Phytophthora root rot of pistachio

The disease known as Phytophthora root rot of pistachio is caused by the oomycete Phytophthora pistaciae. This pathogen is a soil-borne organism that primarily attacks the root system and the crown of the tree. It is recognized as one of the most destructive diseases affecting pistachio production globally, particularly in orchards with poor drainage.

The primary host for this pathogen is the Pistachio tree (Pistacia vera). The organism invades the plant through wounded roots or the bark at the root collar. Trees that are subjected to excessive moisture, either through over-irrigation or prolonged waterlogging, are significantly more susceptible to infection by this fungus-like pathogen.

Initial symptoms include reduced vigor, yellowing of leaves (chlorosis), and premature defoliation. As the infection progresses, lesions appear on the crown near the soil line, which often develop into dark, sunken cankers. If the lesion circles the trunk, the tree loses the ability to transport nutrients and water, eventually resulting in tree decline and death.

Environmental conditions are crucial for the development of Phytophthora pistaciae. The pathogen produces motile zoospores that require water to swim through soil pores and infect healthy tissues. High humidity, heavy clay soils, and poor irrigation management create ideal conditions for the rapid spread and proliferation of the pathogen within an orchard.

Economic damage caused by this disease can be catastrophic, often necessitating the removal of entire blocks of trees. Control strategies are focused on the following practices:

• Selection of resistant rootstocks during planting.
• Improvement of orchard drainage systems to prevent water accumulation.
• Irrigation scheduling that prevents the trunk and root collar from staying wet.
• Sanitary disposal of infected plant debris to reduce the inoculum level in the soil.
• Application of targeted systemic fungicides containing fosetyl-aluminum or metalaxyl as part of an integrated management program.