Reduce tree vigor through reduced late season irrigation

Reducing the vigor of the trees by making sure that the root zone is dry prior to the arrival of freezing temperature appears to be an effective counter measures to WJTD.   On deeper, heavier soils, the last irrigation should probably occur about the third week of August.  On shallower, lighter soils the grower can probably wait longer and may want to reduce water more slowly.  Cutting off the water will reduce late-season growth, but not as much as some fear.  Don’t try to make up for a late planting or budding by pushing the trees into the fall, especially in a low-elevation area.  Not getting the tree up the stake the first season is not the end of the world.  Cutting of the irrigation water really means cutting off the water.  Since pistachio trees, on the existing rootstocks, do not appear to become anything like fully dormant until the end of December, do not irrigate until January, once you shut off the water in August.  In saline areas, wait until the new year to begin leaching salt. Current observations suggest the tree is more in danger of frost, than salt.  Of course, in a high rainfall year, cutting off irrigation will be less effective, since the orchard may remain wet.  However, wet air associated with precipitation, is more difficult to cool, and frost is less of a problem.  Additionally, as was the case for the 2011 crop year which was harvested late, reducing irrigation too much too early could reduce the split nut percentage in 5th leaf trees and older that are in their early nut-bearing years.

The reason that first and second-leaf trees appear less at risk from WJTD than slightly older trees is that their root systems are not that well established yet under drip systems, and the tree may be able to be put into drought quicker by irrigation shutoff.

Reduce tree vigor through nitrogen management

Do not over apply nitrogen to juvenile trees.  Test the soil to see how much nitrogen is present and then estimate how much to add so that little N remains in the soil at the end of the season. Pistachio is an effective scavenger of nitrogen due to its extensive root system so a margin exists if your nitrogen application is too low.  Cut off nitrogen applications in late June or early July.  In deep, heavy soils that store large volumes of water, it may be difficult to encourage earlier dormancy through water stress.  This is especially true for 5th or 6th leaf trees that will be harvested and are being fully irrigated in August and September to encourage shell splitting.  For these older juvenile trees, nitrogen management, other than defoliation,  may be the only tool available to reduce tree vigor in the fall.

Avoid budding late-planted rootstocks late

Trying to get newly planted trees grafted and up the stake may be a mistake in low elevation areas.  If you bud them late, you will have to irrigate the trees late to ensure the buds take.  This places trees at increased risk from freezing.  It may be better to hold off budding until next year, and reduce irrigation earlier, to decrease tree vigor.

Choice of rootstock

Rootstocks that by heritage contain 50% P. integerrima and 50% P. atlantica heritage appear to be more frost tolerant.  These rootstocks would include UCB1 seedlings, Duarte clones, and Pioneer Platinum  clones.  However, choice of rootstock is not enough.  All of these rootstocks are vigorous and additional measures will be required.

Leaf canopy defoliation

Early defoliation with foliar zinc sulfate sprays or by some other means has been suggested as a means of encouraging dormancy for many years.  However, trying to find scientific research supporting this practice in pistachio has not been easy.  It does make sense that defoliating the tree should send a message to the tree to slow down.  Since freeze events can occur, especially at low elevations at the end of October, trees should probably be defoliated in mid-October.  Defoliation should be done in conjunction with soil drying.  We don’t want to defoliate too early, as the leaves may regrow, which may reinvigorate the tree, creating a condition that we are trying to avoid.  We have be experimenting with defoliating trees, but have not yet found optimum conditions to determine how well defoliation works on its own, separate from the effects of soil drying.  Having a dry root zone and a defoliated canopy should take the vigor out of even the most stubborn tree.

Tree Wraps?

Research conducted last year demonstrated that tree wraps may have some use in protecting the portion of the tree covered by the wrap.  Tree wraps composed of ¾ inch, closed cell, black foam, tightly fit to the trunk with black Gorilla tape kept the trunk approximately 4ºF warmer, 12 inches above ground level than the outside temperature.  Protect the graft union if possible.

What to do if the trees freeze?

Previous experience has shown that the trunks, including the rootstock and scion, of frozen trees, may sunburn in the spring, after the loss of the shading canopy.  Whitewash the trunk, especially on the south and west sides of the tree, to prevent sunburn.

Do not try to prune the tree during the winter or early spring.  Damaged trees will likely continue to die back during the hotter temperatures of May and June.  By late May or June, you should have a pretty good idea if a tree is worth saving.

If the tree has died back to the lower trunk, it is probably best to replace the tree or rebud on a sucker originating from below ground level.  Often the trunks are severely damaged, and a stronger tree will likely result if no old scar tissue exists between the producing canopy and the roots. 

Trying  to rebud a severely WJTD-affected  tree too high up the damaged rootstock will likely result in a tree with reduced future bearing potential.  Trees that have lost the leaf canopy are susceptible to sunburn in May and June. Whitewash trunks to reduce sunburn in late spring and summer.