Septobasidium

The primary symptom is the appearance of thick, leathery, or velvety growths on the bark of trunks and branches. These growths can vary in color from light tan to dark brown or charcoal grey.

These fungal mats are firmly attached to the bark, making them look similar to crustose lichens. Upon close inspection, the mat is found to cover the underlying insect colonies.

As the fungus expands, the bark in the affected areas may become roughened or cracked. This physical change interferes with the normal appearance and potential health of the bark surface.

The signs are often most visible during the growing season when the fungal mats appear distinct against the healthy, smooth bark of younger branches.

In cases where the fungus is widespread, it can cover large sections of the scaffold branches, giving the tree an unhealthy appearance and indicating an underlying pest infestation.

The disease is caused by fungi of the genus Septobasidium, which belong to the class of Basidiomycetes. These fungi are unique as they are obligate symbionts with scale insects and soft scales.

The fungus does not infect the wood deeply but forms thick fungal mats, known as stroma, that cover colonies of insects. This structure provides a stable home for the insects while the fungus feeds on their byproducts.

The biology of the pathogen is strictly linked to the lifecycle of the host insects. The fungus spreads through spores that travel via wind or insects, landing on new bark surfaces to begin forming a new colony.

Since the fungus relies on the presence of scale insects, its distribution is directly related to the density and health of these insect populations within an orchard or garden environment.

Although it is often considered a minor disease compared to others, its persistence on the bark can lead to long-term issues for the tree's overall vigor if the insect population remains unchecked.

Septobasidium thrives in environments with high humidity and consistent rainfall, as the fungus requires a moist surface to expand its mycelial network across the bark.

The presence of scale insects is the most critical condition for the development of the fungus. Without the insects to provide a feeding site and foundation, the fungus cannot thrive.

Poorly pruned trees with dense foliage create microclimates characterized by reduced airflow and higher humidity, which are ideal for the rapid spread of the fungal mats.

Warm spring and summer temperatures accelerate the growth of both the insects and the fungal structures, often leading to visible infestations appearing within a single season.

Neglected orchards, where trees are not regularly pruned or treated for pests, are significantly more susceptible to the colonization of Septobasidium on tree trunks.

The harm caused by Septobasidium is largely due to the physical covering of the bark, which may inhibit the tree's natural respiratory processes and hinder growth.

A major problem is that the fungal mat acts as a protective shield for scale insects, preventing insecticides from reaching the pests and making chemical control efforts far less effective.

Over time, the affected branches may show signs of stunted growth, reduced vigor, and premature drying, which can negatively impact overall fruit production.

The tree's health is further stressed by the combined feeding of the insects and the colonization by the fungus, potentially reducing its resistance to other secondary diseases.

In severe cases, if a large portion of the tree's bark is covered, it can lead to the dieback of branches, necessitating severe pruning and potentially reducing the tree's lifespan.

Effective control of Septobasidium focuses on eliminating the scale insect population, as the fungus cannot survive or spread without its insect host.

• Regular pruning of heavily infested branches to remove the fungal mats and the underlying insects.
• Manual scrubbing of the bark to remove old, thick layers of fungal growths and debris.
• Targeted application of horticultural oils during the dormant season to suffocate scale insects.
• Using systemic insecticides at the correct timing to manage insect population outbreaks before the fungal mats can expand.

Maintaining proper tree structure through thinning cuts improves airflow, which helps keep bark surfaces drier and less suitable for fungal colonization.