Oregon State University - "What is Phytophthora? Phytophthora (Fy-TOFF-thor-uh) species make up a group of microorganisms that are important plant pathogens. The name Phytophthora derives from Greek and literally means “plant destroyer.”
Phytophthora species resemble fungi but are not. While they are most closely related to aquatic organisms, such as brown algae and diatoms, they can also exist on land. However, Phytophthora organisms are often referred to as water molds because they do need water to complete their life cycle.
Phytophthora disease is the number-one disease of nursery crops nationwide."
Notorious Phytophthora diseases include rhododendron root rot, sudden oak death, and potato late blight.
Some species of soil-inhabiting pathogens in the genus Phytophthora cause root and crown rot diseases of herbaceous and woody plants. According to UC IPM, "Almost all fruit and nut trees, as well as most ornamental trees and shrubs (including many California natives), can develop Phytophthora rot." During the first half of 2015 thirty-three Phytophthora species were identified in container stock samples from twenty nurseries. Seventy-five percent of the participating nurseries tested positive for at least one Phytophthora species. Detection has also occurred in native habitat restoration areas with devastating effect. Since inadvertent inoculation of native habitats by Phytophthora-infected nursery stock could severely impact wildlands, the native plant community is leading the battle. A "Working Group for Phytophthoras in Native Plant Habitats" has formed to better understand Phytophthora and develop best management practices to minimize the spread of disease. "In general, Phytophthora (now considered a water mold or oomycete) requires warm, moist soils in order to cause disease. The gardener will reduce the threat of Phytophthora by avoiding prolonged saturation of the soil. Irrigate only as much and as often as necessary. If using a drip system, place the emitters at least a foot away from the trunk. Avoid planting susceptible species on poorly drained or shallow soils." - UC IPM
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