Pistachio

Pistachio (Pistacia vera L.) is the only species within the genus Pistacia with edible fruit. It originated in the south-central Asia regions of Kyrgyzstan, Turkmenistan, Tajikistan, Uzbekistan, and Kazakhstan. From there, it was expanded to Syria, Europe, and only recently to the U.S., South America, Australia, and China. In 2021-2022, the U.S. produced 67% of the world’s pistachios, followed by Iran (17%) and Turkey (11%). Other Pistacia species are used as rootstocks, i.e., P. terebinthus, historically used under rainfed conditions as it is the most rustic one, yet it is highly susceptible to Verticillium wilt, the deadliest pistachio disease. The introduction of resistant P. integerrima rootstocks managed this disease. Today, the primary rootstocks are hybrids of P. atlantica and P. integerrima. Currently, they have the best salt, cold, and Verticillium resistance, vigor, and productivity. Pistacia species are recalcitrant to rooting and historically have been propagated as seedlings. Recently, micropropagation has resulted in rapid propagation of clonal rootstocks. Pistachio is hardy, xerophilous, and tolerates drought, high temperatures, and salinity. The main environmental limitations for pistachio production are the low number of chilling units during winter, low heat during summer, high relative humidity, and waterlogging. High relative humidity favors the spread of diseases attacking the foliage. The most devastating disease is Botryosphaeria panicle and shoot blight, which has caused significant crop losses. Another major disease is Septoria leaf spot that causes severe premature defoliation and weakens the trees. Waterlogging can lead to abiotic (hypoxic soil condition) and biotic stress, by increasing susceptibility to Phytophthora and Armillaria root rots. Pistachios are commonly planted on 5.2 x 6.0 m spacing with 308 trees per ha. Since pistachio is dioecious, 1 male tree is planted to every 24 female trees, every fifth row every fifth tree. Synchrony of bloom time between male and female cultivars is essential. The scion is T-budded in early summer. Young trees are trained as an open vase with upright shape and clear trunk to facilitate mechanical harvesting and induce lateral branching in this apically dominant tree. Mature pistachio pruning is less demanding and severe than other fruit tree crops. Care should be taken during heading cuts, to be conducted above at least one vegetative leaf bud to retain the upright habit of the tree. Pistachio is often planted on marginal soils and under rainfed conditions. Irrigation can substantially increase yields, but pistachio’s full evapotranspiration requirement is very high (about 1270 mm per season per year) and often not available where this crop is grown. Regulated deficit irrigation can be successfully applied through reduction of irrigation during shell hardening. Salinity levels above 6-8 dS m-1 will significantly impact productivity, but still pistachio is among the most salt-tolerant tree crops. The fruit (drupe) grows following a typical double sigmoid pattern: first, the pericarp (hull) and the endocarp (shell) rapidly expand; then shell lignification starts; and in July, the embryos (kernels) start to grow. This last period is critical for pistachio since the growing kernels compete with other organs for energy, metabolites, and nitrogen, and deplete reserves of the shoot. In a year with high yield, this competition will precipitate reproductive bud abscission (up to 90% of the buds will drop) impacting next year’s production. For this reason, pistachios tend to alternate between high and low crop years. Fruit ripens at the end of summer and is characterized by the development of separation of the suture line, leading to seed dehiscence (shell split) and the reduction of chlorophyll in the hull and the kernel. Timely harvest is essential for high quality nuts. Nuts harvested too early are non-split and immature, often including an undeveloped kernel. Problems associated with late harvest are hull cracking, shell staining, hull and kernel deterioration, reduced sensorial properties, fruit shedding, mechanical injurie,s and insects’ attacks. Late harvest may lead to kernel decay by Aspergillus spp. that contaminates the early-split and hull cracked nuts, leading to aflatoxin contamination, which is managed at a certain extent with the use of atoxigenic strain technology. As the most damaging insect attacking pistachio, Amyelois transitella, navel orange-worm (NOW), third generation emerges in early to mid-August and lays eggs in the sutures of the early-split nuts; it can be controlled by winter sanitation and early harvest. Harvest is generally fully mechanized with different types of trunk shakers. In California, side-by-side harvesters are used, with two separate machines, capable of semicontinuous harvest that takes only 1.8 min per tree. Harvested nuts should be brought to the transformation facility as fast as possible to minimize shell staining, a common defect in pistachios. Other defects that reduce pistachio value are non-splitting, nuts with hull adhering to the shell, insects damage, blank percentage, and small size. Once nuts reach the transformation facility, they are separated from the hulls and dried to 4% to 6 % moisture content. Dried nuts can be held at 20°C and 65% to 70% relative humidity for up to 1 year. Pistachio is sold as a snack, with the shell roasted and salted, or as a kernel for the processing industry.