Black walnut (Juglans nigra L.) is native to much of the eastern United States and is highly valued for its nuts and timber. Black walnut trees are monoecious with one to four pistillate flowers in a cluster at distal ends of the current season’s shoots. Each pistillate flower produces one orthotropous ovule surrounded by the integument (Schaffer et al., 1996). Within the ovule, an eight-nucleate embryo sac is formed. Staminate flowers generally have 20 to 30 sessile stamens and occur on catkins that emerge from dormant buds on the previous season’s growth above leaf scars (Beineke and Masters, 1977). Most commercially grown black walnut cultivars are protogynous with pistillate bloom in late April to early May and pollen shed usually a few days thereafter in Missouri (Warmund and Coggeshall, 2010). After flowers are wind-pollinated, fertilization occurs 2 to 5 d later followed by a period of rapid expansion of the fruit (Funk, 1979). In early July, shell hardening occurs and the embryo and cotyledons enlarge. Black walnut kernel development begins adjacent to the testa and continues inward until the exocarp is filled (Van Sambeek and Rink, 1982). Several factors influence kernel filling, including previous and current year’s cropload, tree nutritional status, and environmental factors, especially drought (Crane, 1949). Black walnut fruit generally attain most of their size by mid-August and kernel fill is completed by late September or early October (Brawner and Warmund, 2008; Funk, 1979). Fruit are then harvested, hulled, and dried in-shell before cracking for commercial sales.
Ambers is a commonly used term for poorly filled, shriveled kernels with a dark brown or black-colored pellicle inside black walnut fruit (Funk, 1979; Gibson and Kearby, 1976) (Fig. 1). Walnuts with ambered kernels are unmarketable, resulting in economic loss to commercial growers (Biggs, 2011). Although symptoms of ambered kernels have been previously described, there has been limited research on the cause of this problem. Stoke (1941) reported that shriveled black walnut kernels have lower oil content than non-shriveled kernels. In a later study, Mangoff (1980) found that ambered kernels typically had less stearic and oleic acid than non-ambered kernels. Gibson and Kearby (1976) reported that ambered kernels were not associated with feeding by walnut husk maggots (Rhagoletis completa Cresson) in the fall. More recently, ambered kernels were found in shelled nuts of all cultivars examined, including ‘Emma K’, ‘Kwik Krop’, ‘Sparrow’, ‘Surprise’, ‘Schessler’, ‘Davidson’, ‘Thomas’, ‘Sparks 127’, ‘Hare’, ‘Tomboy’, and ‘Bowser’, in a black walnut repository at New Franklin, MO (Biggs, 2011). At another site, it was discovered that not all fruit produced on a single terminal fruiting shoot (with apparently healthy foliage) had ambered kernels on ‘Football’ trees, which suggests that these symptoms are not associated with a nutritional disorder (M.R. Warmund, unpublished data).
Other nut tree species produce discolored kernels, although causal agents are likely different from that for ambered black walnut kernels. Persian walnut cultivars such as ‘Ashley’, ‘Serr’, and ‘Chico’ produce fruit containing “oilless” nuts with dark, shriveled kernels in the shaded interior of the tree canopy early in the growing season (Grant et al., 1985). Unlike black walnut, Persian walnut fruiting shoots with oilless nuts produce sparse leaves with interveinal chlorosis and these fruiting spurs often die the next winter (Grant et al., 1985). Necrosis has also been reported on the basal area of pecan kernels and it affects certain cultivars such as ‘Pawnee’, ‘Choctaw’, and ‘Oklahoma’ more so than ‘Cheyenne’ (Smith et al., 2007). Although the cause of pecan kernel necrosis is unknown, it does not appear to be the result of stink bug damage, pecan weevil feeding, or a nutritional disorder.
Because of the paucity of information on black walnut ambers, the objectives of this study were to: 1) evaluate the incidence of ambered kernels in fruit of ‘Football’ trees at three sites within a black walnut orchard; 2) determine when symptoms of ambered kernels first appear during the growing season; and 3) ascertain which fruit tissues are associated with ambers.
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