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Richard P. Buchner

California walnut farmers compete with pests and diseases to produce an economically viable crop. Current control strategies work reasonably well for most pest and diseases. However, the future of these techniques is a matter of speculation. This presentation describes current production practices and potential alternatives to “traditional” pest and disease control. Codling moth, walnut husk fly, mites, navel orangeworm, aphids, and scale are typical insect pests that have an impact on California walnut production. Spray decisions using Integrated Pest Management, beneficial insect releases, mating disruption, insect growth regulators, and orchard sanitation offer potential alternatives. Major diseases include: Phytophthora crown and root rot, crown gall, oak root fungus, and walnut blight. Control options include careful site selection and orchard management, resistant rootstocks, competitive bacteria for crown gall control, and copper compounds for walnut blight suppression. Weed growth is related to the amount of light reaching the orchard floor. Mature trees often shade the orchard floor, subsequently reducing the need for weed suppression. Herbicides are typically used for vegetation control. Choice of irrigation system, cultivation, mowing, cover crops, and flaming offer potential alternatives either alone or in combination with conventional herbicides.

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William H. Olson and Richard P. Buchner

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 (Phytophthora spp). Both copper resistant and copper sensitive strains of the walnut blight bacterium are best controlled with combinations of copper bactericides and maneb instead of copper materials alone. A new computer model, Xanthocast, used to forecast the need for walnut blight treatment is under evaluation. Crown gall is managed using a preplant biological control agent and a heat treatment to eradicate existing galls. Phytophthora crown and root rot is dealt with primarily by site selection, irrigation management and rootstock selection.

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Allan Fulton, Richard Buchner, Cyndi Gilles, Bill Olson, Nick Bertagna, Jed Walton, Larry Schwankl and Ken Shackel

Covering a plant leaf with a reflective, water impervious bag ensures that equilibrium is reached between the nontranspiring leaf and the stem, and appears to improve the accuracy of determining plant water status under field conditions. However, the inconvenience of covering the leaf for 1 to 2 hours before measuring stem water potential (SWP) has constrained on-farm adoption of this irrigation management technique. A second constraint has been that the requirement of midafternoon determinations limits the area that can be monitored by one person with a pressure chamber. This paper reports findings from field studies in almonds (Prunus dulcis),prunes (P. domestica), and walnuts (Juglans regia) demonstrating modified procedures to measure midday SWP, making it a more convenient and practical tool for irrigation management. For routine monitoring and irrigation scheduling, an equilibration period of 10 min or longer appears to be suitable to provide accurate SWP measurements. Based on the large sample sizes in this study, we estimate that measurement error related to equilibration time for SWP can be reduced to an acceptable level [0.05 MPa (0.5 bar)] with a sample size of about 10 leaves when using a 10-min equilibration period. Under orchard conditions where tree growth and health appears uniform, a sample of one leaf per tree and 10 trees per irrigation management unit should give an accurate mean indicator of orchard water status. Under more variable orchard conditions a larger sample size may be needed. Midmorning and midday SWP both exhibited similar seasonal patterns and responded alike to irrigation events. On some occasions, midday SWP was accurately predicted from midmorning SWP and the change in air vapor pressure deficit (VPD) from midmorning to midday, but both over- and underestimate errors [to 0.3 MPa (3.0 bar)] appeared to be associated with unusually low or high diurnal changes in VPD, respectively. Hence, direct measurement of SWP under midday conditions (about 1300 to 1500 hr) is still recommended.

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Richard P. Buchner, William H Olson, Vito S. Polito and Katherine Pinney

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 pollen tube growth. Results indicate that Streptomycin inhibits pollen tube growth, which precludes fertilization. This pattern of development and timing of nut drop following Streptomycin application at full bloom is similar in all ways to unpollinated walnut flowers. Nut growth and development appear normal for 3 to 5 weeks; then nuts abort. If Streptomycin became available for walnut blight control, sprays timed to coincide with pistillate bloom and pistillate flower receptivity should be avoided.

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Stephen M. Southwick, James T. Yeager, Joseph Osgood, Richard Buchner, William Olson and Maxwell Norton

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 the highest yield efficiency of any tested rootstock. Several selections had characteristics that would make expanded planting worth considering.

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Richard P. Buchner*, Allan Fulton, Bruce Lampinen, Ken Shackel, Terry Prichard, Larry Schwankl, Sam Metcalf and Cayle Little

Ninth leaf California Chandler Walnuts (Juglans regia) on Northern California Black (Juglans hindsii) or Paradox (English/black hybrid) rootstock were irrigated to achieve three levels of Midday Stem Water Potential (MSWP). Target potentials were: 1) low water stress (average MSWP of -3.2 bars); 2) mild water stress (average MSWP of -6.2 bars); and 3) moderate water stress (average MSWP of -7.3 bars). Stem Water Potential was measured midday (12-4 pm) by placing leaves inside water impervious, light blocking foil bags. Leaves remained bagged for at least ten minutes to achieve equilibrium. Bagged leaves were removed, placed inside a pressure chamber and stem water potential was measured at endpoint. Data are presented for the 2002 and 2003 seasons. Withholding irrigation water had a significant impact on `Chandler' growth, productivity, and profitability particularly on young, vigorously growing trees. Chandler/Black appears to be more tolerant to water stress compared to Chandler/Paradox For Chandler on Paradox, water stress significantly reduced growth, yield, price per pound, percent edible kernel, and resulted in darker kernels. In addition, water stress significantly increased the total percent offgrade. Withholding irrigation does not appear to be a good strategy in young, vigorously growing `Chandler' orchards. Mature trees and trees grafted onto Northern California black rootstock may be more tolerant of moisture stress.

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Richard P. Buchner, Joseph H. Connell, Brent A. Holtz, Jeffery L. Olsen, William H. Olson and William Reid