Symptoms and Signs
Roots infected with Armillaria mellea have white to yellowish fan-shaped mycelial mats between the bark and the wood. Dark brown to black rhizomorphs can sometimes be seen on the root surface, especially during wet springs.
Wilted, downward-hanging foliage is often the first obvious symptom of Armillaria root rot. Other symptoms include foliage yellowing, leaf drop, and dieback of upper limbs. During the rainy fall and winter, groups of short-lived mushrooms often grow around the base of Armillaria-infected trees.
Comments on the Disease
Although pistachio is susceptible to Armillaria root rot, the disease is infrequently reported. The relative susceptibility of pistachio rootstocks is unknown. The fungus survives on dead roots.
Armillaria root rot is only occasionally a problem in pistachio orchards.
Management of this disease can be difficult and resistant rootstock may offer the best protection. Research indicates that Pistacia terebinthus and P. atlantica x P. integerrima hybrids are tolerant but P. atlantica and P. integerrima are susceptible. However, because pathogenicity and virulence in the natural population of the pathogen ranges from weak to severe, disease response may vary with different combinations of rootstocks and pathogen. Exposing an infected crown may help individual trees with the disease.