Septobasidium pilosum

Septobasidium pilosum is a unique fungus within the order Septobasidiales, known for its intricate symbiotic relationship with scale insects (Coccoidea). As a basidiomycete pathogen, it is commonly observed as a felt-like or crusty growth on the bark of woody plants, effectively acting as an external parasite that shelters its insect hosts.

The primary symptom of infestation is the presence of distinctive, fuzzy, or leathery mats on branches, which serve as a protective cover for scale insect colonies. This condition typically affects various horticultural crops, including apple, pear, and citrus species. The fungus forms a complex, multi-layered structure that protects the insects while drawing nutrients from the honeydew produced by the scales.

The life cycle of Septobasidium pilosum is intrinsically linked to the reproductive cycles of its insect hosts. When the scale insects settle on the bark, the fungal mycelium grows over them, creating a sheltered environment. Basidiospores produced by the fungus are dispersed by wind or carried by the mobile crawler stages of the insects to new colonization sites on the same or adjacent trees.

High humidity and poor air circulation within the tree canopy are primary environmental factors that promote the development of this fungus. In regions with mild, humid climates, the fungus can persist year-round, gradually expanding its coverage across the limbs. If left unchecked, the accumulation of fungal mats can disrupt the bark's normal respiratory functions.

While the fungus itself does not directly rot the tree tissue, its presence indicates a severe underlying infestation of scale insects, which can significantly weaken the host plant. Effective control requires a multifaceted approach focused on both the insect vector and the fungal growth. Key management practices include:

• Scrubbing or pruning affected branches to remove fungal mats.
• Applying dormant oil sprays to target and suffocate scale insect populations.
• Implementing regular fungicide applications to prevent spore germination.
• Improving overall tree vigor through proper fertilization and canopy thinning.