Septobasidium tanakae

Pathogen and disease type. Septobasidium tanakae is a fungal disease belonging to the class Basidiomycetes. Unlike typical plant pathogens that invade living tissue directly, this fungus maintains a symbiotic relationship with scale insects. The fungus forms dense, crust-like mats on the bark of trees, which serve as protective shelters for colonies of these insects, while the fungus derives nutrients from the insect’s honeydew and the tree's surface.

Host plants and symptoms. The disease primarily affects fruit trees such as apple, pear, and various citrus species. The most distinctive symptom is the appearance of thick, leathery, or felt-like mats on branches and trunks. These structures often appear brownish or grayish and can expand over several seasons, covering significant areas of the bark. Unlike lichen, these growths are directly linked to the presence of scale insect infestations.

Environmental conditions. The development of Septobasidium tanakae is heavily dependent on the climate and the host insect population. Humid and warm environments are optimal for the growth of the fungal mats. The spores are primarily dispersed by wind and rain splashing, as well as through the movement of scale insects, which act as vectors for the fungus. High-density planting without proper airflow contributes to the rapid spread of the disease.

Economic impact. The damage caused by this fungus is primarily physiological. By covering large surfaces of the bark, the fungal mats restrict the tree's ability to undergo gas exchange, which is essential for healthy bark respiration. Weakened trees show reduced vigor, stunted growth, lower fruit yields, and increased susceptibility to other opportunistic pathogens and environmental stressors.

Protection and prevention. Effective management of the disease involves several key strategies:

• Thorough pruning and removal of infested limbs to reduce fungal inoculum.
• Control of scale insect populations using dormant oils or systemic insecticides.
• Applying copper-based fungicides to the bark during the dormant season to sanitize the surface.
• Ensuring proper orchard sanitation to prevent the spread of spores to healthy trees.