Pistachio Lower Limb Dieback Observations from 2006
The 2006 season was marked by deficient winter chilling (600-700 hours at or below 450F), Freezing temperatures in mid-February (24-260F), and cold, wet spring weather with rainfall in April equal to the entire annual amount (7 inches) for Fresno. Pistachios not receiving dormant oil applications, which aid in overcoming inadequate chilling, leafed out and bloomed as much as 21 days later than oiled trees. Pathologists report that Verticillium wilt symptoms are typically are more frequent in cold, wet springs. Vert "strikes" are thus more common on susceptible perennial plants such as almond, olive, and pistachio grown on P. atlantica and, to some degree, P. integerrima rootstocks. During the summer, several growers with mature, dense canopied orchards reported the gradual death of lower limbs up to three inches in diameter. I investigated one such orchard and, having found canker-like symptoms in limb cross sections, submitted samples to UC pathologist, Dr. Themis Michailides. He recovered verticillium in all the samples, plus two wood rotting fungi. At this point, Themis is not sure which disease was the primary infection. These trees were on P. integerrima, a rootstock with relatively high tolerance for vert. Due to the shaded position of the affected limbs, it is reasonable to hypothesize that they produce insufficient photosynthates for self-support. Thus, their weakened state predisposes them to decay fungi. Several of the cankers were observed radiating from an old pruning wound toward the limb center. The verticillium staining was also in older wood tissue rather than what was created this season. Hence, it is possible that these limbs succumbed from a progression of infections over their lifespan. Growers observing considerable loss of low limbs are encouraged to examine cross sections at various locations along the limb's length and compare symptoms to the photos on my website under "Pistachio low limb death". Notify me and I will attempt to get a limited number of samples cultured.