Pistachio Tree Diseases

A pistachio tree (Pistacia vera) may have to fend off some fairly treacherous diseases in its lifetime. Where the tree grows as a perennial, across U.S. Department of Agriculture plant hardiness zones 4 through 9, it is susceptible to fungal pathogens that cause illness or death. These pathogens can mount a sneak attack from underneath the soil or a bold offensive maneuver in the open air.

Although some fungal pathogens live in soil and attack a pistachio tree's roots, the above-ground symptoms they cause usually are noticed first. When disease hits a pistachio at its foundation -- the roots -- it hinders the tree's ability to absorb water and minerals from the soil.

Pathogen/causal agent: Verticillium dahliae.

Symptoms: A characteristic symptom of Verticillium wilt  -- also called "one-sided chlorosis" -- causes yellowing on only one side, which gives the tree a lopsided look. The disease also causes black rings or streaks inside the trunk or branches. An affected tree may have wilting leaves and may not be revived even when it has enough water.

Control: Preventing Verticillium wilt is the primary form of control because once a pistachio is infected, no fungicide can cure it. Choose a pistachio tree that is grafted onto a Verticillium wilt-resistant rootstock to improve the tree's chances of surviving the disease. 

Pathogen/causal agent: Armillaria mellea.

Symptoms: Armillaria root rot is commonly called "honey mushroom rot" because of the honey-colored mushrooms that may grow around the base of an affected tree. Sheets of white fungal growths in fan-shaped patterns grow between the infected tree's bark and outer wood.

Control: Once this disease takes hold, it's difficult to control. If it's practical to do so, remove and destroy all the diseased roots.

Some fungi live above ground, typically overwintering on fallen leaf litter from infected plants. As part of their life cycle, these fungi produce spores that become airborne when the wind blows or when rain splashes them onto plants.

Pathogen/causal agent: Septoria spp.

Symptoms: Circular or angular spots appear on the leaves of a tree that has Septoria leaf spot, and leaves fall from the tree.

Control: Rake, remove and destroy all fallen leaves from around the tree. Prune all of its infected branches, and don't use overhead irrigation.

Disinfect all pruning tools by soaking them for five minutes in a solution that is 1 part household pine-oil cleaner and 3 parts water. Rinse the tools with water or let them air-dry before using them.

Pathogen/causal agent: Alternaria spp.

Symptoms: Alternaria late blight causes dark circular or angular spots on a pistachio tree's leaves, and leaves fall from the tree. Also, black lesions appear on the tree's nuts.

Control: Rake, collect and destroy all fallen leaves from around the tree. Also, prune all affected branches. Don't use overhead irrigation.