Symptoms and Signs
Phytophthora-infected trees have reduced growth, thin canopies, and often die. If the disease progresses rapidly, trees may die in 1 or 2 years. Roots rotted by Phytophthora are dark and trees affected for long periods by Phytophthora root rot may have few root hairs. The bark of the root crown turns dark as the infection spreads; a hand tool can be used to reveal the boundary between healthy (whitish) and diseased portions (dark) of the root crown bark.
Comments on the Disease
Phytophthora species require free moisture in the soil to produce and disperse swimming zoospores, which are the main infective propagules of the pathogen. Consequently, frequent and prolonged periods of soil water saturation due to over irrigation, excessive rainfall, or poor soil water drainage favor Phytophthora root and crown rots. The pathogen can live independently in the soil and survive long periods of dryness as oospores or chlamydospores. Phytophthora species that infect olive can vary in their aggressiveness and responses to soil moisture and temperature conditions.
Careful soil water management is the best foundation for control of Phytophthora root and crown rot. Cultural practices that avoid prolonged and frequent water saturation of soil, especially near the tree root crowns, can help to minimize disease losses.
- Plant trees on berms, shorten irrigation times, and improve soil water penetration and drainage to reduce losses caused by Phytophthora.
- Do not place irrigation emitters too close to the trunk of trees; the goal is to provide accessible water to the root system without overwetting the soil around the root crown.
- Avoid planting olives trees in low areas and in heavy clay soils.
Phytophthora-resistant olive rootstocks have not been identified.