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  • Author or Editor: Mark V. Coggeshall x
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Eastern black walnut (Juglans nigra L.), is a native tree species valued both for its timber and nuts. Individual trees require excellent soils with adequate moisture for maximum wood and nut productivity. The vast majority of black walnut nutmeat production is centered in the western part of the species’ native range and is predominantly derived from wild, unimproved sources. Historically, the size of this crop has ranged from 4.5 to 15.9 million kilograms (fresh weight, without hull) with less than 1% obtained from improved cultivars. Alternate bearing is common in this species, primarily as a result of susceptibility to walnut anthracnose Gnomonia leptostyla (Fr.) Ces. & De Not. Significant reductions in nut productivity caused by walnut curculio (Conotrachelus retentus Say) are common for unimproved, wild trees. Based on a collection of 65 nut cultivars established by the University of Missouri, black walnut exhibits significant genetic variation for a range of commercially important nut traits, including precocity, percent kernel, nut bearing habit, anthracnose tolerance, season length, and yield efficiency. Exploiting this variation through traditional plant breeding techniques will result in new, improved varieties in the future. Defining improved cultural practices that will significantly impact yield and nut quality remains an area of active investigation. Such knowledge will need to be combined with improved cultivars that are well adapted to local growing conditions to ensure the commercial success of this species over the long run.

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A study was conducted to: 1) evaluate the use of a durometer for determining husk softening and the date of black walnut harvest and 2) elucidate the relationship between husk hardness, kernel color and weight, and date of harvest. Thirty nuts were randomly collected weekly from mature `Sparrow', `Emma K', `Kwik Krop', and `Football' trees from 1 Sept. to 13 Oct. 2004. Husk denting, hardness, and color measurements were recorded immediately after harvest. Husk denting is the method commonly used by growers to determine the optimum time of harvest. Nuts were then hulled within 48 hours and the in-shell fresh weights were recorded. After drying under natural conditions for 5 weeks, kernel color and weights were assessed. On 15 Sept., 99% of `Sparrow' husks dented with a mean durometer value of 54. On 29 Sept., ≥99% of `Emma K' and `Kwik Krop' husks dented with mean durometer values of 63 and 68, respectively. By 13 Oct., 80% of `Football' husks dented with a mean durometer value of 74. From the first harvest date to the time of maximum denting, kernel weight and color (L*, hue values) of `Sparrow' and `Emma K' generally increased. The L* and hue values of `Kwik Krop' were inversely related to increased kernel weight over time. Kernel color of `Football' remained relatively constant as kernel weight increased over time.

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Grafting failure of Chinese chestnut (Castanea mollissima Blume) trees is common. Some researchers have suggested that secondary phloem fibers in chestnut rootstock stems may interfere with graft union formation. Others suggest that high rootstock moisture content may reduce grafting success. This study was conducted to ascertain the precise arrangement of secondary phloem fibers in Chinese chestnut seedling rootstocks and to determine if they are associated with grafting failure. We also investigated the effects of moisture content of rootstocks and container growing medium on grafting success. Nodes on Chinese chestnut seedlings had 2/5 phyllotaxy with five alternate buds present in two revolutions around the stem. Vascular cambium in 1-year-old rootstock stems was arranged in a regular five-lobed cylinder in cross-sections. Primary phloem fibers were present in a discontinuous ring and secondary phloem fibers were usually clustered outside the cambium between the lobes. Grafts were successful when scion buds were placed on the rootstock disrupting secondary phloem fibers with cambia of the budded tissues aligned. Containerized rootstocks at 56% and 62% growing medium moisture content before grafting had 25% graft union success rates, whereas non-irrigated plants at 48% moisture had a 75% success rate.

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The walnut twig beetle [WTB (Pityophthorus juglandis Blackman)] is the primary insect vector for a pathogen that causes thousand cankers disease (TCD), a disease complex that leads to mortality in species of walnut (Juglans L.). We performed field and laboratory trials to determine if reproduction by WTB varies between two black walnut (Juglans nigra L.) parent trees of a full-sib mapping population of 323 offspring, and between black walnut and butternut (Juglans cinerea L.). These two tree species are native to eastern North America. In field trials, we found no significant differences in colonization density or mean number of adult offspring per female among branch sections from black walnut parent trees or among branch sections from black walnut and butternut, respectively. In laboratory trials with controlled colonization densities of WTB, we found that significantly fewer adult offspring developed in branch sections of the black walnut maternal ‘Sparrow’ parent than the paternal ‘Schessler’ parent over three summer months and one winter month. In the field, high colonization densities likely limited reproduction due to increased intraspecific competition beneath the bark. In the laboratory, where we established a lower colonization density, reproduction was likely influenced by differences in host quality. In laboratory trials, no differences were detected in the number of adult offspring emerging from black walnut and butternut accessions. This finding suggests that butternut is a suitable host for WTB. Future screening of the full-sib mapping population of 323 offspring of black walnut parent trees for WTB resistance is a warranted next step in developing alternative management strategies for TCD in black walnut.

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Eastern black walnut (Juglans nigra) is a large tree endemic to the eastern United States and is highly sought after for its timber products and uniquely flavored nuts. The University of Missouri Center for Agroforestry in New Franklin, MO, USA, hosts an eastern black walnut cultivar repository. This collection supports an ongoing breeding program to improve economic performance for nut production (kernel weight, nut quality, precocity, and yield). In 1996, 54 cultivars were grafted and planted in a series of experimental orchards for evaluation. From 2001 until 2015, trees were evaluated for 12 phenological and eight nut quality/yield traits. Economically relevant traits including kernel weight, kernel percentage, and precocity (total nuts produced age 6 to 10 years) ranged from 1.1 to 8.6 g, 5.3% to 39.3%, and 16 to 1522 total nuts per tree, respectively. Kernel percentage was positively correlated with kernel weight (r = 0.51) and precocity (r = 0.38). Precocity was negatively correlated with the first (r = −0.39) and last (r = −0.30) female bloom. Principal component analysis and biplot analyses revealed high levels of variation among the cultivars. The first two components explain 43.4% of the total variation. Nut dimensions (nut length and nut thickness), nut and kernel weight, and kernel percentage are the largest drivers of variation in the collection. Eigenvectors for precocity and kernel percentage load together and are orthogonal to kernel weight, suggesting these three important traits can be improved simultaneously. Also, nut length loads with kernel weight, providing a candidate indirect selection parameter to increase kernel weight. These data inform strategies for crossing scheme design, expectations for multitrait genetic gain, complementary hybridization, and identifying unique recombinants.

Open Access