Prosthemium

The primary symptom of Prosthemium infection is the formation of distinct, depressed, or sunken necrotic patches on the bark. These spots often start small and expand slowly, changing color to brown or black.

As the infection progresses, the bark on the affected areas tends to split and peel away. Close inspection reveals small, dark, pimple-like structures on the surface, which are the fungal pycnidia releasing spores.

Affected trees often show signs of branch dieback. This starts from the tip of the branch and works its way down to the trunk. The foliage on infected branches often yellows and wilts due to the disruption of water and nutrient transport.

If the infection occurs on the main trunk or at the base of branches, it can cause girdling, which leads to the sudden death of all distal plant tissues and significantly weakens the overall structure of the tree.

In humid conditions, the pycnidia may exude a slimy mass containing millions of spores, which is a clear diagnostic feature when observed under magnification.

The causative agent of the disease is the fungus of the Prosthemium genus, classified within the Ascomycota phylum. These fungi are known for colonizing the bark and cambium layers of various woody plant species.

The biological cycle of the fungus involves the development of pycnidia, small fruiting bodies where spores are matured. These pycnidia are typically found within the crevices or necrotic areas of the bark.

The pathogen functions as a necrotrophic parasite, obtaining nutrients by killing the cells of the host plant. It remains dormant during the cold season within infected twigs and branches, waiting for spring temperatures.

Transmission occurs through spores being dispersed by wind and rainwater splash. Once on a receptive host, the spores germinate and enter the plant tissues through mechanical wounds such as cracks, pruning cuts, or frost damage.

The resilience of the fungus allows it to persist in dead wood for long durations, making it a persistent threat in forests, orchards, and urban landscapes.

High humidity and moderate temperatures are the most critical factors for the development and spread of Prosthemium. Extended periods of rainfall during spring significantly increase the rate of spore germination.

Physiological stress, such as drought, nutrient deficiency, or winter injury, renders a tree much more susceptible to invasion. A weakened immune system makes it difficult for the plant to compartmentalize the infection.

Mechanical damage to the bark is the primary entry point for the fungus. Frost cracks and sunscald are particularly dangerous as they create open areas that remain moist and protected from direct sunlight.

Crowded plantings that prevent proper air circulation around the crown lead to a high-humidity microclimate, which creates an optimal environment for the fungi to settle and thrive.

The presence of contaminated plant debris from previous seasons serves as a continuous reservoir for the pathogen, allowing new infections to start as soon as environmental conditions become favorable.

Prosthemium causes significant harm by destroying the protective bark layer, which leads to branch dieback and an overall loss of tree vigor. Over time, this leads to a sparse, unattractive canopy.

Chronic infection poses a serious threat to the longevity of the plant. If the fungus reaches the main stem, the tree may eventually succumb to the infection, leading to total loss of the specimen.

In commercial orchards and nurseries, the disease can cause severe economic losses, requiring the removal of infected stock and increasing expenditures on preventive maintenance and fungicides.

The disease also facilitates secondary attacks by wood-boring insects. Weakened trees, unable to produce defensive resin or sap, become prime targets for pests that eventually complete the destruction of the wood.

In public landscapes, the presence of diseased branches creates a safety hazard, as dead limbs can break off unexpectedly during high winds, posing risks to people and property.

Sanitation is the cornerstone of control. All infected branches must be pruned well below the site of the necrosis into healthy wood, and the pruning tools should be disinfected with alcohol between cuts.

Preventing physical damage to the bark is essential. White-washing tree trunks in late autumn or early spring helps protect against sunscald and reduces the number of entry points for the pathogen.

The use of copper-based fungicides is recommended for preventive treatment, particularly during the dormant phase of the plant. These applications help reduce the concentration of spores on the bark surface.

Ensuring the tree is well-nourished and adequately watered helps it maintain a strong defensive response, preventing successful fungal colonization even if spores are present.

• Monitor the bark for necrotic spots every spring.
• Properly dispose of infected branches by burning.
• Improve air circulation through thinning cuts.
• Choose resistant cultivars when planting new trees.
• Sterilize pruning equipment to avoid cross-contamination.