California Pistachios: Points To Remember Going Into Summer

    The cool spring has contributed to slower crop development. I have mentioned in many past Task Lists that spring temperatures have a greater effect on date of maturity than summer heat, because cool spring weather does not optimize carbon fixation.

    The lower rate of carbon accumulation cannot be “made up for” by hot summer temperatures because the plant has to limit its water loss for survival.

    Thus, the rate of photosynthesis does not continue to increase with temperature. In fact, it is possible that super-hot days in the summer may burn extra fixed carbon from higher respiration rates. This may all seem like science Mumbo Jumbo to you, but it is actually very important that you understand how heat affects your ability to produce a big crop.

    Spring temperatures could collectively result in some pistachio orchards not being ready to harvest until mid-September. We will know more about maturity when we determine the shell hardening date.


    Average pistachio water use (ETc) for June 1-15 is 4.00 inches and June 16-30 is 4.6 inches. As you know, it has been cool this spring, thus reducing water use by as much as ten percent. Research by Dr. Goldhamer indicates regulated deficit irrigation (RDI) during growth stage 2 (late-May to late-June) can be safely implemented at 50% of full ETc on deeply rooted trees with no adverse crop effect.

    • This would mean one rather than two June irrigations of four to five inches.
    • Be sure to meet full ETc by the beginning of nut filling.
    • Do not consider RDI if you are on shallow soil and are already struggling to adequately irrigate during kernel filling.


    Research now suggests that 1,000 pounds of dry, in-shell pistachios requires 28 pounds of actual N. About 25 pounds of N is needed for tree development. Kernel filling begins in late June, and is the most demanding sink for N.

    For that reason, I suggest that 75% of your total nitrogen management program be applied from late June to early August, when demand is greatest. Your nitrogen management program should include tissue, soil and water analysis to quantify all sources of N and insure that excessive nitrates are not accumulating in the soil from over fertilization.

    Potassium (K) uptake is also very high during kernel filling. Research by Drs. David Zeng and Patrick Brown indicate potassium applications up to 200 pounds actual K per acre applied in equal splits over the months of May through August significantly increased yield, split-nut percentages, nut weight and reduced blank and stained nuts.

    Reduced staining was associated with less alternaria leaf infections at harvest. This research was conducted on San Joaquin, Yolo and Arbuckle soil series.

    The greatest response to K fertilization was on the San Joaquin soil series, which is lower in total K and less likely to bind the applied K to the clay types in that soil. Young alluvial soils, such as those on the Westside, typically run very high in available potassium and are less likely to require as much supplementation.

    Zeng and Brown suggest the August tissue level for K should be about 1.7% for optimum plant performance. The high fixation capacity of some soils requires large K applications to saturate the soil exchange sites and increase K tissue levels.

    Growers using surface irrigation should therefore band the application. This saturates the exchange complex of the clay and provides more K in soil solution for uptake.

    Three continuous years of potassium chloride application did not elevate chloride in the leaf tissue. However, consider orchard health, soil permeability, salinity, stratification and deficit irrigation before performing large-scale KCL applications. Siddiqui and Brown calculate the annual K requirement at 25 pounds per 1000 pounds in-shell ACP weight.


    Gills mealybug crawler emergence has been slowed by cool spring weather in many locations this year. Check with your crop consultant. Early to mid-June is typically when most of the crawlers have moved out from under the adult females.

    This treatment is important in orchards with significant pistachio Gills mealybug, since control programs for the second generation have been less effective. The next opportunity for control should be in late July.

    Be sure to discuss your choice of insecticide with your processor before treating to insure there are no concerns about acceptable residues. In addition to birds, pistachio mealybug is readily spread by harvesting equipment, so growers are advised to inspect the harvesters upon arrival to minimize the need for this expensive treatment.

    Also, watch for light browning of the nut rachis and fruit from citrus flat mite. This often goes undetected until economic injury has occurred. Control is easily achieved from 30-40 pounds of dusting sulfur per acre or 15- 25 pounds of wettable sulfur. Finally, keep your eyes and ears pealed for stinkbugs and leaffooted bugs, which could become significant in June, prior to kernel development.

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