Arthoniales lichens

Arthoniales is an order of fungi that form symbiotic relationships with algae, known as lichens. In an agronomic context, they are categorized as epiphytes that colonize the bark of fruit trees. While they do not directly infect living plant tissue like parasitic fungi, their excessive presence is a bioindicator of low orchard vitality and poor air circulation.

The biology of Arthoniales involves living on the surface of the bark, obtaining moisture and minerals from the air and dust rather than the tree itself. However, by covering the tree surface, they block lenticels, which are essential for gaseous exchange in the bark, potentially causing long-term physiological stress to the host tree.

Fruit crops commonly affected include pome fruits like apples and pears, as well as stone fruits. The issue is more prevalent in older orchards where canopy density is high and standard maintenance protocols, such as regular pruning or bark scrubbing, have been neglected over several years.

Symptoms appear as crustose or foliose patches on the trunk and primary branches. The color ranges from grey to green or brownish tones. Over time, the bark becomes brittle, and the expansion of the lichen layer provides shelter for overwintering pests, such as aphids and scale insects, which can cause significant damage to the plant.

• High humidity levels in the orchard microclimate.
• Poor ventilation due to overgrown crowns.
• Acidic soil conditions.
• Lack of routine tree trunk hygiene and sanitation.

Effective protection measures involve mechanical removal of lichens during the dormant season and improving overall orchard hygiene. Pruning the tree to improve light penetration and airflow is the most effective preventative measure. Chemical control using copper-based fungicides during the dormant period is highly successful in preventing colonization.