Septobasidium pseudopedicellatum

The fungus does not penetrate deep into the plant tissues. Instead, it grows on the surface of branches and trunks, forming dense, leathery, or felt-like structures known as stroma, which can cover large areas of the tree bark.

The biology of this fungus is intrinsically linked to scale insects (Coccidae). The fungal mat provides the insects with physical protection against predators, parasites, and chemical sprays, while the insects provide the fungus with nutrients from the plant sap.

As a basidiomycete, it reproduces via basidiospores. These spores are disseminated by wind, rain splashes, and contact with insects, enabling the spread of the fungus throughout the orchard under favorable humid conditions.

Identification is based on the presence of these distinctive brown to grey patches on the bark. Because they are often mistaken for lichens or epiphytes, close inspection is required to reveal the associated scale insect colonies hidden underneath.

The primary hosts of Septobasidium pseudopedicellatum are citrus species, including oranges, grapefruits, and lemons. It can also occur on various evergreen broadleaf trees in tropical and subtropical climates.

The damage caused is primarily through the physical smothering of the bark, which interferes with gas exchange and respiration of the woody tissues, leading to a general decline in the tree's physiological health.

The most significant harm is indirect: the fungal mat acts as an impenetrable shield for scale insects, allowing them to feed undisturbed. This results in heavy infestation levels that stunt growth and reduce fruit yield and quality.

Affected trees may show symptoms of chlorosis, leaf drop, and dieback of the smaller branches. The overall vitality of the tree decreases significantly over time, making it more susceptible to other diseases and environmental stress.

In commercial orchards, the presence of this fungus complicates pest management programs, as standard contact insecticides often fail to reach the scale insects protected by the thick fungal layer.

The most common sign is the development of firm, leather-like, or felt-like patches on branches and trunks. These patches can range from light brown to dark grey and often have an irregular, crusty appearance.

A key indicator is the association with scale insects. If you peel back or examine the edges of the fungal growth, you will find active scale insects, which explains why these colonies are so difficult to eradicate.

Unlike many other fungal growths, the Septobasidium stroma is firmly attached to the bark and does not wash off easily with rain. Its texture is more consistent and robust than that of typical lichen growth.

During periods of high humidity, the fungal layer may appear more swollen or active. Surrounding areas of the bark may also appear stressed, with premature cracking or discolouration visible on the affected woody parts.

Persistent re-infestation of scale insects in specific areas of the tree canopy is a strong indicator of the presence of this fungus, which provides a sanctuary for the pests to survive control efforts.

Control strategies must prioritize the eradication of the scale insects. Since the insects are essential to the fungus's life cycle, managing the insect population leads to the natural decline of the fungal colonies.

Sanitation is crucial. Heavily infested branches should be pruned and removed from the orchard to reduce the inoculum level and improve the airflow and sunlight penetration within the canopy.

Mechanical cleaning of the bark using stiff-bristled brushes is an effective way to remove the fungal stroma and expose the insects, making subsequent chemical control treatments much more effective.

Applying copper-based fungicides during the dormant season can help inhibit the growth of the fungal mats and prevent the spread of spores, providing a cleaner surface for the tree to grow new shoots.

• Regular pruning to maintain tree vigor
• Targeted use of systemic insecticides
• Application of horticultural oils to suffocate scale
• Frequent monitoring during the peak insect breeding season