecan rootstocks through seedstock selection.

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Tommy E. Thompson and L.J. Grauke

Putative resistance to the blackmargined aphid (Monellia caryella Fitch) in `Pawnee' pecan [Carya illinoinensis (Wangenh.) K. Koch] was first noted in greenhouse tests by rating cultivars for relative amounts of honeydew on adaxial leaf surfaces. This resistance was confirmed in two field tests monitored from mid-June to mid-October. `Pawnee' supported significantly lower aphid populations during every rating period when relatively large numbers of these insects were present. `Navaho' also showed resistance, with `Desirable' having intermediate resistance and `Stuart' being very susceptible. Insect populations were also monitored on the four quadrants of each tree, with quadrant effect being significant in only one test. This test had the highest populations on the western quadrant and lowest populations on the eastern quadrant. In determining individual clone resistance, it is recommended that the general orchard aphid infestation level be determined so that only two or three well-timed clonal ratings are needed. We also recommend that all sides of the tree be monitored.

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Tommy E. Thompson and L.J. Grauke

Thirty-six cultivars and 948 seedlings from 15 controlled crosses in the U.S. Dept. of Agriculture-Agricultural Research Service pecan [Carya illinoinensis (Wangenh.) K. Koch] breeding program at Brownwood, Texas, were rated for susceptibility to nut scab [Cladosporium caryigenum (Ell. et Lang.) Gottwald] to determine heritability of this trait. Differences between parents and progenies, and within progenies, were highly significant. Within most families, a complete range of resistance reactions were evident, from fully susceptible to fully resistant. Heritability of resistance was determined by regressing individual progeny values on female, male, and midparent values, with the midparent heritability estimate being the highest (0.54). This moderate level of additive gene action and the identification of superior parents in this study will contribute to the efficiency of breeding resistant cultivars.

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Tommy E. Thompson and L.J. Grauke