Pistachio powdery mildew

The causal agent of this disease is the ascomycete fungus Erysiphe pistaciae. It is an obligate parasite that specifically infects pistachio trees. The fungus colonizes the external surfaces of the plant parts, creating a network of hyphae that extracts nutrients directly from the epidermal cells of the host through specialized structures called haustoria.

The primary host for this pathogen is the true pistachio tree (Pistacia vera). The disease manifests most aggressively on young, succulent tissues, including leaves, growing shoots, and developing inflorescences, thereby significantly impacting the tree's vegetative development and reproductive capacity.

Symptoms appear as characteristic white, powdery patches on both the upper and lower leaf surfaces. As the infection progresses, these white patches become denser and may turn grayish or brownish. Heavily infected leaves become brittle, curl inward, and eventually drop prematurely, leading to significant defoliation of the tree canopy by mid-summer.

The spread of Erysiphe pistaciae is highly dependent on environmental conditions, particularly humidity and moderate temperatures that favor conidial germination. Wind-dispersed conidia serve as the primary source of spread during the growing season. In the winter, the pathogen survives as cleistothecia within the leaf litter and debris left on the orchard floor.

The economic damage caused by this disease is substantial, as premature defoliation inhibits the tree's ability to store reserves, leading to smaller nut size and reduced yields in subsequent seasons. Effective disease management requires integrated cultural and chemical practices:

• removal and destruction of fallen leaves to eliminate overwintering sites;
• thinning the tree canopy to increase airflow;
• application of sulfur-based fungicides or systemic triazole treatments at the first signs of infection.