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.