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Stephen M. Southwick, W. Krueger, J.T. Yeager, and J. Osgood

French prunes (Prunus domestica L.) on myrobalan seedling rootstock were planted in 1981 in an east-west direction with 4.9 m between rows and 2.7 m between trees on a poorly drained Class II soil in Glenn County, CA. A randomized complete block design was used with 8 trees per plot. Trees were pruned by hand to an open-center tree form or pruned by machine to a pyramid form in the dormant or summer season resulting in 6 pruning treatments. This high density system has led to high yields of good quality fruit (9.18 dry tons/acre in 1989, sized at 78 fruit per pound). Hand pruning led to higher yields, larger fruit, lower drying ratios and a greater dollar return per acre than any of the machine pruned trees. Dormant machine pruning led to larger fruit produced than those trees pruned in the summer by machine. Mechanical pruning may be possible for short time periods, but continued practice led to smaller fruit with lower yields than hand pruning. Certain locations within the tree canopy had smaller fruit size and it is within those lower locations where fruit size needs to be improved. These and additional experimental results obtained from 1987 through 1989 growing seasons will be presented.

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J. Edstrom, W. Krueger, J. Connell, W. Micke, J. Osgood, W. Reil, and J. Yeager

In 1979 a Nonpareil-Price almond orchard, was planted 2.2m × 6.7m (270 trees/acre). Four pruning treatments were imposed on the hedgerow planting at the end of the first year. 1. Interplanted: Trees trained to 3 scaffolds then standard pruned 2nd-6th years. Alternate trees were whisked back during 7th and 8th years and whisked trees removed after 9th year. 2. Permanent Hedge: Trees trained to 3 scaffolds and standard pruned throughout. 3. Two Scaffold Hedge: Heavy 2nd and 3rd year training required to form 2 main scaffolds growing into the row middles then standard pruned. 4. Unpruned Hedge: Trees trained to 3 scaffolds then no further pruning. Treatment with alternate trees whisked back had 15% reduced yield each year following whisking. Removing these heavily pruned alternate trees at the 9th year then reduced yields an additional 30%. Now, three years after removal, yield still lags by 18%. Accumulating six years yield data shows no differences between the three treatment maintained as hedgerows. However, whisking and removing alternate trees resulted in 2000 lbs less yield over the 6 year period.

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J.P. Edstrom, J. Connell, W. Krueger, W. Reil, J. Hasey, and J. Yeager

Four tree training methods have been evaluated since 1979 in California for their affect on yield of “Nonpareil” ctv. almond [Prunus dulcis (Mill.) D.A.Webb] in a tightly spaced “Nonpareil”/”Price” ctvs 1:1 hedgerow planting. Four variations of open center training began at the first dormant pruning in a 2.2 × 6.7-m spacing (667 trees/ha): 1) Temporary Hedge—trees trained to three primary scaffolds, standard pruned with alternate trees gradually whisked back to allow space for permanent trees and then removed at 8th year leaving 4.4 × 6.7-m spacing(333 trees/ha); 2) Permanent Hedge—trees trained to three scaffolds, standard pruned at 2.2-m spacing; 3) Two-Scaffold Hedge—Trees trained into “perpendicular V” two scaffold configuration, standard pruned at 2.2-m spacing; 4) Unpruned Hedge—Trained to three scaffolds then left essentially unpruned at 2.2-m spacing. Replicated yield data accumulated over 15 years shows no difference in production between the three permanent 2.2-m hedgerow methods. Yield for the Temporary Hedge, however, declined 30% the year following alternate tree removal. Adequate canopy expansion resulted in some regained nut production, but yields never recovered and remain 20% below the permanent hedge treatments 13 years post-removal. Observations indicate considerable loss of fruitwood has occurred in the lower canopy of all three 2.2-m hedge treatments, especially in the Unpruned but good commercial production has been maintained at 2400 to 3000 kg/ha The size of almond kernels was not affected by training method. Trunk circumference was affected by treatment. Trees in Temporary Hedge plots grew sustantially larger after alternate tree removal than trees in all 2.2-m hedge treatments that were equal in size.

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J.H. Connell, F. Colbert, W. Krueger, D. Cudney, R. Gast, T. Bettner, and S. Dallman

A well maintained orchard floor is critical for insuring year-round orchard access and a clean almond harvest operation. This study compared three methods of orchard floor vegetation management over a 4-year period. The objective of this study was to evaluate cost effective vegetation management programs for difficult to control summer annual weeds while maintaining the population of desirable winter annual species. Common purslane (Portulaca oleracea) can be a major summer weed problem that interferes with almond harvesting operations. A low rate residual herbicide program controlled purslane more consistently than mechanical or chemical mowing programs. Desirable winter weed cover was preserved in all three management systems. The costs for each program were similar; however, there was a reduction in the number of operations required for both chemical mowing and low rate residual programs compared to the mechanical mowing program.

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K.A. Shackel, B. Lampinen, S. Southwick, W. Olson, S. Sibbett, W. Krueger, J. Yeager, and D. Goldhamer

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Kenneth A. Shackel, B. Lampinen, S. Southwick, D. Goldhamer, W. Olson, S. Sibbett, W. Krueger, and J. Yeager

Prunes trees are believed to be relatively tolerant of water stress, and because prune fruit are dried, a low fresh to dry weight ratio of the fruit will reduce energy requirements for fruit drying and will represent an economic benefit to the grower. In previous research, we found that, under some orchard conditions, irrigation deprivation was associated with a number of economically beneficial effects, including a lower fresh to dry weight ratio of the fruit, increased return bloom, and final saleable crop yield. Analysis of these results was complicated by the effects of irrigation on alternate bearing, and the fact that tree water stress could be substantially different under different soil conditions for the same level of irrigation deprivation. Taking these factors into account, however, indicated that economic yield in prune could be maintained or increased by managing trees at a moderate level of water stress. An experiment was established to determine whether midday stem water potential could be used to guide irrigation and achieve a target level of water stress during the growing season, and whether a moderate level of water stress would be economically beneficial to prune production. By managing prune trees at a moderate level of water stress (midday stem water potential reaching about –1.5 Mpa by the end of the season) over 3 years, an average savings of 40% in applied irrigation water was obtained. Modest increases in return bloom, and an improved fruit dry to fresh weight ratio, occurred in moderately water stressed trees, although overall yield was not changed. The substantial savings in water, without reducing yield, should represent a net economic benefit to growers, depending on the price they pay for water.

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S.M. Southwick, S.A. Weinbaum, T.T. Muraoka, W.R. Krueger, K.A. Shackel, and J.T. Yeager

Leaf dry weight per leaf area (LDW/LA); weight of leaf N per unit leaf area (LN/LA); leaf dry weight (LDW); and fruit quality, particularly sugar per fruit (SF); fruit fresh weight (FFW); and fruit dry weight (FDW) were measured over a range of daily average incident photosynthetic photon flux values (PPF) (50 to 1000 μmol·s-1·m-2) in 7-year-old prune (Prunus domestics L. syn. `Petite d'Agen') tree canopies. Linear or curvilinear relationships between these leaf attributes and fruit characteristics were significant over the PPF range. Analysis of LDW/LA or LN/LA may be used to indicate tree canopy locations in which fruit size and quality is limited by suboptimal PPF.

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Kenneth A. Shackel, H. Ahmadi, W. Biasi, R. Buchner, D. Goldhamer, S. Gurusinghe, J. Hasey, D. Kester, B. Krueger, B. Lampinen, G. McGourty, W. Micke, E. Mitcham, B. Olson, K. Pelletrau, H. Philips, D. Ramos, L. Schwankl, S. Sibbett, R. Snyder, S. Southwick, M. Stevenson, M. Thorpe, S. Weinbaum, and J. Yeager

To be useful for indicating plant water needs, any measure of plant stress should be closely related to some of the known short- and medium-term plant stress responses, such as stomatal closure and reduced rates of expansive growth. Midday stem water potential has proven to be a useful index of stress in a number of fruit tree species. Day-to-day fluctuations in stem water potential under well-irrigated conditions are well correlated with midday vapor-pressure deficit, and, hence, a nonstressed baseline can be predicted. Measuring stem water potential helped explain the results of a 3-year deficit irrigation study in mature prunes, which showed that deficit irrigation could have either positive or negative impacts on tree productivity, depending on soil conditions. Mild to moderate water stress was economically beneficial. In almond, stem water potential was closely related to overall tree growth as measured by increases in trunk cross-sectional area. In cherry, stem water potential was correlated with leaf stomatal conductance and rates of shoot growth, with shoot growth essentially stopping once stem water potential dropped to between −1.5 to −1.7 MPa. In pear, fruit size and other fruit quality attributes (soluble solids, color) were all closely associated with stem water potential. In many of these field studies, systematic tree-to-tree differences in water status were large enough to obscure irrigation treatment effects. Hence, in the absence of a plant-based measure of water stress, it may be difficult to determine whether the lack of an irrigation treatment effect indicates the lack of a physiological response to plant water status, or rather is due to treatment ineffectiveness in influencing plant water status. These data indicate that stem water potential can be used to quantify stress reliably and guide irrigation decisions on a site-specific basis.