Arthoniomycetes

Arthoniomycetes are a class of fungi that frequently appear as lichenized organisms on the surface of woody plant bark. The primary sign of their presence is the formation of spots, crusts, or patches of varying configurations on tree trunks and main branches.

Visually, these infestations appear as flat, dry, or slightly raised patches that may have a gray, whitish, or yellowish tint. Unlike active parasitic fungi, they do not cause tissue rot directly, but they significantly alter the aesthetic and surface condition of the bark.

Upon close inspection, one can identify the fruiting bodies, known as apothecia, which appear as small black streaks or dots embedded within the lichen thallus. Over time, these formations can cover significant areas of the trunk, creating a dense, crusty layer.

Plants affected by these fungi often appear weakened, and their bark may lose its natural elasticity. It is critical to distinguish these surface formations from deep necrosis caused by dangerous pathogens, as Arthoniomycetes are primarily epiphytic organisms.

In neglected orchards, these fungi may persist for years, gradually expanding across aging trees, which often serves as a indicator of reduced tree vitality and overall health.

The agents of this process are members of the class Arthoniomycetes, belonging to the division Ascomycota. These are symbiotic organisms, most of which associate with algae to form a lichenized thallus.

A biological hallmark of this class is the specific type of fruiting body, the apothecia, which often take on an elongated shape. This is a key morphological marker that distinguishes them from other fungal species commonly found on bark.

Most species in this class lead a saprotrophic or epiphytic lifestyle, using tree bark exclusively as a substrate for attachment. They lack specialized apparatus for penetrating living plant tissues, which distinguishes them from true parasitic pathogens.

Spores of the fungus are dispersed by wind, rain, or insects, settling on moist bark surfaces. Under favorable conditions, they germinate, forming new colonies that eventually develop into visible patches or crusts.

The scientific importance of these fungi in agronomy lies in their role as bioindicators of environmental conditions, often highlighting issues like poor air circulation or excessive shade within the tree canopy.

The primary factor promoting the development of Arthoniomycetes is high humidity and poor light penetration in the canopy. They thrive in orchards where trees are planted too densely, restricting airflow.

Congested canopies create a microclimate with stagnant air, which is ideal for maintaining the moisture levels required for thallus hydration. Lack of regular pruning acts as a direct stimulus for the expansion of these organisms.

The presence of old, dead bark with cracks provides an ideal texture for spore attachment. Fungi often colonize areas where bark surfaces remain damp for extended periods after rain or morning dew.

Poor orchard hygiene and improper irrigation methods (such as overhead sprinkling that keeps the trunks wet) can facilitate the accelerated development of these lichen colonies.

In ecologically pristine areas with high moisture, the growth of many lichen species is vigorous; however, Arthoniomycetes demonstrate high adaptability to various garden environments.

The direct harm caused by Arthoniomycetes to woody plants is considered minimal as they do not feed on plant sap. However, their mass proliferation can negatively impact the overall physiological state of the tree.

A dense layer of lichen can obstruct the pores (lenticels) of the bark, which hinders natural tree respiration and slows down gas exchange. This imposes additional stress on the plant, especially during drought or dormancy periods.

In areas where these fungi accumulate, they often provide shelters for wintering garden pests, such as scale insects, spider mites, or leafroller larvae. Consequently, they act as an indirect factor in raising the infection pressure in the orchard.

Visual damage to the bark reduces the aesthetic value of the trees and complicates the monitoring of early stages of other, more dangerous diseases like black rot or cytospora canker, which may hide beneath the lichen layer.

The presence of a significant amount of fungi on a tree often signals to the grower that the plant requires revitalization, rejuvenating pruning, and improved cultural practices.

The best method for management is regular sanitary and rejuvenating pruning, which ensures proper ventilation and illumination of all tree parts, preventing moisture stagnation.

Mechanical cleaning of trunks and main branches to remove dead bark and lichen patches using wire brushes or scrapers allows for the effective removal of fungal colonies without damaging living tissues.

After mechanical cleaning, it is advisable to apply a whitewash to the trunks and bases of main branches using a lime solution with added copper sulfate to disinfect the surface and prevent re-infection.

• Implementing annual canopy pruning.
• Ensuring proper irrigation regimes.
• Timely disinfection of bark damage.
• Maintaining overall tree immunity through fertilization.

In cases of severe infestation, the use of copper-based fungicides in the early spring before bud break is permissible, which can significantly reduce the spore population in the garden environment.