Phytophthora root rot of Douglas fir

Pathogen and disease type. The disease is caused by the oomycete Phytophthora pseudotsugae, which is classified as a soil-borne pathogen responsible for root rot. It infects the vascular and root tissues of host plants, ultimately preventing the uptake of water and essential nutrients, which leads to the decline of the plant.

Host plants. The primary host for this pathogen is the Douglas fir (Pseudotsuga menziesii). It is particularly destructive in forest nurseries, where it causes significant mortality among seedlings and transplants, creating challenges for reforestation and ornamental plant production.

Symptoms and signs. The visual symptoms start with needle chlorosis and yellowing, eventually turning brown. As the disease progresses, the tree wilts and dies. A key diagnostic feature is observed in the root system: the feeder roots are destroyed, and the root bark turns dark and necrotic, showing signs of advanced decay.

Environmental conditions. Development of Phytophthora pseudotsugae is highly dependent on moisture. Waterlogged, poorly drained soils provide the ideal medium for the production and dispersal of zoospores. These mobile spores swim through the water film to infect healthy root tips, particularly during cool and wet seasons.

Control and prevention measures. Managing this disease requires an integrated approach focusing on site hygiene and environmental control:

• Improving soil drainage to prevent stagnant water.
• Implementing strict quarantine and sanitation protocols in nurseries.
• Using pathogen-free potting media and certified nursery stock.
• Applying targeted systemic fungicides containing metalaxyl or fosetyl-aluminum where necessary.
• Removing infected plants and surrounding soil to limit the spread of the pathogen.