Passalora sequoiae blight

Passalora sequoiae blight is a significant fungal disease primarily affecting various species of the Cupressaceae family. The pathogen, Passalora sequoiae (formerly known as Cercospora sequoiae), is responsible for severe needle drop and branch dieback in ornamental conifers and plantation trees.

The host range is extensive, including prominent species such as Sequoia sempervirens, Sequoiadendron giganteum, numerous Juniperus species, Thuja (arborvitae), and Chamaecyparis. It is a common problem in nurseries where high-density planting and favorable microclimates facilitate the rapid spread of fungal spores.

Symptoms typically manifest as small, chlorotic spots on the needles, which eventually transition into a distinct brown or reddish-purple necrosis. The infection usually starts from the lower, inner branches, where humidity is highest, and progresses upward. In severe cases, the tree exhibits significant thinning of the canopy and may lose all needles on infected branches.

The disease thrives under conditions of high relative humidity, frequent rainfall, and moderate temperatures. Spores are primarily disseminated by water splash during rain events and wind currents. Moist conditions are essential for spore germination and colonization of new tissue, making late spring and autumn critical periods for infection cycles.

To effectively manage the disease, it is crucial to employ an integrated pest management (IPM) approach. Key measures include removing and disposing of infected plant material to reduce the inoculum, improving airflow within the canopy via pruning, and applying preventive fungicide treatments during periods of active growth and favorable weather conditions for the pathogen.