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Brent A. Holtz

Pistachio (Pistacia vera) was successfully introduced into California and initially touted as a tree nut crop with no disease or insect pests. Unfortunately, these expectations were dashed as a number of diseases and pests followed commercial plantings, making plant protection practices integral to production. Verticillium wilt (Verticillium dahliae) devastated early plantings but is now controlled with the use of resistant rootstocks. Botryosphaeria blight (Botryosphaeria dothidea) and alternaria late blight (Alternaria alternata) are recently arrived foliar fungal diseases that blight fruit clusters and defoliate trees, respectively, and multiple fungicide applications are needed for control. The conversion to low volume irrigation systems, specifically to drip or buried drip, has reduced disease. Pruning out botryosphaeria blight infections has reduced overwintering inoculum and disease, while current research aims at accurately predicting infection events to increase fungicide efficacy. A number of hemipteran insect pests have been associated with epicarp lesion: spring treatments have been replaced with dormant carbaryl and oil applications which are less toxic to beneficial insects while controlling phytocoris (Phytocoris californicus and P. relativus) and soft scale pests. Early season insect damage can be tolerated because trees compensate by maturing a higher percentage of remaining fruit kernels. Some mirid (Calocoris spp.) pests can be effectively reduced by eliminating alternate hosts in an effective weed control program. If lygus (Lygus hesperus) populations are present, weeds should not be disturbed from bloom until shell hardening to prevent movement by insects into the trees where feeding can result in epicarp lesion. Stink bugs (Pentatomidae) and leaffooted bugs (Leptoglossus clypealis and L. occidentalis) can penetrate the hardened shell and cause internal nut necrosis along with epicarp lesion. Trap crops are used to monitor pest populations in order to develop treatment thresholds. Degree-day based timing of treatments increase insecticide efficacy for the control of navel orangeworm (Amyelois transitella) and obliquebanded leafroller (Choristonuera rosaceana), but navel orangeworm populations are more effectively managed by destroying unharvested over wintering fruit. Bacillus thuriengiensis sprays, liquid-lime-sulfur, and biological control show promise in controlling obliquebanded leafroller.

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Craig E. Kallsen, Dan E. Parfitt and Brent Holtz

Alternate bearing (alternating years with high and low yields) is a prominent characteristic of ‘Kerman’ pistachio (Pistacia vera L.), the primary California cultivar. The degree of alternate bearing is described by alternate bearing index values from 0 (identical yields every year) to 1 (complete alternate bearing). Two separate and replicated trials designed to evaluate selections from a breeding program were conducted in the southwest (Kern County) and northeast (Madera County) areas of the San Joaquin Valley of California. Yields from the scion genotypes ‘Kerman’, ‘Golden Hills’, ‘Lost Hills’, ‘B5-8’, and ‘B19-1’ on PG1 rootstock were measured from 5- to 9-year-old trees in Kern County and from 5- to 7-year-old ‘Kerman’, ‘Golden Hills’, ‘Lost Hills’, and ‘B5-8’ trees on PG1 and UCB1 rootstock in Madera County. In Kern County, average annual yields among genotypes varied from a low of 208 to a high of 5273 kg·ha−1. Differences in the alternate bearing indices among genotypes were significant and ranged from 0.10 for ‘Lost Hills’ to 0.80 for ‘B19-1’. A similar pattern was observed for alternate bearing indices at the Madera County trial. In this younger trial, scion genotype had more influence on alternate bearing indices than did rootstock. Marked differences in the intensity of alternate bearing of young trees in these two trials suggest that alternate bearing might be amenable to selection in breeding programs. However, the observation that ‘B5-8’, with an alternate bearing index of 0.74, varied significantly from its female parent ‘Kerman’ at 0.36 suggests that inheritance is complex.

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Dan E. Parfitt, Craig E. Kallsen, Brent Holtz and Joseph Maranto

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C.E. Kallsen, Brent Holtz, Lou Villaruz and Chris Wylie

Mature `Kerman' pistachio (Pistacia vera L.) trees, located on the west side of the San Joaquin Valley of California on an alkaline clay-loam soil, were fertigated in 1997 and 1998 with three combinations of zinc (Zn) and copper (Cu) sulfate. Seasonal applications of Zn and Cu were injected at three separate times starting in spring and ending in summer. Efficacy of the treatments were compared by measuring the concentration of Zn and Cu in leaf tissue. Fertigated treatments were compared to trees receiving no supplemental Zn or Cu and to trees receiving a single foliar treatment of Zn and Cu each year. Soil Zn and Cu concentrations increased in the fertigated plots. Plots receiving the highest rates of Zn and Cu showed the greatest increase. However, at the recommended August leaf sampling period, only the foliar treated pistachio trees showed a significant increase in leaf Zn and Cu. The results demonstrate that after two seasons, a foliar application of Zn and Cu in April, two to three weeks after flowering, increased leaf-tissue concentrations of Zn and Cu to sufficiency levels. These foliar applications were more effective than fertigating with Zn and Cu sulfate.

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Dan E. Parfitt, Craig Kallsen, Joseph Maranto and Brent Holtz

‘Golden Hills’ is a new female pistachio (Pistacia vera L.) cultivar released by the California Agricultural Experiment Station in 2005. The nuts mature uniformly and are harvested 2–4 weeks earlier than ‘Kerman’. In-shell nut yields in the first 4 years are equal to those of ‘Kerman’, but the percentage of edible split nuts is higher. ‘Golden Hills’ flowers earlier than ‘Kerman’ and has not demonstrated symptoms of insufficient chilling in Kern Co. or in Madera Co., Calif. It appears to be a good early maturing cultivar, with uniform harvest period and little navel orangeworm infestation.

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Qiupeng Zeng, Patrick H. Brown and Brent A. Holtz

A field experiment was conducted from 1996 to 1998 to examine the effects of K fertilization on leaf K, nut yield, and quality in pistachio (Pistacia vera L.). There were six treatments, including four annual rates of K application (0, 110, 220, and 330 kg·ha-1) and three K sources (K2SO4, KCl, and KNO3). Pistachio trees exhibited highly fluctuating seasonal leaf K levels. Leaf K concentration was low (<10 g·kg-1) during spring flush, increased dramatically during fruit development, and declined rapidly after harvest. Leaf K concentration increased following K fertilization. Potassium fertilization at the rate of 110 to 220 kg·ha-1 K significantly increased nut yield and quality, but nut yield tended to decrease when the annual rate exceeded 220 kg·ha-1 K. There were no significant differences among the K sources in their effects on leaf K concentration, nut yield, and quality. The use of KCl as a K source for 3 years did not increase leaf Cl concentration. There was a significant, positive correlation between nut yield and leaf K concentration during nut fill. The critical leaf K value for optimal pistachio production determined from 3 years' cumulative data was 16.9 g·kg-1. For sustained production in highly productive pistachio orchards, we recommend annual application rates of 110 to 220 kg·ha-1 K, using K2SO4, KCl, or KNO3.

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Dan E. Parfitt, Craig E. Kallsen, Brent Holtz and Joseph Maranto

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