Septobasidium pedicillatum

Septobasidium pedicillatum is a basidiomycetous fungus known for its complex symbiotic relationship with scale insects on the bark of woody plants. Unlike typical parasitic fungi that invade plant tissues directly, this species forms thick, felt-like crusts on the surface of branches. These crusts provide a protective environment for scale insects, while the fungus benefits from the substances secreted by the insects during their life cycle.

This disease is commonly observed on various orchard crops, including citrus, apples, and pears. While it is not always immediately lethal, its presence is a clear indicator of a wider infestation of scale insects. The fungus is most prevalent in humid, subtropical climates where tree bark stays damp for extended periods, allowing the fungal mats to expand effectively.

Symptoms are easily identified by the presence of leathery or velvety patches on the branches. These fungal mats can range in color from gray to reddish-brown and are often mistaken for harmless lichens. Upon closer inspection, one can find the scale insects living underneath the fungal layer, effectively shielded from environmental stressors and many types of traditional pesticide applications.

The development and spread of Septobasidium pedicillatum depend on high humidity and the presence of a host population of insects. Spores are released and spread via wind and rain splash, settling into cracks or crevices in the bark. Once established, the fungus grows slowly, but given time and favorable conditions, it can encircle entire branches, significantly hindering the tree's health.

The damage caused by this fungus is primarily due to the impact of the protected scale insect populations and the restriction of bark respiration. Management and control measures should focus on the following approaches:

• Targeting the underlying scale insect population with systemic or contact insecticides.
• Applying dormant oil sprays to smother overwintering insects and fungi.
• Pruning and destroying severely infected branches to reduce the inoculum.
• Promoting good airflow throughout the canopy to decrease local humidity levels.