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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.
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.