Wall lichen

An important clarification for agronomists: Verrucaria muralis (Wall lichen) is not biologically a plant disease in the classical phytopathological sense. It is a species of lichen belonging to the Verrucariaceae family, leading an epilithic lifestyle. It does not parasitize the tissues of agricultural crops but develops on mineral substrates such as limestone, brick, or concrete, occasionally colonizing tree species with an alkaline bark reaction.

The agent of this growth is a symbiotic organism consisting of a fungus and an alga. In agronomic practice, this lichen is sometimes mistakenly identified as a pathogen because it covers the bark of trees or garden structures with dense dark spots. However, Verrucaria muralis does not penetrate the cambium and does not feed on plant sap, therefore, it does not cause necrosis or systemic plant diseases.

External signs of this organism on plants manifest as small, sunken fruit bodies (perithecia) that look like black dots. On stony surfaces in the garden, the lichen forms a thin, grayish or whitish crust. The main potential issue is creating a micro-environment for pests that may hide under the lichen crust within bark crevices, potentially sheltering insects during the dormant season.

The development of this lichen occurs extremely slowly under conditions of high humidity and stable substrate pH. Verrucaria muralis prefers shaded areas of the garden where poor ventilation and excessive moisture prevent surfaces from drying quickly. It serves as a bioindicator of air quality, as it is sensitive to high levels of chemical pollutants and sulfur compounds.

Protection and preventive measures are focused on maintaining the sanitary condition of the garden:

• Mechanical cleaning of tree bark using stiff brushes during the tree dormant period.
• Whitewashing trunks with lime, which creates an alkaline barrier unsuitable for lichen development.
• Regular pruning of the canopy to improve aeration and light penetration, reducing surface moisture.
• Preventive spraying with iron sulfate solution to discourage colonization by epiphytes.