What is in this article?:
- This past winter, as was the case the two previous winters, juvenile pistachio trees demonstrated considerable blackening of the trunk, gumming, early leaf-out and dieback in a number of orchards in the southern San Joaquin Valley.
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 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.