Orchard hedgerow production systems have been used successfully in fruit and nut crops in California for decades to enhance yield, particularly in the early years of production. English walnuts (Juglans regia) are compatible with hedgerow techniques under prime soil conditions but are thought to require deep well drained soil to be commercially productive. Combining the production techniques of micro-irrigation, close spacing, minimal pruning and frequent fertilization in almonds has improved yield substantially on soils exhibiting a shallow, course textured topsoil underlain with a dense clay layer. Paradox hybrid rootstock (J. regia × J. hindsii) has shown greater tolerance to root lesion nematodes and heavier textured or poorly drained soils than Northern California Black (J. hindsii). Fourteen years of evaluation (1986-99) using `Chandler' and `Howard' Ctvs English walnuts in a replicated field trial on marginal soil has shown that 1) yields of 6700 kg·ha-1 (inshell) are attainable under these substandard soil conditions 2) Paradox hybrid rootstock out-yields Northern California Black by 30% on both cultivars tested, 3) kernels of high commercial quality for can be produced for both cultivars and 4) slip plow soil modifications may not improve tree growth, yield or crop quality in drip irrigated walnut hedgerow plantings.
English walnut (Juglans regia, L.) is a monoecious species bearing staminate and pistillate flowers separately on the same tree. Walnuts are generally self-fruitful, cross-compatible and dichogamous, having incomplete overlap of pollen shed and female receptivity. It is this characteristic which led to the recommendation that about 10% of the trees in a commercial planting be a cultivar with a pollen shed period overlapping pistillate flower receptivity of the main cultivar. Excessive pollen load has been implicated in the `Serr' cultivar in pistillate flower abortion (PFA), the loss of the female flowers early in the season before fruit drop due to lack of pollination. PFA can be reduced and yield improved in `Serr' orchards by reducing pollen load. This can be accomplished by pollinizer removal, or catkin removal at the beginning of pollen shed by mechanical shaking. In years of significant bloom overlap between staminate and pistillate bloom, PFA can be further reduced and yield improved by removing `Serr' catkins. PFA occurs to a lesser extent in other cultivars such as `Chico', `Chandler', `Vina' and `Howard'. This information has led to the reevaluation of pollinizer recommendations. Research focused on optimum pollinizer levels in `Chandler', a cultivar of increasing importance to the California walnut industry, has been inconclusive. Lack of pollinizers may impact yields to a greater extent in the in the northern San Joaquin Valley and Sacramento Valley than in the southern San Joaquin Valley. In any case the previously recommended 10% appears to be excessive. Two to three percent is probably adequate to limit losses due to lack of pollination without resulting in excessive PFA, and is currently being recommended by extension farm advisors and specialists. Factors to consider when determining the number of pollinators to plant include: cultivar susceptibility to PFA, walnut pollen load in the area and local pollination and fruit set experiences.
Abstract
Principal hosts of the misletoe Phoradendron flavescens (Pursh.) Nutt. that infests black and English walnut trees in California are common cottonwood and California buckeye, but it also occurs on many other trees. The mistletoe usually becomes established in new growth or roughened areas along the older stems and crotches of branches of the host trees (2).
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 shell, nut size, nut weight, percent kernel potassium, percent light-colored kernels, percent edible kernel, percent kernel yield, or percent shriveled kernel. Trees with leaf potassium levels at or above 1.5% July leaf potassium produced 80 pounds per tree more yield than trees with leaf potassium levels at or below 1.0% July leaf potassium levels. These data indicate that good tree potassium status influences tree size and tree productivity. Also the walnut husk is an important sink for the accumulation of potassium. Currently recommended adequate potassium levels for walnut appear to be lower than what this study indicates.
The ability to remove the pistillate flowers and young nuts from precocious lateral-bearing English walnut (Juglans regia L.) cultivars during the first several years following planting would be useful both in reducing competition with vegetative growth and eliminating potential infection sites for blackline disease (cherry leafroll virus). Applications of ethephon shortly after full bloom were shown to effectively remove all or most pistillate flowers depending upon spray timing and ethephon concentration. Moderate phytotoxicity and reduced seasonal growth limit the usefulness of this technique in the field. Removal of staminate flowers (catkins) prior to pollen release may reduce the excessive pistillate flower abscission of the `Serr' cultivar. Applications of ethephon shortly before the onset of pollen shedding were shown to be ineffective in catkin removal.
In both laboratory and field experiments, excessive pollen has been found to be a major cause of pistillate flower abscission (PFA) and reduced yields of sensitive English Walnut cultivars (CVs) (especially “Serr”). In the field, PFA and reduced yields develop when substantial overlap of male and female walnut bloom occurs. PFA and poor yields can be further aggravated when pollenizing CV's have been included into an orchard to maximize pollen availability for the commercial CV Field experiments, conducted in 1992 and 1993, demonstrated that mechanically shaking trees to remove male flowers pre-bloom from either pollenizer CV's or the main CV reduced pollen load, PFA, and substantially improved yields.
Walnut husk fly (Rhagoletis completa) is a major pest of English walnuts in California. Research was initiated in 1995 to evaluate reduced-risk pesticides. The first effective control measures were found in 2001—spinosad plus bait and kaolin (Surround). Spinosad was registered for organic use in 2003 as GF-120 NF (a formulation of spinosad and bait) and Entrust (a spinosad formulation to be mixed with existing baits). GF-120 is recommended for low to moderate populations while Entrust + bait is recommended for higher populations. Entrust + bait is price competitive with the standard malathion + bait. Kaolin is also effective for walnut husk fly control; however, control is dependent upon adequate coverage. In 2004, varietal susceptibility was assessed. The cultivars `Payne', `Serr', `Pedro', and `Hartley' were found to be highly susceptible while `Chandler' and `Howard' were somewhat resistant. Infestation in `Chandler' began later in the season than in other cultivars. Quality parameters were also measured. Samples of 100 infested and uninfested `Tulare' walnuts were compared at harvest. There were significant differences in adhering hull, percentage of edible yield, and relative value between infested and uninfested nuts. The relative value of infested nuts was 28% less than uninfested nuts. There were minimal differences in percentage of mold and kernel color. Walnuts sprayed with kaolin had larger nuts, fewer external defects, and an earlier harvest timing.
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.
Abstract
Spray application of 2-chloroethylphosphonic acid (ethephon) to walnut trees caused dehiscence of hulls and enabled complete nut removal with a single mechanical shaking operation as much as 3 weeks earlier than normal. Walnuts were hullable and the quality was generally superior to that of the controls. In some instances considerable leaf fall resulted from the treatment.
Abstract
The time of initiation of pistillate flowers was determined for each of 7 cultivars of walnut (Juglans regia L.). Flower formation (flattened apex) was evident during the first 2 weeks of June for ‘Chico’, ‘Vina’, ‘Serr’, ‘Tehama’, and ‘Pedro’ and mid-July for ‘Hartley’ and ‘Franquette’. Initiation of floral parts also occurred at different times depending on the cultivar. Pistil development began in February for ‘Chico’ and in April for ‘Franquette’. Fertilization occurred throughout April for most of the cultivars, but in the middle of May for ‘Franquette’.