Phytophthora lateralis

Pathogen and disease type. Phytophthora lateralis is a virulent oomycete pathogen known for causing lethal root and crown rot in conifer species. As a soil-borne water mold, it invades the root system of susceptible hosts, causing extensive necrosis that eventually disrupts water and nutrient transport, inevitably leading to the death of the infected plant.

Affected host plants. The primary and most susceptible host is the Lawson cypress (Chamaecyparis lawsoniana). However, the pathogen can also infect other species within the Cupressaceae family. Because these trees are widely used in nurseries, ornamental gardening, and forest plantations, the spread of this pathogen can lead to significant ecological and economic consequences.

Symptoms and signs. Symptoms usually appear as a gradual chlorosis of the foliage, which turns from a healthy green to a dull yellow and then bronze-brown. A key diagnostic feature is found at the base of the stem: if the outer bark is removed, the inner bark and cambium display a distinct reddish-brown discoloration, contrasting with healthy tissue. This lesion often eventually encircles the stem, cutting off the supply chain from the roots.

Conditions for development and spread. The pathogen thrives in cool, moist, and poorly drained soils. It produces motile zoospores that swim through water-saturated soil or surface runoff to infect new hosts. Long-distance dispersal occurs primarily through the movement of infested soil or plant materials, such as nursery stock, machinery, footwear, and vehicles that carry contaminated mud into previously unaffected areas.

Pathogenicity and control measures. This disease is notoriously difficult to manage once established, as chemical control options are very limited or ineffective for infected trees. Preventive measures are the only reliable defense strategy:

• Sourcing certified disease-free nursery stock.
• Ensuring excellent soil drainage in planting sites.
• Strict sanitation protocols for cleaning tools and equipment.
• Prompt removal and destruction of symptomatic plants, including the entire root system and surrounding soil.