Pyrenothrix nigra

The primary symptom of infection is the appearance of specific black spots or crusts on the bark of branches and trunks, often resembling a dark, primitive lichen.

Affected areas look like small, dark, slightly raised structures that are hard in texture and firmly attached to the bark surface of the host plant.

Under magnification, one can identify the fruiting bodies of the fungus (apothecia or pseudothecia), which are circular and have a distinct coal-black coloration.

The disease is often accompanied by the gradual dying of bark sections, which can lead to localized necrosis of the cambium layer beneath the fungal colonies.

The progression of the disease is usually slow, meaning visual signs may remain undetected by growers until the bark becomes significantly hardened and colonised.

The pathogen responsible is the microscopic fungus Pyrenothrix nigra, which belongs to the group of lichenized or semi-lichenized fungi, often mistakenly identified as simple epiphytes.

This species has a close biological association with the bark of woody plants, using it as a primary substrate for attachment and nutrition in a parasitic or symbiotic relationship.

The fungal mycelium penetrates the outer layers of the bark, forming specialized structures that allow it to withstand significant fluctuations in humidity and temperature.

The feeding habit of this organism is typically classified as weak-parasitic, where the fungus thrives on weakened bark tissues without causing immediate mortality to the tree.

The reproduction of the pathogen occurs via spores, which are dispersed by wind or water droplets to adjacent branches or trees within the immediate plantation area.

The development of Pyrenothrix nigra is directly dependent on environmental humidity, causing the fungus to spread most actively in dense and poorly ventilated tree canopies.

Shaded sides of trunks, where moisture remains for longer periods after rainfall, provide the ideal environment for the pathogen to colonize and form its black crusts.

The presence of weakened trees with pre-existing bark damage significantly facilitates spore establishment, creating entry points for subsequent mycelial growth.

Prolonged periods of moderately warm and rainy weather create favorable conditions for massive spore production and the expansion of the infected area within the orchard or forest.

Poor agricultural practices, such as lack of regular sanitation pruning and insufficient light penetration into the canopy, promote a microclimate suitable for the fungus.

The danger posed by Pyrenothrix nigra lies primarily in the clogging of bark pores, which disrupts the tree's normal gas exchange processes and inhibits growth.

Massive colonization by the fungus can contribute to premature aging and cracking of the bark, which opens the way for secondary infections and wood-boring pests.

Although this fungus rarely causes the rapid death of a mature tree, it significantly reduces its overall vigor, weakening the plant's resistance to more aggressive pathogens.

For fruit crops, the presence of widespread lesions reduces the market value of nursery stock and mature trees, which is critical for commercial gardening and nurseries.

Long-term parasitism depletes the tree's energy reserves, leading to a noticeable decrease in annual shoot growth and lower fruit yields over time.

The primary control method involves regular sanitary pruning, aimed at removing heavily infected branches and thinning the canopy to increase airflow.

Preventive whitewashing of trunks with lime solutions containing fungicides helps prevent spore attachment and improves the tree's overall phytosanitary status.

Application of copper-based products during the early spring season effectively suppresses spore development and prevents the further spread of the infection throughout the garden.

Maintaining the overall health of trees through optimal irrigation and fertilization is crucial, as healthy plants possess higher natural resistance to such fungi.

• Mechanical removal of dead bark patches.
• Ensuring proper spacing between trees for ventilation.
• Balanced phosphorus and potassium fertilization.
• Prompt fungicide treatment upon detecting the first signs.