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’.