This past winter (2011-2012), as was the case the two previous winters, juvenile pistachio trees (i.e. trees 1 to 6 years old) demonstrated considerable blackening of the trunk, gumming, early leaf-out and dieback in a number of orchards in the southern San Joaquin Valley.  The purpose of this newsletter is to share some of the current thinking on the cause of the problem and what might be effective in reducing the incidence in the future. The timing of the release of this newsletter is important.  Observations made last winter in low-elevation orchards with high sodium levels suggested that shutting off the irrigation water beginning in late August and not reirrigating until January or later may reduce the incidence and severity of winter juvenile tree dieback (WJTD).

Description of the Problem

Juvenile pistachio trees appear to be in excellent health as they go into fall dormancy. Initial winter juvenile tree dieback occurs sometime in the time period between early November and spring leaf out in trees one to six years old. Leaves on branches affected before the trees begin to defoliate in the fall remain attached to the branch delaying leaf drop. Black mold may grow on damaged bark, especially on the trunk between the lower portions of the scaffold branches and the graft union.  The rootstock does not usually demonstrate mold growth.  These moldy areas will appear wet. Often white beads or small ribbons of dried sap, apparently originating fairly shallowly in the bark, are visible on the outside of the tree in the moldy areas. Initially, in the moldy areas of the tree, there appears to be little if any damage to the bark and cambium layer below the outer skin of the bark. Trees showing mold growth and gumming may leaf out earlier than unaffected trees. Later, in early spring, more severe dieback of the branches may become apparent in some of the trees showing the most pronounced of these early symptoms.  The dieback begins at the branch tips and affected areas of the scion and rootstock.  Most of these trees that demonstrate this gumming, blackening bark and early leaf-out recover quickly, and show little short or long-term damage.

However, the worst WJTD occurs in trees showing none of the early symptoms described above.  Damage to these trees is not obvious until May, when they begin to leaf out after the first run of warm to hot spring temperatures.  In trees third-leaf or older, the new growth, typically may push only from the lower scion or rootstock and this growth occurs later than in unaffected trees.  Affected 1st and 2nd leaf trees may simply die without pushing any new growth.  Some affected trees show minimal dieback, while many trees in the worst affected blocks die back to the rootstock, or the entire tree is killed.  Kerman, Peters, Kalehghouchi and Golden Hills cultivars growing on UCB1, UCB1-clonal material, and PGI rootstocks have been affected. Reported incidences of WJTD increase greatly soon after extended freeze events.  

Additional observations of WJTD

Additional observations of WJTD are as follows:

a) trees go into dormancy apparently vigorous and healthy;

b) topographically low-elevation orchards have most, but by no means all, of the WJTD, but damaged trees are often not in the lowest areas of a given orchard;

c) rootstock starch concentrations are much higher during early spring at leaf out in WJTD trees than in unaffected trees;  

d) high sodium levels in the soil and water, and soil characteristics such as presence of hardpans, appear to increase WJTD;

e) sodium concentration is highly elevated, when measured in mid-May, in the scion and rootstock bark of WJTD-severely affected trees growing in salt-impacted soils;

f)  the scion is usually more adversely affected than the rootstock, but the entire tree may be killed;

g)  A tree with WJTD is often immediately adjacent to an unaffected tree, although typically, affected trees appear in clumps;

h)  Trees uninjured the previous year may be injured the subsequent year.

i.) juvenile trees are much more susceptible to WJTD than are bearing trees, and usually third, fourth, and fifth leaf trees appear most at risk.

j.) the most vigorous rootstocks, cultivars and growth stages of the pistachio tree appear most susceptible to WJTD.

Probable cause  

Probable cause  

The cause of WJTD is probably freeze damage, although freeze damage has not been proven and the extreme variability in symptoms within an orchard remains puzzling.  Considerable debate remains about the cause of the WJTD and how much of the problem may be related to salinity or disease.  I will discuss what I think is happening and what the grower might do if contemplating planting new pistachios or managing existing juvenile pistachio trees, especially at a low-elevation location where cold drainage may be more of a problem. This hypothesis is based on many discussions and initial research results involving a number of crop consultants, growers, and my coworkers in UCCE, with special involvement of Blake Sanden, Irrigation and Soils Farm Advisor in Kern County, and crop consultants Carl Gwilliams, Mike Harvey, Carl Fanucchi, Alan Scroggs and others.  I would like to stress that many of whom would likely disagree with some to many of my deductions.  Discovering the cause of WJTD remains a work in progress.  The following discussion is provided with the important caveat that future research may change the exact cause and corrective recommendations considerably. 

Current hypothesis and reasoning

Winter juvenile tree dieback is not new to the San Joaquin Valley.  In the past, we have called this problem freeze damage. What is new is the extent of the problem.  I believe the problem has become bigger because more pistachio trees are being planted in low-elevation areas of the San Joaquin Valley at the same time that we have had several low-rainfall years.  Cold air drains into low elevation areas, and low rainfall translates into reduced fog. The absence of fog means drier air with low dew point temperatures.  Dry air means temperatures get colder faster in the evening, resulting in lower lows and longer durations of cold temperatures, with much more potential to damage vigorously growing young trees.  The most obvious cause of the dieback during the winter is freeze damage.   Reports of WJTD always spikes shortly after a major early freeze event.  Investigations of WJTD affected trees for disease and the soil for harmful levels of heavy metals, have found nothing suggesting these are causal factors. Juvenile trees are affected at both low elevations and high elevations, as long as freezing temperatures are present.

I believe a large part of the WJTD problem is associated with the rootstocks currently used in the pistachio industry, specifically, the P. integerrima component. This observation does not mean that we need to throw these rootstocks out! The discovery by University of California researchers that Pistacia integerrima was resistant to Verticillium Wilt disease was of tremendous importance to the California pistachio industry.  The lack of resistance to Verticillium Wilt disease in the existing rootstocks of the time, was allowing Verticillium Wilt to destroy the industry.  Trees are still dying from Verticillium Wilt in San Joaquin Valley orchards planted to P. atlantica and what is called P. terebinthus in California.  Rootstocks with Pistacia integerrima have been and will continue to be a critical component of the California pistachio industry.  However, P. integerrima is not a perfect match for the San Joaquin Valley, which is to be expected, since this species of pistachio is adapted to a relatively high elevation, subtropical area of the Indian subcontinent.  Juvenile P. Vera scions (i.e. the commercial nut bearing portion of the tree) such as ‘Kerman’ grafted onto rootstocks having P. integerrima heritage, do not appear well-suited to respond to environmental cues that winter is approaching.  These cues include shorter day length and reduced night-time temperatures.  If water is available, these trees continue to grow vigorously into the fall and winter, since temperatures remain warm throughout the fall.  In fact, trees on our commonly used rootstocks, no matter which one, do not appear to go fully dormant until the end of December.  In general, we have observed in orchards planted to various rootstocks, that the greater the amount of P. integerrima heritage, the greater the susceptibility to frost damage.

Some qualitative starch analysis of the rootstock of unaffected and trees with WJTD conducted after leaf-out in 2011, demonstrated that unaffected trees contained almost no starch below the graft union either in the trunk or roots, while severely impacted trees were loaded with starch.  This suggests that all of the trees in the orchard were healthy going into the winter, and had stored large amounts of starch in the scion trunk, roots and above ground portions of the rootstock.  The trees not damaged by frost were able to mobilize these starch reserves in the spring and use them to produce a new leaf canopy.  Starch stored in trees that had died back, and now had too few unfrozen growing points left, remained stored in the rootstock instead of being used to make a new leaf canopy.  In some frost-damaged trees, there was a clear line of demarcation between no starch in the scion directly above the graft union and a large concentration of starch in the rootstock directly below the graft union.

The clear line of demarcation at the graft union also suggests that the graft union, the place where P. integerrima melds with P. vera, is a weak point with respect to frost tolerance.  In Montana and Minnesota, where the temperatures get very cold, it is very difficult for any grafted landscape plants to survive the winters.  In cold climates, the graft union is a point of weakness in susceptibility to frost.  The xylem (i.e. the water conducting tissues) are the most susceptible structures in the trunk to frost damage and the xylem vessels most at risk appear in the tree appear to be those at the graft.  In freeze-damaged orchards, it is not uncommon to see rootstocks regrowing, minus the scion, which has died.  Starch reserves in the insulated roots below ground level, allowed the rootstock to regrow.  The P. vera scion did not have that option.  As part of my research efforts in the past, I have been involved in a number of seedling breeding plots, where P. vera are grown on their own roots.  I have not seen a P. vera tree on its own roots dieback as a result of frost damage.

How cold temperatures become and when the low temperatures occur in the fall appears to dictate symptoms.  Freeze damage can be roughly divided into two types -  ‘mild’ and ‘severe’. 

Mild WJTD

For vigorously growing young trees, temperatures just below freezing in late October or mid-November appear to be sufficient to freeze small-diameter shoots and branches.  Typical symptoms include black mold, bark wetness and white gum on the lower scaffold branches and trunk above the graft union and are observable shortly after the freeze event.  These symptoms appear to only be associated with juvenile trees that have not yet entered dormancy in the fall.  Once the trees have dropped leaves and are in the initial stages of entering dormancy, freeze damage is probably not observable until leaf-out in the spring. If the freeze occurs before leaf drop, which is usually the case with mild WJTD, the leaves of frozen branches remain on the tree and are very distinctive after the leaves of unfrozen branches lose their leaves.  Trees showing these symptoms in the fall often leaf-out and flower, if old enough, earlier in the spring than unaffected trees. However, with the first string of hot days in May, affected outer branches may die back, unable to keep up with the transpirational requirements as a result of damage to the vascular tissue.  However, frequently, no die back occurs and the tree will grow normally.

Severe WJTD

Severe WJTD appears to occur after mid-November through mid December.  Usually temperatures do not drop low enough, even at low elevations, prior to mid-November to cause severe WJTD.  Trees severely affected by WJTD in late November or mid-December do not exhibit attached leaves, mold growth and gumming after the freeze event.  These trees, however, are even more heavily damaged than those showing symptoms, and may only leaf-out from the lower scion and rootstock of the trunk in the spring, well after the unaffected trees have produced a normal canopy of leaves.  For trees to be severely damaged by frost, temperatures usually fall to 22 ºF , with temperatures below freezing for an extended part of  a night during the period from late November through mid-December. Please note that temperatures discussed in this article were measured at four (4) feet above ground level.  In the early morning, before sunrise, temperatures one (1) foot above ground level in areas were cold air ponds, can be three to four degrees F. colder than those measured at 4 feet. Generally, trees appear to be dormant enough by late December, that they are no longer easily damaged by frost.

The chart above was prepared from temperature data measured 4 to 5 feet above ground level in two 5th-leaf orchards during the night of December 6-7, 2012.  On several other nights in early December, temperatures fell to similar levels. The trees in the warmer orchard showed no WJTD, while those in the colder orchard had approximately 20% severe WJTD. 

Development of dormancy

Development of dormancy

The reason for differences seen in freeze-related symptoms appears to be related to the stages a tree goes though in becoming dormant.  Dormancy should be thought of as a time of reduced metabolic activity not as a time of no metabolic activity.  Most trees from colder climates become dormant beginning at the outer periphery of the tree and then sequentially toward the trunk. In late October or early November, no part of the tree is dormant, and freeze damage is dependent on the diameter of the wood.  By late November to mid-December, the outer branches have begun to go dormant, so do not appear to be damaged by extended low temperatures as much as the scaffold branches and trunk, which have not yet begun dormancy.  Generally, then, the cause of the dieback, is damage to the water-conducting vessels, as a result of cold temperatures.  Basically, the tree dies back due to dehydration, or inability to supply water for maintenance and growth of the tree.

In many plants, when a plant part is damaged or wounded by something like frost, mechanical damage, an herbivore, or insect infestation; ethylene (a plant growth regulator) is released by damaged cells.  Ethylene induces a stress response in the plant, which results in cells adjacent to the wound site sealing of the damaged area.  For example, parenchyma cells associated with the xylem create tyloses, which affectively seal off the xylem, preventing excessive loss of water by the plant.  Interestingly, applying ethephon to the canopy and trunk of a pistachio tree at concentrations greater than 500 ppm before it is dormant appears to mimic the symptoms of gumming and early leaf out, seen in mildly frozen pistachio trees. If a tree is frozen, but not so badly that the upper canopy is destroyed, we would expect to see early leaf-out as a symptom as leaf and flower buds are affected by the ethylene gas being released by damaged cells.  Trees that do not show the early symptoms of bark blackening, early leaf-out and gumming,  but only sprout from the lower trunk in May, have probably been so badly damaged by frost, that the cells of the tree are no longer able to respond to wound-related plant growth regulators, or the frost damage occurred later, and the tree was sufficiently dormant that cells were no longer responding to ethylene.

Trees that are less than one-year old and up to 2nd leaf, when frost affected, are often killed outright by the frost.  The rootstock is killed, followed shortly by the scion.  The trees usually begin dying back in May with the first warm temperatures and the dry or drying leaves will be obvious from a distance.  At this time, if you scratch the bark, the bark of the rootstock will be gray to black, while that of the scion will still be green.  There appears to be insufficient root mass and carbohydrate storage in very young trees to support regrowth of suckers, and the rootstock dies.  Older trees, with there more extensive root systems, appear to have sufficient carbohydrate reserves to survive and push new suckers which the grower can graft to, to regrow the tree.  In general, it may be easier to encourage dormancy in very young trees, since they have not yet developed extensive root systems, and are more susceptible to drought-induced reductions in vigor and earlier dormancy.

As mentioned above, excessive tree vigor going into fall appears to have a strong association with WJTD.  Trees older than seven or eight years appear to be largely immune from the problem, but, of course, if temperatures get cold enough, anything can freeze.  Perhaps bearing large yields of nuts or prolific amounts of pollen reduces tree vigor, or something about the rhythmicity of the bearing cycle once fully established, allows trees to become dormant earlier. 

The Kalehghouchi variety is probably the most vigorously growing cultivar we have.   We noticed this season at a couple of sites, that this cultivar appears to be slightly more freeze susceptible than other cultivars growing adjacent to it.  Also, we have noticed that trees that are fully irrigated late into the season appear to be more at risk of WJTD.    Juvenile pistachio trees show very little sign of  entering dormancy on their own if sufficient water is available.  In the San Joaquin Valley, there is often very little transition in temperature in the fall.  Temperatures can easily be in exess of  80º F in October and early November and suddenly fall to below freezing, especially in lower elevation areas.  The variability of the degree of WJTD in an orchard may reflect differences in individual tree vigor within the orchard. 

High soil salinity, especially in association with low-soil calcium appears to be associated with WJTD.  However, soils do not have to be saline for WJTD to occur.  Separating the effects of salinity from late-season soil-water storage is difficult.  Areas with high salinity are usually associated with silty, poorly draining soils and shallow hardpan or high water tables.  Poorly draining areas of the field, also tend to be saline, since it is more difficult to leach salts in these areas.  Low calcium in plants has been associated in the literature with increased sensitivity to frost for a number of plant species including trees.  Differences in calcium concentration between affected and unaffected plant parts has been found in our testing, but this may simply be the result of sodium being higher in poorly drained areas of the field where greater damage is found.  Since pistachio excludes salts from its cells, we have been unable to establish much difference in plant sodium concentration between plant parts of affected and unaffected trees shortly after a freeze event when the trees begin showing the first symptoms.  The concentration of sodium is much higher in the bark and outer wood near the cambium of the trunk of WJTD trees in May after leaf out. However, bark sodium concentration does not appear to be higher shortly after the freeze event.  This suggests that sodium is leaking out of the frozen xylem vessels in the trunk and into the wood and bark as the flow in the xylem responds to the increased transpirational demand on the tree in the spring, and is not higher in these tissues prior to the freeze event.

Reduce incidence of WJTD

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.