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Gale H. McGranahan, Demetrios G. Voyiatzis, Peter B. Catlin and Vito S. Polito

The role of pollen in abscission of pistillate flowers of Persian walnut (Juglans regia L.) cv. Serr was investigated over a 4-year period by controlled pollinations and pollen counts. Self-pollen, pollen from other walnut selections or cultivars, or dead pollen was applied at high and low doses to pistillate flowers enclosed in pollination bags. Unbagged, open-pollinated flowers and bagged, nonpollinated flowers served as controls. In all cases, presence of pollen significantly increased the probability of pistillate flower abscission (PFA). Dead pollen resulted in as much PFA as live pollen. Counts of pollen grains confirmed that PFA-type flowers had significantly more pollen than normal flowers. In the fourth year `Serr' pollen was applied to unbagged flowers of `Serr' and ten other Persian walnut cultivars, and the amount of PFA on the artificially pollinated flowers was significantly higher than on the open-pollinated flowers, while the control flowers dusted with talc or pine pollen had almost no PFA. These results clearly indicate that excess pollen is involved in pistillate flower abscission in `Serr' walnut and suggests that other cultivars may also be sensitive to pollen load. This phenomenon may have implications in the biology of selfing and evolution.

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Keith E. Woeste, Gale H. McGranahan and M.N. Schroth

Walnut blight of English walnut (Juglans regia L.), incited by Xanthomonas campestris pv. juglandis (Pierce) Dowson, causes significant crop loss in California. To assess levels of resistance in walnut germplasm, leaves and nuts of mature walnut genotypes were inoculated with X. campestris pv. juglandis. Significant differences were found among cultivars in size and frequency of lesions on leaves and in frequency of abscission of diseased leaves. Cultivars also varied in frequency of abscission of nuts following infection and in marketability of infected nuts. Afthough there was considerable variation in disease levels over 2 years, leaves of PI 159568 consistently received significantly higher disease ratings than leaves of `Chandler' or `Adams'. Nuts of `Adams', `Payne', PI 18256, and `Sinensis 5' abscised less frequently following inoculation than nuts of other cultivars. In addition, the quality of infected nuts that did not abscise was consistently better for PI 18256 and `Sinensis 5'. The rank of cultivars for levels of disease in inoculated leaves was not significantly correlated with the rank of cultivars for frequency of infestation of dormant buds associated with infected foliage. The apparent resistance of walnut germplasm may be affected by the abscission or necrosis of infected tissues.

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Janet Caprile and Steve Grattan

In areas of California with a high incidence of walnut blackline disease, walnut orchards are increasingly being planted on English walnut (Juglans regia) rootstocks that are tolerant to the virus. There is limited documentation on the salt tolerance of this rootstock. This work was done to quantify the response of English walnut rootstocks to high boron (B), chloride (Cl), and sodium (Na) and to compare this to the more common rootstocks, Northern California Black (J.hindsii) and Paradox (J. hindsii × J. regia). The trial was configured as a randomized complete-block design with 20 plots. Plots consisted of three proximate, matched `Chandler' trees, each on a different rootstock. Leaf samples over a 2-year period showed that trees on the English rootstock had a significantly higher salt uptake than trees on Paradox, which had a significantly higher uptake than trees on Black. Bark cores showed little difference in B uptake between any of the rootstocks. Bark cores also showed that all rootstocks accumulated significantly more B than the scion portion of the tree, indicating that B transport may be limited by the graft union. Trees on English rootstock had significantly more B in the scion bark cores than trees on either Black or Paradox, indicating that rootstocks with a J. hindsii parent may be better at reducing salt transport across the graft union.

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Asadolah Aslani Aslamarz, Kourosh Vahdati, Majid Rahemi and Darab Hassani

The objective of this work was to determine the chilling and heat requirements of Persian walnut cultivars and genotypes using excised twigs. The experiment was carried out from Nov. 2006 and 2007 to Mar. 2007 and 2008. One-year-old twigs were prepared from four cultivars and four domestic genotypes of Juglans regia L. After leaf fall, the twigs were taken and placed in plastic bags and kept at 4 ± 1 °C to stimulate 400 to 1500 chilling hours. After chilling, the excised twigs were transferred to the greenhouse with a natural photoperiod and a temperature from 18 to 27 °C. The evaluation of budbreak was made three times a week and the number of accumulated growing degree hours (°C) was determined until the buds reached the balloon or green tip stage. The chilling requirements were lowest (400 h) for catkins and highest (1000 h) for lateral buds. The Serr cultivar and ‘Z30’ genotype had the lowest chilling requirements (650 and 650 h). ‘Lara’, ‘Z63’, ‘Z53’, ‘Pedro’, and ‘Z67’ showed intermediate chilling requirements with values of 900, 900, 800, 750, and 750 h, respectively. Finally, ‘Hartley’ completed its dormancy after an accumulation of 1000 h, being the walnut cultivar with the highest chilling requirement in our study. As the final result, the cultivars and genotypes were classified into three groups based on their heat requirements: low requirement (‘Z30’ and ‘Serr’), medium requirement (‘Z53’, ‘Z67’, ‘Lara’, and ‘Pedro’), and high requirement (‘Hartley’ and ‘Z63’).

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William H. Krueger

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.

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Bruce D. Lampinen, Vasu Udompetaikul, Gregory T. Browne, Samuel G. Metcalf, William L. Stewart, Loreto Contador, Claudia Negrón and Shrini K. Upadhyaya

A mobile platform was developed for measuring midday canopy photosynthetically active radiation (PAR) interception in orchards. The results presented are for almond (Prunus dulcis) and walnut (Juglans regia), but the mobile platform can be used in other orchard crops as well. The mobile platform is adjustable to accommodate orchard row spacing from 4.8 to 7.8 m and is equipped with a global positioning satellite (GPS) receiver and radar for positional assessment as well as three IR thermometers for measuring soil surface temperature. Data from the mobile platform are logged at 10 Hz and stored on a data logger. Custom software has been developed to process the data. The mobile platform was used extensively for mapping midday canopy PAR interception in almond and walnut orchards in 2009 and 2010. The mobile platform produced comparable results to those collected with a handheld light bar with the advantage of being able to cover much larger areas and compare these data to mechanically harvested yield data over the same area. For almond orchards, midday canopy PAR interception peaked at ≈70% at an orchard age of ≈12 years. For walnut orchards, midday canopy PAR interception continued to increase to ≈15 years of age and peaked at a level above 80%. The mobile platform was also able to follow seasonal development of midday canopy PAR interception in young and mature orchards. This technology has potential for evaluating new varieties in terms of productivity per unit PAR intercepted, in evaluating hand pruning or mechanical hedging practices in terms of impact on PAR interception/productivity as well as evaluating effectiveness of insect or disease management treatments. It also has potential as a reference point for grower self-assessment to evaluate orchard canopy development compared with other orchards of similar variety, spacing, etc. Finally, this technology could be used as ground truth referencing for remotely sensed data.

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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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Janine K. Hasey, Dave Ramos, Warren Micke and Jim Yeager

In a comparison of six walnut rootstocks either nursery-grafted or field-grafted to `Chandler' (Juglans regia), the highest-yielding trees after 9 years are on either seedling or clonal Paradox rootstocks. Trees growing on both Paradox rootstocks had higher yield efficiency than trees on the black rootstocks in both 1995 and 1996. Since 1993, relative tree size based on trunk circumference has not changed: southern California black (J. californica), seedling Paradox and northern California black (J. Hindsii) have remained significantly larger than clonal Paradox, Texas (J. microcarpa) or Arizona (J. major) black rootstocks. The smaller size of clonal as compared with seedling Paradox trees might be explained by a delay in field grafting success. Although both northern and southern California black rootstock trees were significantly larger than clonal Paradox trees, they did not differ significantly in yield and had significantly lower yield efficiency in 1996. Clonal Paradox trees have significantly smaller nut size than northern California black rootstock trees that can be explained by its higher yield efficiency. An adjacent trial planted in 1991 compares micropropagated `Chandler' on its own root vs. `Chandler' on seedling Paradox rootstock. In 1995 and 1996, own-rooted `Chandler' had significantly greater trunk circumference, yield, and yield efficiency than did `Chandler' on Paradox rootstock. Many of the trees on Paradox rootstock are growing very poorly compared to the own rooted trees. This could be due to diversity within the Paradox seed source. If own-rooted `Chandler' trees become commercially available, they may have potential in areas where other rootstocks are undesirable because of hypersensitivity to cherry leafroll virus.

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William H. Olson and David Ramos

The Persian or English walnut (Juglans regia) is widely cultivated, with commercial production in France, Italy, Turkey, China, and the United States. Practically all of the U.S. production of Persian walnuts is in the central valley of California, which now has about 169,000 bearing acres with an average yield of around one and one-third short tons per acre. Many orchards produce over two tons, and three tons per acre are common in many modern plantings. Walnuts have two major outlets: the exported in-shell market (about 35% of production) and the domestic shelled market (about 68% of production). A cooperative handles about half the crop, while several independent handlers sell the remainder. Walnuts are sensitive to both low and high temperatures. Temperatures in excess of 90 °F will begin to sunburn nuts. Freezing temperatures will damage tender growth in the spring and fall. Dormant trees can tolerate 15 °F without injury if soils are moist. Dry winter soils and cold temperatures cause winter kill. A minimum of 800 hours of winter chilling are required to avoid delayed bud break and poor crops. Walnuts do best on deep, medium textured, well drained soil. Under these conditions, both rootstocks, the Northern California Black Walnut (J. hindsii) and Paradox (J. regia x J. hindsii), do well. Under less favorable soil conditions, Paradox is the preferred rootstock. A mature walnut orchard requires 4 to 4.5 acre-feet of water per acre per year if the trees are to produce the maximum number of high quality nuts possible. Hartley, preferred for its in-shell quality, is the leading cultivar, with about 30% of the acreage. In recent years, the Chandler variety has accounted for most new plantings. It is known for high kernel quality and yields. Yield factors include: bearing habit, bearing area, flower differentiation, fruit set, nut size, kernel percentage, and kernel quality. Major insect pests of walnut include codling moth, navel orangeworm, and walnut husk fly. The major diseases are walnut blight, deep bark canker, Phytophthora, and blackline. Major research efforts include the walnut breeding program, which includes blackline and Phytophthora susceptibility of new cultivars and root-stocks, codling moth and walnut husk fly control, epidemiology and control of walnut blight, pruning and planting strategies, and clonal propagation.

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Xiaobo Song, Shengke Xi, Junpei Zhang, Qingguo Ma, Ye Zhou, Dong Pei, Huzhi Xu and Jianwu Zhang

Walnut ( Juglans regia ) is one of the oldest cultivated fruits and an economically important tree, now widely cultivated in temperate regions for both its nuts and timber ( Chen et al., 2014 ). In China, this species has been cultivated for at