Phytophthora root rot of trees

Phytophthora lacustris is a species of pathogenic oomycetes responsible for serious diseases in various tree and shrub species. Initially identified in aquatic environments and riparian ecosystems, this pathogen is now recognized as a significant threat to forest health and ornamental plant nurseries. It belongs to a complex of Phytophthora species that thrive in moisture-rich habitats.

The pathogen primarily targets the root system and the root collar of susceptible hosts. Woody plants such as alder, willow, beech, and various conifers are particularly vulnerable. Phytophthora lacustris infection frequently leads to root decay, disrupting the tree's ability to uptake water and nutrients, which eventually leads to the decline and death of the plant.

Visible symptoms often include yellowing of leaves (chlorosis), wilting, and premature foliage drop. In advanced stages, dieback of twigs and branches becomes apparent. A critical diagnostic feature is the presence of cankers or weeping lesions on the lower trunk, often characterized by a dark, viscous exudate. Peeling back the bark in these areas reveals necrotic, reddish-brown tissue beneath.

Development and spread are heavily dependent on water availability. The motile zoospores of the pathogen travel through saturated soils and flowing water, infecting plants through fine roots or stem injuries. Periods of excessive rainfall, flooding, or poorly drained sites significantly increase the risk of infection, as the pathogen exploits wet conditions to colonize new hosts rapidly.

The economic and ecological impact of Phytophthora lacustris includes stunted growth, loss of biodiversity, and mortality in nursery stocks. Management and prevention strategies are essential to mitigate its impact:

• Improving soil drainage and preventing waterlogging in planting sites.
• Strict hygiene measures, including the use of clean nursery stock and disinfected tools.
• Removal and destruction of infected material to prevent the spread of inoculum.
• Application of phosphite-based fungicides to protect vulnerable trees in high-risk areas.