Phytophthora root rot of cypress

Phytophthora austrocedri is a destructive soil-borne oomycete pathogen responsible for significant dieback in various conifer species. Unlike many other Phytophthora species, this pathogen exhibits a high level of host specificity towards members of the Cupressaceae family, causing irreversible vascular damage to the infected hosts.

The primary hosts of this pathogen include various species of Cupressus, Juniperus, and Libocedrus. Its impact is particularly severe on Lawson cypress and other ornamental junipers widely planted in commercial nurseries and landscaped environments. Once established in a site, the pathogen can persist in the soil for several years.

The disease primarily manifests as root rot and stem cankers at the base of the trunk. As the infection progresses and girdles the stem, the transport of water and nutrients is severed. Typical symptoms include foliage chlorosis, which progresses to a distinct bronze or brown color, eventually leading to branch dieback and total tree mortality.

The development and spread of Phytophthora austrocedri are driven by environmental conditions, particularly water saturation in the soil. The pathogen produces motile zoospores that require water to swim through the soil profile and reach the roots of healthy trees. Movement is often exacerbated by contaminated irrigation water, human activities, and the movement of infested nursery stock.

Effective management requires an integrated approach focusing on exclusion and prevention, as curative treatments for established infections are generally ineffective. Recommended best practices include:

• Sourcing healthy, pathogen-free nursery stock from reputable suppliers.
• Implementing strict drainage management to avoid waterlogging in the root zone.
• Sanitizing pruning tools and machinery between different areas.
• Prompt removal and destruction of symptomatic plants and infested root systems.
• Monitoring irrigation systems to prevent the recycling of potentially contaminated water.