When the last truckload of pistachios rolls out of the orchard, it’s not the time to relax—it’s time to prepare for a successful harvest next year.
Guidelines for postharvest pistachio management by University of California (UC) Davis researchers cover all postharvest activities necessary to ensure a quality crop for the next year. Pest control and crop advisors also have their own checklists that cover specific challenges and growing conditions in their individual orchards.
Evaluate IPM Plan
Since insect pests and diseases are often the cause of poor yields and quality in a nut crop, a postharvest evaluation of the current integrated pest management (IPM) plan for an orchard is in order. Spray application times, additional control activities and monitoring records prior to control and after should be reviewed.
Orchard sanitation remains the cornerstone of pest management in pistachios, as it is proven to reduce numbers of navel orangeworm (NOW) in an orchard, making additional control measures more effective. Sanitation has been a challenge in some pistachio orchards due to the physical shape of pistachio shells and uneven floors that make mummy removal difficult. Smoothing out ruts to remove hiding places for nuts can make sanitation more effective. Shaking and removing or destroying unharvested nuts reduces overwintering sites for navel orangeworm as well as inoculum sources for Botryosphaeria panicle and shoot blight. Pest control advisors will evaluate the NOW pressure by cracking mummy nuts to look for larvae.
Scouting for mealybug infestations should also be done postharvest. Infestations of Gill’s mealybug are easy to find from early fall through mid winter when numbers are at their highest. Before trees become dormant, look for sooty mold on leaves and for mealybugs within the clusters. Once leaves have fallen, look for white aggregations of mealybugs on the tree trunks and the undersides of main scaffolds. If mealybugs are found, record the locations for evaluation the following spring.
The UC guidelines also recommend no postharvest treatments in the orchard because this is when predators are most active, no damage occurs to the crop in winter, and there is already very high winter mealybug mortality.
Fungal Infections
Fungal infections in trees become apparent after harvest. UC IPM guidelines recommend scouting orchards for infected, dead and dying branches soon after harvest.
Pruning out and destroying these affected branches reduces inoculum sources of Botrytis blossom and shoot blight and Botryosphaeria panicle and shoot blight. Depending on the number of infected trees in an orchard block, the prunings should be shredded or removed from the orchard site.
In late August through September, Botryosphaeria infected fruit are covered with pycnidia—black structures containing fungus spores. They have a silvery color in contrast to the non-invaded blighted fruit, which are brown. Infections on leaves also start as small black lesions that coalesce and cause leaf flight. From August through October, infected trees will have large necrotic lesions on leaves of male and female trees.
When disease incidence is low, pruning blighted shoots and panicles, shoots with cankers, dead and dying wood, and removing infected wood from the orchard can help reduce or eliminate inoculum for a few years. During late summer, after harvest, and during dormancy infected areas should be pruned two inches past the blighted margins.
Removing and destroying unharvested nuts and mummies as soon as possible after harvest will reduce sources of inoculum. Orchard sanitation can also help reduce the incidence of Botrytis blossom and shoot blight, another fungal disease in pistachio.
Pruning blighted shoots, shoots with cankers and dead or dying branches postharvest will reduce the sources of inoculum.
Weeds
Weeds, especially in younger orchards where canopies have not completely shaded out the row middles, can become established if not kept under control. Weed species in an orchard need to be identified so the correct timing and control materials can be determined.
Surveying for weeds in your orchard postharvest accomplishes two things. Summer weeds that escaped the current weed control program can be identified and adjustments can be planned to control these species next year. Fall monitoring will also pick up any winter species that are emerging.
Ideally, tree rows are weed-free, whereas weeds growing in row middles may be beneficial in reducing erosion, soil compaction, water runoff, and sediment runoff to creeks and streams that ultimately impairs water quality. However, perennial weeds are problematic and should be kept from establishing in row middles.
The postharvest weed survey can be done after the first rains of the fall. Check to see the effectiveness of any preemergent herbicide applications. Check the resident weed/ground cover in row middles for perennial seedlings. Finally, record weed infestation on a weed survey form and use the map to show areas of problematic weeds.
Nutrition
Finally, a word about postharvest nutrition. Meeting a tree’s nutritional needs prior to dormancy can mean better yields at the next harvest. After harvest, the tree’s root hairs are foraging for nutrition to enhance photosynthesis and gain mass for carbohydrate storage. Building structure in those root hairs requires a lot of calcium. Certified crop advisor Rich Kreps said that a good postharvest program may seem a bit expensive, but an increase in production and orchard health can greatly increase that return on investment.
Kreps emphasizes that it is imperative to address calcium (Ca) and two of the big three nutrients (phosphorus P and potassium K) to enhance an early pistachios harvest nutrition program.
“ To simplify it, P is the energy producer, K moves the nutrients and Ca will allow your soil to flocculate (add pore spaces) while giving the needed nutrient for cell wall construction as your roots flush.”
Combined with proper irrigation strategies, Kreps added that trees would have the ability to quickly recover and exacerbate photosynthesis. After rehydration and assimilation if these three nutrients, micronutrient deficiencies can be addressed with foliar applications and fertigation. Make sure those three nutrients are in plant ready forms and applied to the root zone with a carbon source. Your trees will respond quickly.
To find an interactive nitrogen management spreadsheet go to http://ucanr.edu/p/41860.
