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  • Author or Editor: Kitren Glozer x
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Production of extra-early cling peach varieties in California typically results in a $988 per hectare loss for Sacramento Valley growers, based on a 2004 University of California cost analysis study. This net loss is due to a number of factors, including lower yields than late-harvested peaches; and pruning, thinning, and harvest labor. The estimated cost per hectare to hand-thin extra-early varieties is $1515, which is 31% of all cultural costs. A conservative estimate for machine thinning with transportation costs would be $136 per hectare, a cost savings of about 90%. Machine thinning operates at about 200 trees per hour with two persons (operator and supervisor), compared to four to six trees per hour with two hand-thinners. In recent years, equipment to mechanically thin and harvest has become more sophisticated. We evaluated different types of equipment and settings in two experimental orchards trained in two pruning systems in 2005. We compared effects of crop load and variability in fruit development at time of shaking, as well as the timing of shaking with respect to fruit growth after bloom and compared mechanical and hand thinning. We found an optimum “window” for mechanical thinning based on fruit size and crop load, with tree architecture less important than these factors. Machine-thinning with follow-up hand-thinning reduced the thinning time by 30% to 41%. When machine thinning without follow-up hand-thinning was compared to hand-thinning alone, total yield was improved by 22% and salable yield was improved by 18% in the machine-thinned trees. The net increase in undersized yield in the machine-thinned only treatment was less than 6%.

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Pear growers in California's Sacramento River Delta and, to some extent, other pear-growing areas in California, use dormant oils for pest control and dormant bud growth stimulation. It is generally believed that well-timed applications can advance flowering, improve uniformity of flowering and fruit ripening, and improve vegetative budbreak. Traditionally, dormant oils have been applied in late December to mid-January, based on experience and calendar date. However, bud development and full bloom dates may differ from year to year, with variable weather cycles and chill accumulation experienced by the plant. In the 2004–05 dormant season, some dormant oil applications timed at intervals calculated by chill portions (defined by the Dynamic Model) advanced and compressed the bloom period. Fruit size (diameter and weight) and total estimated yield were improved by dormant oil treatments applied within a certain range of chill portion accumulation without reduction in total number of fruit per tree, while the percentage of undersized fruit was decreased by 65% to 83% when compared to the untreated control. The use of chill portions and the Dynamic Model to time applications of dormant oil appears to benefit fruit quality. Although chill was not limiting in the trial dormant season, there may be benefit even in years when chill accumulation is adequate.

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While rest-breaking agents have become commonly used in California cherry production, application timing continues to involve a certain amount of uncertainty from year to year. In order to use any chilling model adequately and thereby schedule rest-breaking treatments, both the beginning point of dormancy and the beginning point of chill accumulation must be understood. One method of testing dormancy onset is tree defoliation, which may be used to alter the pattern of budbreak and regrowth in spring. Defoliation is used in many tropical and subtropical fruit-growing regions to promote budbreak and flowering in species that are not adapted to less than adequate chilling conditions. Recent trials in California compared hand defoliation to applications of urea and zinc sulfate to determine effects on budbreak and flowering of sweet cherry, as well as to better identify entry into dormancy. Chemical applications were at concentrations lower than those used to effect complete defoliation. We found that chemical applications tended to advance bloom and that the most effective timings were consistent, based on chill portion accumulation and the Dynamic Model. In one of two years, chemical treatments tended to decrease floral bud death and increase fruit set when compared to hand defoliation and untreated trees.

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Use of rest-breaking chemicals may partially substitute for chill requirement in “French' prune. Many California prune growers use oil in the dormant season to tighten and advance bloom, with application timing judged by experience and calendar date. Other rest-breaking agents have become commonly used in California cherry production and their application is generally timed by chill portion accumulation, calculated by the Dynamic Model. We evaluated the effects of treatments of dormant oil or CAN17 (calcium ammonium nitrate) + Entry on budbreak and bloom progression in `French' prune with applications timed at regular intervals. While most treatments improved fruit set and reduced reproductive bud death, an optimum range for both types of rest-breaking treatments was found for advancement and compression of bloom. All rest-breaking treatments advanced fruit maturity equally, compared to the untreated control, as measured by fruit firmness. Although chill hour (hours ≤7°C) calculations might also be used for timing these treatments, when chill portion and chill hour accumulations are compared for the 2004–05 dormant season at several different sites, differences from site-to-site are small for chill portions, and much greater for chill hours. This fact supports experimental evidence from numerous California trials in sweet cherry in which rest-breaking treatment timings based on the Dynamic Model tend to be more consistent than the timings based on the “chill hour” model.

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Many commercially grown stone fruit including apricots (Prunus armeniaca L.), peaches and nectarines [P. persica (L.) Batsch], plums (P. salicina Lindl., P. domestica L.), prunes (P. domestica L.), and pluots (P. salicina × P. armeniaca) have a tendency to produce high numbers of flowers. These flowers often set and produce more fruit than trees can adequately size to meet market standards. When excessive fruit set occurs, removal of fruit by hand thinning is necessary in most Prunus L. species to ensure that remaining fruit attain marketable size and reduce biennial bearing. Over the years there have been numerous attempts to find chemical or physical techniques that would help to reduce the costs associated with and improve efficiencies of hand thinning, however, alternate strategies to hand thinning have not been widely adopted for stone fruit production. In the past 10 years, several chemical treatments have shown promise for reducing hand thinning needs in stone fruit. Management of flowering by chemically reducing the number of flowers has been particularly promising on stone fruit in the Sacramento and San Joaquin Valleys of California. Gibberellins (GAs) applied during May through July, have reduced flowering in the following season in many stone fruit cultivars without affecting percentage of flowers producing fruit. As a result, fruit numbers are reduced, the need for hand thinning is reduced and in some cases eliminated, and better quality fruit are produced. There are risks associated with reducing flower number before climatic conditions during bloom or final fruit set are known. However, given the changes in labor costs and market demands, the benefits may outweigh the risks. This paper reviews relevant literature on thinning of stone fruit by gibberellins, and summarizes research reports of fruit thinning with GAs conducted between 1987 and the present in California. The term thin or chemically thin with regard to the action of GA on floral buds is used in this paper, consistent with the literature, although the authors recognize that the action of GA is primarily to inhibit the initiation of floral apices, rather than reduce the number of preformed flowers. At relatively high concentrations, GA may also kill floral buds. Chemical names used: gibberellic acid, potassium gibberellate.

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Moderate California winters often result in delayed, erratic or extended bloom, inadequate overlap with pollenizers, poor leafing-out, low fruit set, and irregular fruit maturity. In recent years, use of rest-breaking agents has become commonplace in California sweet cherry (Prunus avium L.) culture, mainly to promote earlier bloom and fruit maturity, but also to promote uniform flowering and overcome lack of marginal chilling. Variable responses by different cultivars and in different seasons may be due to different chilling requirements, despite little variation in genetic background for chill requirement in California's commercial cultivars. Other sources of variation include the activity of the rest-breaking agent used, concentration and carrier volume. A minimum amount of effective chill appears to be required for a given cultivar before rest-breaking agents can be effectively applied. This threshold, as exhibited by degree of response to treatment, can be an important indicator of when to spray. Method of measuring chill accumulation, and thus, timing of applications, varies by region and historic acceptance. California's tree fruit industry typically uses the 45 °F “chill hour” model. The Utah Chill Unit Model and the Modified 45 °F Chill Hours Model had not been thoroughly tested under California conditions nor with the rest-breaking chemicals that are in use today in California. We tested our research results against these models and the Dynamic Model developed in Israel and concluded that the Dynamic Model provided the best explanation of responses in our experimental trials. We have been developing recommendations for application of rest-breaking chemicals based on Dynamic Model chill portion accumulation.

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Prune (Prunus domestica) producers have an increased incentive to produce larger fruit. Potassium is thought to be critical in prune production and the fruit utilize large amounts of K. Growers have been fertilizing heavily with K to maximize fruit size. We conducted a survey of 16 `French' prune orchards in 1998 and 1999 growing seasons. Low (≈1.0% mid-July) leaf K concentration is associated with leaf chlorosis, early leaf drop and shoot dieback, with symptoms pronounced in the upper canopy, particularly with heavy cropping. The survey orchards were chosen to represent a range of leaf K within and among orchards, and among counties. At harvest, fruit drying ratio, dry yield per tree, and dried fruit size were determined in order to develop relationships between fruit quality and yield, and leaf K over the growing season. To date we have determined the following: 1) spring (May 1998 and April–May 1999) leaf K concentration is correlated with mid-summer (mid-July 1998) and early summer (late June 1999) leaf K concentration, respectively; 2) mid-April to mid-May leaf K concentration is being maintained by growers due to thinning and fertilizing, and to light cropping in 1998, at adequate to above adequate levels (2%) that increase through the growing season; 3) no beneficial relationship between fruit size, drying ratio or dry yield has been found with spring or summer (June or mid-July) leaf K concentration above 2%, either when evaluating all surveyed orchards together or as individual orchards.

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The California table olive (Olea europaea L.) industry relies exclusively on hand harvesting of its primary Manzanillo cultivar. Increased harvesting costs have intensified industry interest in identifying an abscission agent that can be used with developing mechanical harvesting technologies to increase removal rates. Table olives are harvested immature green at horticultural maturity but before physiological maturity. The goal of this research was to reevaluate the potential of ethylene-releasing compounds (ERCs) as olive-loosening agents and to screen additional candidates previously shown to accelerate citrus fruit abscission. Eleven compounds were screened at two separate table olive-growing sites (Fresno and Tehama counties) in California in September until Nov. 2006. Compounds were applied at various concentrations alone or in combination. Fruit detachment force (FDF) and percent fruit drop were measured and leaf loss assessed. Of the compounds evaluated, the ERC ethephon (2-chloroethyl phosphonic acid) and 1-aminocyclopropane-1-carboxylic acid were the most efficacious. In whole tree applications, concentrations of ethephon or 1-aminocyclopropane-1-carboxylic acid above 1000 mg·L−1 reduced FDF to less than 50% of the untreated control within 17 days, but leaf drop increased with increasing concentrations. Addition of 1-methylcyclopropene reduced efficacy of ethephon and delayed leaf drop. Monopotassium phosphate + ethephon (4% and 1000 mg·L−1, respectively) reduced FDF and leaf loss was equivalent to the ethephon alone treatment. Compounds such as methyl jasmonate, coronatine, dikegulac, MAXCEL, traumatic acid, and 5-chloro-3-methyl-4-nitro-1H-pyrazole were not efficacious.

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Table olives (Olea europaea) traditionally are hand harvested when green in color and before physiological maturity is attained. Hand harvesting accounts for the grower's main production costs. Several mechanical harvesting methods have been previously tested. However, tree configuration and fruit injury are major constraints to the adoption of mechanical harvesting. In prior work with a canopy shaker, promising results were attained after critical machine components were reconfigured. In this study, stereo video analysis based on two high-speed cameras operating during the harvesting process were used to identify the sources of fruit damage due to canopy-harvester interaction. Damage was subjectively evaluated after harvest. Fruit mechanically harvested had 35% more bruising and three times as many fruit with broken skin as that of hand-harvested fruit. The main source of fruit damaged in the canopy was the strike-impact of fruit by harvester rods. Implementation of softer padding materials were effective in mitigating fruit injury caused by the impact of rods and hard surfaces. Canopy acceleration was correlated with fruit damage, thus restricting improvements needed for fruit removal efficiency through increased tine frequency.

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