Symptoms and Signs
Generally Verticillium wilt causes a rapid desiccation and death of one or more scaffolds or the entire tree, usually in late spring or early summer. The first symptoms are interveinal patches of yellowing or scorching of the leaves on affected branches.
In some instances, however, it may also cause a condition known as thin leaf decline where the disease develops slowly over several years before the tree becomes economically unproductive or dies. Thin leaf decline is characterized by slow loss of vigor, reduction in growth and yield, and gradual thinning of the canopy until most of the remaining leaves are clustered in tufts at the ends of branches and shoots.
Comments on the Disease
Verticillium wilt affects a large number of herbaceous and woody species. The causal fungus, Verticillium dahliae, infects susceptible plants through the roots. Pistachio trees of any age are subject to attack. The disease has been most destructive where pistachio trees were grown in fields previously planted to other susceptible crops such as tomato, cotton, melons, or peppers.
The fungus survives as microscopic, black resting structures (microsclerotia) capable of surviving in soil for many years. When a susceptible plant is grown in soil infested with the fungus, the microsclerotia germinate and infect the plant.
It invades and colonizes the plant's vascular system, plugging the xylem and preventing or reducing the transport of water from the roots to the above-ground portion of the tree. If dead or dying branches are cut in cross section, a darkened ring of plugged xylem tissue can be seen.
Verticillium wilt is favored by cool temperatures. Extended spring weather and mild summers often are accompanied by severe losses to this disease. The fungus apparently is eliminated from aboveground portions of trees in hot summer weather. Repeated attacks of wilt apparently represent new infections each year.
The best defense against Verticillium wilt is the use of the resistant rootstock Pioneer Gold, Pistacia integerrima, PGII or Platinum (a P. integerrima x P. atlantica hybrid), or UCB I (a P. atlantica x P. integerrima hybrid). Pistacia atlantica and P. terebinthus rootstocks are very susceptible and should be avoided where Verticillium is present. During nut development, monitor trees for Verticillium wilt and note trees for future removal.