Frequently, the presence of plant-parasitic nematodes in the soil have not been considered an issue in development of new pistachio plantings in the Central Valley, but researchers are concerned that could change. Altered production practices and different pre-crops may expose such new plantings to nematode species not previously encountered.
UCCE Nematology Specialist Andreas Westphal points out several possibilities for increased risk potential for damage to young trees: pistachio trees being planted in former vineyard ground that can contain a whole mix of plant-parasitic nematodes or following walnut that frequently carry infestations with root lesion nematode, the currently most notorious nematode pest of nut crops in California. There remains the risk that some UCB-1 clones have higher susceptibility than others.
Westphal stresses that the nematode/pistachio rootstock interaction is not fully understood at this time. Trials are being done to examine the host susceptibility to root lesion nematode on currently available pistachio rootstocks, including multiple clones of UCB-1. Other trials address the question whether Xiphinema index, the notorious dagger nematode that can transmit fan leaf virus in grapes, impacts pistachio growth.
When pistachio trees were first being planted in the San Joaquin Valley in the 1970s, it was believed that plant-parasitic nematodes were not a problem in pistachio. In addition, the rootstock commonly used at that time, Pistachia atlantica, appeared to be resistant to root lesion nematode. When Verticillium wilt threatened the pistachio industry, the hybrid rootstock UCB-1 was developed. It is marketed as seeded rootstocks but more frequently as various clones. Half of its parentage is P. atlantica, so despite the differences within this species, there may be limited susceptibility. The second crossing partner may have brought some susceptibility into the offspring. Westphal said such variability was suggested by data from offspring screens from this cross provided by his predecessor Dr. Mike McKenry. Westphal established ongoing trials to determine the response of commercial UCB-1 clones to root lesion and root-knot nematodes.
“We have seen that the rootstocks grown out as trees for the most part are performing well in the presence of nematodes. Data are just beginning to come in to try to quantify measurable damage. Because of the really slow juvenile development, we’re not there yet to fully quantify plant damage by root lesion nematode on pistachio. It appears though that different UCB-1 clones respond to infections by root lesion nematodes differently in how much nematode reproduction they allow and how much their growth performance suffers.”
Looking for field symptoms in pistachio would be similar to scouting in other young nut tree plantings in plant-parasitic nematode infested soils. Typical for symptoms of nematode infestations on the field level, affected trees would be in spotty areas across the orchard and growth would be quite uneven. Not all trees would be affected. These symptoms may be difficult to detect because of the intensive pruning and training strategies in pistachio. The affected trees with reduced vigor and poor growth would also have smaller trunk circumference compared to non-affected trees.
