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These studies show that early walnut harvest minimizes insect-damaged kernels and maximizes quality and resulting kernel value. Direct relationships exist between harvest date and nut quality. Differential in value within walnut growing... more
These studies show that early walnut harvest minimizes insect-damaged kernels and maximizes quality and resulting kernel value. Direct relationships exist between harvest date and nut quality. Differential in value within walnut growing districts is directly dependent on time elapsed between date of maturity and harvest. Temperatures during harvest may account for differences in value between districts. Once nuts are removed from the tree, picking should proceed as rapidly as possible to maintain optimum kernel color and value. Rate of loss in kernel quality is directly proportional to increasing air temperature. Kernel temperature of about 140°F appears to be the threshold for onset of kernel darkening, and is reached in the sun at air temperature above 90°F and in the shade at air temperatures of 104°F or greater. Once threshold kernel temperatures are attained, substantial loss in value can occur within nine hours. Threshold kernel temperatures are reached fastest when nuts are h...
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Not AvailableFirst paragraph follows: Timing walnut harvest so that nuts are removed as soon as kernels are mature is an important concept both in terms of kernel quality and in the prevention of insect damage, particularly that from... more
Not AvailableFirst paragraph follows: Timing walnut harvest so that nuts are removed as soon as kernels are mature is an important concept both in terms of kernel quality and in the prevention of insect damage, particularly that from navel orangeworm.
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Ten new marianna root-stocks [Prunus cerasifera Ehrh. × P. munsoniana Wight & Hedr.(?)] derived from open pollination of `Marianna 2616' (M series) were planted in 1987 and evaluated at four commercial orchard locations in California... more
Ten new marianna root-stocks [Prunus cerasifera Ehrh. × P. munsoniana Wight & Hedr.(?)] derived from open pollination of `Marianna 2616' (M series) were planted in 1987 and evaluated at four commercial orchard locations in California (Tehama, Butte, Sutter, and Merced counties) with `Improved French' prune (P. domestica L.) as the scion. These rootstocks were compared to three standard rootstocks: `Marianna 2624', myrobalan seedling (P. cerasifera Ehrh.) and `Myrobalan 29C'. Leaf potassium (K) and nitrogen (N), tree growth, fruit production and fruit quality were measured. Selection M40 in particular had high leaf N, high leaf K (equal to `Marianna 2624' and better than the myrobalan standards), higher yield efficiency per tree, fruit size, drying characteristics, and few root suckers when compared to the three standard rootstocks. M40 is being considered for patent and release by the Pomology Department at the University of California, Davis. Selection M58 had t...
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Six years of previous research in a 12-year-old English walnut orchard, with a history of potassium deficiency, created a large number of trees with different potassium status. This provided the opportunity to study the long-term effects... more
Six years of previous research in a 12-year-old English walnut orchard, with a history of potassium deficiency, created a large number of trees with different potassium status. This provided the opportunity to study the long-term effects different potassium status has on English walnut trees growth, productivity, and nut quality. Walnut trees with a history of potassium deficiency, adequacy or luxury continued in this mode during this evaluation. Positive correlations existed between July leaf potassium levels and tree trunk sectional area (TCSA), visual potassium status, percent husk potassium, yield per tree, and tree yield per TCSA. These positive correlations suggest July leaf potassium levels of 1.4% to 1.5% as being adequate. This is higher than the 1.2% leaf potassium level currently recommended as being adequate for a July sample. Poor or no correlations existed between July leaf potassium levels and percent shell potassium, shell weight, shell breaking force, percent broken...
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Annual pruning was compared with nonpruning for 8 years and to two biennial pruning treatments for 4 years in a mature full-canopied `Ashley' walnut (Juglans regia L.) orchard. Light penetration and nut distribution through the canopy... more
Annual pruning was compared with nonpruning for 8 years and to two biennial pruning treatments for 4 years in a mature full-canopied `Ashley' walnut (Juglans regia L.) orchard. Light penetration and nut distribution through the canopy was improved by pruning. Nut size and percent edible kernel was consistently lower in nonpruned trees than in trees pruned annually or biennially. Yield from annually pruned trees was not significantly different from that of the nonpruned trees because of the removal of fruitful spurs. Yield of biennially pruned trees was similar to annually pruned or nonpruned trees in the year following pruning, but yield was usually greater during years in which trees were not pruned.
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English walnut (Juglans regia) producers in California compete with many insect and disease pests to produce an acceptable crop. Traditional control strategies work reasonably well for most pests. However, environmental concerns, loss of... more
English walnut (Juglans regia) producers in California compete with many insect and disease pests to produce an acceptable crop. Traditional control strategies work reasonably well for most pests. However, environmental concerns, loss of certain pesticides and new or impending regulations threaten the use of many traditional techniques for control of many of the pests. Codling moth (Cydia pomonella), walnut husk fly (Rhagoletis completa), and walnut aphid (Chromaphis juglandicola) are the major insects that affect California walnut production. Control strategies that use integrated pest management programs, beneficial insects, mating disruption, insect growth regulators, improved monitoring techniques and precise treatment timing based on the insect's life cycle are leading edge techniques currently available for insect control in walnuts. Major diseases include walnut blight (Xanthomonas campestris pv. juglandis), crown gall (Agrobacterium tumefaciens) and crown and root rot (P...
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Walnut Blight caused by the bacteria Xanthomonas campestris pathovar juglandis is a very destructive disease for California walnut production. Streptomycin is an effective disease control material; however, Streptomycin sprays can result... more
Walnut Blight caused by the bacteria Xanthomonas campestris pathovar juglandis is a very destructive disease for California walnut production. Streptomycin is an effective disease control material; however, Streptomycin sprays can result in significant nut drop 3 to 5 weeks after spray application. We investigated the basis for walnut drop following applications of Streptomycin (Agrimycin) for walnut blight control. Flowers and developing nuts were collected from four treatments, plus an unsprayed control. 200 ppm Streptomycim was applied at 1) budbreak; 2) pre, full, and post-bloom; 3) postbloom; 4) budbreak and postbloom; 5) untreated control. Samples were collected regularly beginning at the first budbreak spray and extending through the period of nut drop. Samples were fixed and prepared for histological examination. In treatments with a high incidence of nut drop, the embryo failed to develop. Examination of the stigma and style in flowers from these treatments showed inhibited...
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Initial leaf tissue-analysis indicated that the degree of distortion and dieback in a young walnut orchard was correlated with decreasing amounts of Cu in the leaf. Complete correction of Cu deficiency was obtained for two years when high... more
Initial leaf tissue-analysis indicated that the degree of distortion and dieback in a young walnut orchard was correlated with decreasing amounts of Cu in the leaf. Complete correction of Cu deficiency was obtained for two years when high rates of Kocide 101 were used or when low rates were applied repeatedly each year. Soil treatments gave partial correction; soil injected treatments showed continued improvement over time. Tissue analysis for Cu correlated well with the degree of distortion and dieback in the trees. Critical Cu levels in the walnut kernel were 4 ppm and 3 ppm in the leaf. Kernel and leaf tissue levels were highly correlated. Shriveling of the kernels was the main nut quality symptom associated with Cu deficiency. High rates of foliar or a combination of foliar and soil treatments may give the best results in young trees. Once trees are in production, the standard yearly Cu program for walnut blight control should provide adequate Cu deficiency correction.