L.J. Grauke

Geographic patterns of genetic variation in native pecans

A structured collection of 80 seedling pecan trees [Carya illinoinensis (Wangenh.) K. Koch], representing 19 putatively native pecan populations across the species range, was evaluated at three plastid and 14 nuclear microsatellite (simple sequence repeat, SSR) loci. Data were analyzed using a priori population designations and also within a Bayesian framework, in which individuals were assigned to clusters regardless of population of origin. Population genetic analyses using a priori populations, clusters based on chloroplast microsatellite data (cpSSR), and clusters based on nuclear microsatellite data (nSSR) yielded consistent results. For all groupings, cpSSR variation exhibited more geographic structure than the nSSR data. Furthermore, cpSSR microsatellite diversity decreased with increasing latitude, but this pattern was not observed with the nuclear data. Contrasting patterns in plastid and nuclear genetic diversity demonstrate unique aspects of postglacial recolonization reflected in the movement of seeds versus pollen. These data suggest that plastid SSRs are useful tools for identifying population structure in pecan and hold promise for ongoing efforts to identify and conserve representative germplasm in ex situ collections.

Expression, purification and crystallization of pecan (Carya illinoinensis) vicilin

Tree nuts are responsible for many cases of severe food allergies. The 7S seed storage protein vicilin has been identified as a food allergen in many kinds of tree nuts. The vicilin protein consists of an N-terminal low-complexity region with antimicrobial activity and a C-terminal domain that forms a trimeric structure that belongs to the cupin superfamily. In this study, vicilin from pecan (Carya illinoinensis) was isolated and was expressed in bacteria for the first time. The cupin structural core of the protein, residues 369-792, was purified by metal-affinity and gel-filtration chromatography to high purity. Vicilin crystals were obtained and the best crystal diffracted to 2.65 Å resolution in space group P212121.

An Overview of the Evolution of the U.S. Pecan Industry

Pecan [Carya illinoinensis (Wangenh.) K. Koch], preferably pronounced as pi-´kan over pi-´kan or ´pē-,kan (Llewellyn 1985), is one of the few native North American plant species that has been developed into a significant agricultural crop. It has also become one of the few indigenous U.S. food crops that is commercially cultivated outside the U.S. (i.e., Mexico, Australia, South Africa, Israel, Brazil, Argentina, Egypt, etc.). As perhaps the economically most significant native contributor to the U.S. agricultural economy, wholesale revenues approximate

Scab Susceptibility of a Provenance Collection of Pecan in Three Different Seasons in the Southeastern United States

200 million annually (USDA 1991) and approaches

Blackmargined Aphid (Monellia caryella (Fitch); Hemiptera: Aphididae) Honeydew Production in Pecan and Implications for Managing the Pecan Aphid Complex in Texas

400 million when all aspects of the industry are included (Crocker 1989).

Relative Susceptibility of Pecan Germplasm to Blackmargined Aphid

Pecan scab (caused by Fusicladium effusum) is the most economically destructive disease of pecan in the Southeast United States. Wet, humid conditions typical of the Southeast are known to provide conditions conducive to epidemics. A provenance collection of pecan from 19 locations representing the native range of the tree is located in Byron, Georgia, and was assessed for pecan scab severity in 1998, 2013, and 2014. There were significant differences among the 19 provenances (F = 5.6 to 62.5, P < 0.0001). Provenances from wetter locations (generally north of Texas) had the greatest proportion of scab resistant trees, while provenances from the drier southern areas (Texas and Mexico) tended to be the most susceptible to scab. The association with rainfall was borne out by correlation analysis (r = -0.625 to -0.823 [P < 0.0001 to 0.004]). Other factors consistently associated with scab severity included leaflet tilt and droop angle (r = -0.533 to -0.883 [P < 0.0001 to 0.02]). Multiple regression analysis demonstrated that leaflet droop angle was a particularly good predictor of provenance susceptibility. Leaflet characteristics vary with provenance location, and whether there is a direct relationship between scab severity and leaflet characteristics is not established. Estimates of heritability were not entirely consistent among years, but different methods were used to assess scab severity in 1998 (a 1 to 5 category scale) compared with 2013 and 2014 (the percent ratio scale). Despite using different methods, there was generally good agreement among years in regard to severity of disease on individual trees. In conclusion, trees from more northern populations (in areas with greater annual rainfall) are most likely to provide valuable and diverse sources of resistance to scab. The provenance collection contains a range of scab-resistant genotypes from diverse locations that can contribute to genetic improvement regarding scab resistance.

Pecans and other hickories (Carya)

Abstract. The blackmargined aphid, Monellia caryella (Fitch), was studied on three cultivars, ‘Cheyenne’, ‘Kiowa’, and ‘Pawnee’, of pecan, Carya illinoinensis (Wangenh.) K. Koch, in the field. Abundance of aphids and natural enemies (lacewings, ladybird beetles, and spiders) was determined twice weekly by directly inspecting foliage on each pecan variety during the summers of 2005 and 2006. Water-sensitive cards were used to measure honeydew deposited. Aphid phenologies were similar among pecan cultivars; however, Cheyenne supported more aphids than did Kiowa or Pawnee. Honeydew production was directly correlated with aphid abundance. Abundance of natural enemies increased during initial stages of aphid outbreak on all pecan cultivars, and the asymptote reached on Cheyenne exceeded the action level of 25+ aphids per leaf. Cheyenne had a lower natural enemy-to-aphid ratio than did the other cultivars, indicating that the functional response of natural enemies to increased aphid abundance was exhausted sooner on Cheyenne than on other cultivars where aphid abundance did not exceed the action level. Honeydew seems to be an attractant for natural enemies, and cost-benefit was calculated to quantify the loss of photosynthates to aphids versus the gain in natural enemies on each pecan cultivar. Cheyenne was the least efficient of the three cultivars in the utilization of this defense mechanism. The energy drain per hectare was calculated using aphid density and by measuring honeydew; data showed the energy drain on Cheyenne was 4–8 fold greater than that on Kiowa or Pawnee by using either method. Conversions of energy drain estimates to nut-equivalents indicated Cheyenne suffered economic damage that warranted treatment whereas Kiowa and Pawnee did not. These results suggested that moderately abundant blackmargined aphids efficiently attracted natural enemies with little risk of economic damage to the crop.

PROVENANCE VARIATION IN PECAN

Abstract. The blackmargined aphid, Monellia caryella (Fitch), is an important phytophage in the pecan, Carya illinoinensis (Wangenh.) K. Koch, agroecosystem where it often is treated with insecticide. Pecan cultivars released by the USDA Pecan Breeding Program vary in susceptibility and risk of damage from the blackmargined aphid. We evaluated a new technique that measures honeydew deposition and found relative differences in susceptibility of a segregating pecan population were identifiable during the course of an outbreak of blackmargined aphids. This provided an efficient method for the simultaneous evaluation of hundreds of segregating pecan trees. Use of this new tool will also aid studies of inheritance, horticultural compatibility, and in determining the relative permanence of this character in pecan improvement.