Thelonectria viridispora

The fungus produces sexual fruiting bodies known as perithecia, which appear as small, bright red or orange-brown clusters on the surface of the infected bark. These are typically visible once the mycelium has established itself within the host tissue.

Inside the perithecia, the fungus develops ascospores. The specific epithet viridispora refers to the distinct greenish color often observed in these spores, which serve as the primary inoculum for the spread of the disease.

Thelonectria viridispora acts as a facultative parasite, surviving in dead or weakened plant tissues. Its survival is aided by both asexual conidial spores and sexual ascospores, allowing it to persist across different seasons.

The pathogen enters the host through physical injuries or natural openings in the bark. Once inside, the hyphae colonize the inner tissues, disrupting vascular transport and leading to localized tissue necrosis.

The first clinical sign of infection is the development of sunken necrotic lesions on branches or the main trunk. The affected bark typically turns brown, dries out, and eventually cracks or peels away from the wood.

In humid conditions, small colorful spore-producing cushions appear on the surface of the lesions. These are often clustered around cracks in the bark, indicating active sporulation and high infectious potential.

Trees often respond to the pathogen by forming callus tissue around the infection site. This results in the development of characteristic swellings, galls, or deep vertical fissures known as target cankers.

Below the necrotic bark, the wood often becomes discolored, fragile, and darkened. If the infection girdles the stem, the transport of water and nutrients is blocked, leading to the rapid wilting and eventual death of the affected branch.

Foliage on the infected parts of the tree shows signs of distress, including premature yellowing, curling, and early leaf drop. In severe cases, the entire portion of the canopy above the infection point will die back.

Excessive humidity and prolonged rainfall are the most critical environmental factors promoting the development and spread of Thelonectria viridispora. Moisture is essential for spore germination and colonization.

The pathogen exploits physical wounds caused by pruning, frost cracking, hail, or insect damage. Any breach in the protective bark layer serves as a primary site for fungal infiltration and subsequent disease progression.

Temperatures between +15°C and +25°C are optimal for rapid mycelial growth. However, the fungus can tolerate a wide range of temperatures, remaining active even during cooler periods in early spring or late autumn.

Orchards with poor air circulation or overcrowding are particularly vulnerable. High planting density prevents the bark from drying quickly after rain, creating a perfect microclimate for the disease to flourish.

Stress factors such as nutritional deficiencies, drought, or poor soil management significantly increase a tree's susceptibility to the pathogen. Healthy, vigorous trees are generally more capable of resisting initial infections.

The disease causes significant damage to horticultural and silvicultural assets by inducing progressive bark necrosis. This leads to the loss of productive branches and reduces overall crop yield.

Chronic infection drains the plant's metabolic resources, leading to physiological exhaustion and decreased vigor. Affected trees become more susceptible to further attacks by secondary pests and diseases.

The structural integrity of the wood is compromised as the fungus breaks down the internal tissues. This increases the risk of limb breakage under wind load or fruit weight, threatening the physical stability of the tree.

In nurseries, the presence of the pathogen can result in high mortality rates for young saplings, causing substantial economic losses. Controlling the spread is vital to maintaining a healthy plantation.

If left unmanaged, the pathogen accumulates in the orchard, necessitating intensive and costly intervention. Effective management is essential to prolonging the economic lifespan of the trees.

The primary control measure is strict sanitation: all infected branches must be pruned back to healthy wood and immediately burned or removed from the site to minimize the spore load.

Disinfection of pruning tools between every single tree is mandatory. Using 70% alcohol or copper-based solutions prevents the mechanical transmission of spores from infected to healthy trees.

All pruning cuts and bark wounds should be treated promptly with wound dressings, horticultural pastes, or fungicidal sealants to create a physical barrier against pathogen entry.

Preventative applications of copper-based fungicides during the dormant season (or early bud break) are highly effective in reducing the overwintering spore population on the bark surface.

Applying lime-based whitewash to trunks and main scaffold branches in autumn helps prevent sunscald and frost cracks, thereby reducing the number of entry points for the pathogen.