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.