Phytophthora disease of alder

Phytophthora disease of alder is a severe condition caused by the oomycete Phytophthora alni. This pathogen is a hybrid species known for its high virulence towards various alder species, specifically the common alder. It acts primarily as a root pathogen, significantly impacting the health and stability of riparian forest ecosystems where these trees are vital.

The disease mechanism involves the infection of roots and the collar area of the tree. The pathogen attacks the vascular tissues, effectively blocking the uptake of water and nutrients from the soil. As the root system deteriorates, the tree loses its ability to sustain itself, leading to a decline in growth and, eventually, mortality.

Typical signs of infection include stunted, yellowish foliage and crown thinning. A diagnostic feature is the presence of necrotic lesions (bleeding cankers) on the bark at the base of the trunk. These lesions are often dark and ooze a tar-like substance, indicating that the pathogen has reached the cambium layer and is destroying the conductive tissues.

Development and spread are heavily dependent on water availability. Phytophthora alni produces zoospores that swim through water-saturated soil and aquatic environments to reach new hosts. Consequently, riverbanks and floodplains are particularly susceptible areas, where the pathogen can spread rapidly downstream or through flooding events.

Management strategies focus on prevention and sanitation. There is no cure for established infections in forest stands, making the early detection of the pathogen essential. Key practices include:

• Strict quarantine of nurseries to prevent the sale of infected seedlings.
• Removal and destruction of infected trees to reduce inoculum levels.
• Avoiding the movement of contaminated soil or water into healthy zones.
• Prioritizing the planting of resistant or less susceptible tree species in high-risk areas.

Long-term protection of alder populations requires integrated forest management and a cautious approach to moving plant material across regions. Monitoring the health of trees near waterways is critical for early identification of outbreaks and the containment of the disease to prevent broader ecological damage.