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R. Scott Johnson, Claude Phene, and Charles Medawar

Six irrigation strategies were imposed on a block of O'Henry peach trees irrigated by fanjets. Treatments received different percentages of ET during the various stages of fruit growth and postharvest. ET was estimated by a large weighing lysimeter containing 2 trees and located in the center of the block. Fruit diameters were measured weekly and final fruit weights were determined at harvest. Adjusted fruit weights were estimated by statistically adjusting each treatment to the same fruit load.

Adjusted fruit weight correlated well with soil water content during the month before harvest but not during early stages of fruit growth. Treatments which applied 50% ET during early stages of fruit growth showed reduced fruit size at that time. However, with applications of 150% ET during the final fruit growth stage, fruit size recovered. Adjusted fruit weight also correlated with measures of tree water status including midday leaf water potential and canopy temperature.

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R. Scott Johnson, Harry Andris, and Shanti Handley

Foliar urea sprays offer an alternative to soil applied fertilizers which could greatly reduce the potential for nitrate pollution of groundwater, The approach in the past has been to apply relatively small doses of urea in order to minimize leaf phytotoxicity. Our approach is to apply relatively large doses in the fall when leaf phytotoxicity is not a serious concern. Results on peach trees in the field indicated rapid uptake of foliar applied solutions of 4.3 to 8.8% urea (w/w) (2.0 to 4.0% N). About 80-90% of the urea was absorbed by the leaf within 24 hours. Leaf N levels suggest the majority of this urea was translocated from the leaf into the tree within 1 week despite damage to the leaf. There were no negative effects on flowering, fruit set and production in the following year as long as a very low biuret formulation of urea was used.

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Franz J. A. Niederholzer and R. Scott Johnson

Urea foliar sprays may be a more efficient and environmentally sound alternative to soil applied fertilizer N in the postharvest period in tree crop production in California. While tree crop sulfur (S) status can interact with tree N status to affect growth, we know of no study assessing tree crop leaf N and S dynamics following fall (postharvest) foliar urea applications. We conducted a field study to measure temporal dynamics of leaf N and leaf S (% dry weight basis) following postharvest urea sprays on prune (Prunusdomestica) and almond (Prunus dulcis). June-budded nursery stock prune (`French' on Myro 29C) and almond (`Price' on Lovell) trees were sprayed to dripping with 6.5% (w/w) and 10% (w/w) standard urea solutions, respectively. Prunes were sprayed on 1 Oct. 2003 and almonds on 18 Nov. 2003. Leaf samples were taken over a 3-week (almond) or 8-week (prune) period, beginning just before treatment. Foliar urea sprays significantly increased prune (23%) and almond (14%) leaf N compared to untreated control within 8 days of application. This affect was transient, as there were no differences in leaf N concentrations between treated and untreated trees at final leaf sampling. Urea sprays did not affect almond leaf S concentration relative to untreated trees. Prune leaf S was significantly reduced compared to untreated trees 8 days after treatment, but only on that sampling date. Remobilization of S from the leaves of control trees of either species was not apparent.

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Kevin R. Day and R. Scott Johnson

Minimal dormant pruning after the first and second growing seasons, followed by standard pruning thereafter, improved total tree yield in the 3rd, 4th, and 5th years after planting. Trees that were pruned in accordance with standard local practice had ≈50% yield compared to minimally pruned trees in years 3 through 5. Fruit from minimally pruned trees was sgnificantly smaller, but mathematical adjustment of crop load indicated that overall yield efficiency was improved in the 3rd and 4th years for trees receiving minimal pruning.

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R. Scott Johnson and Alan N. Lakso

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R. Scott Johnson, Claude J. Phene, and Dale Handley

Generally, water stress reduces yield in annual crops. However, for mature fruit trees, this relationship may not hold in many situations, thus providing the opportunity for saving water without losing production. Indeed, even an increase in productivity may be achieved as we better learn how to manipulate processes within the tree through moderate water stress. Several areas of research have shown promising results. The reduction of irrigation after harvest of early maturing peaches and plums has demonstrated substantial savings of water with no loss of production. Peaches can suffer fruit quality problems such as doubling and deep suturing, but these can be overcome with well-timed irrigations in the previous late summer. Water stress imposed before harvest has also shown some promise. Reports from Australia have demonstrated significant increases in yield and fruit size in peach and pear, although researchers in other locations have generally been unable to replicate these results. The timing and/or rate of stress development appear to be critical factors. Under the right conditions, stress can alter the allocation of resources between vegetative and fruit growth. Before implementation of these practices can be achieved, further research will need to focus on developing good tools for measuring stress in the trees, obtaining a better understanding of adaptation of trees to rapidand slow-developing stress, documenting the effects of stress on vegetative and fruit growth during different times of the season, and understanding the interaction of stress with other factors such as fruit load.

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Kevin R. Day, Ted M. DeJong, and R. Scott Johnson

Previous research with Mayfire nectarine demonstrated that seed length can be used as a developmental marker to predict the optimum date of girdling. Four years of study indicates that seed length also appears to be an effective physiologic marker for integrating early season heat accumulation. Seed length development was more highly correlated with heat accumulation (r=0.936) than with number of days after bloom (r=0.699). However, harvest date is more accurately predicted by number of days between 12mm seed length and harvest (30±1) than by degree-days between 12mm seed length and harvest (337±21).

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Guiwen Cheng, Carlos Crisosto, David Garner, Scott Johnson, and Bob Beede

The roles of genetic makeup and `orchard environment' on fruit skin discoloration (SD) were studied on peaches and nectarines. Our data showed that the SD incidence (SDI) varied among cultivars in the same orchard and orchards with the same cultivar. Differences in SD susceptibility were examined on whole fruit and skin disks by using SD-inducing treatments including combinations of physical injury and dipping in different solutions (pH, and metal ions). High SDI was associated with physical injury, high pH, and high metal ion concentration. With the exception of N, the amount of macro- and micronutrients in unwashed fruit skin was not correlated with the SDI. Preharvest sprays influenced the SDI. The peach variety with the highest SDI had the highest total soluble phenols. Total soluble phenols and anthocyanins, however, did not show a strong relationship with the SDI among all cultivars studied. Lack of correlation with total soluble phenols does not exclude the possibility that SD may relate to specific phenol(s).