Onnia

The genus Onnia belongs to the Hymenochaetaceae family and is known to be a dangerous pathogen causing root and butt rot in woody plant species. Unlike many other saprotrophic fungi, these organisms act as primary parasites, attacking the root system and the lower bole, which leads to the progressive decline and eventual mortality of the infected tree.

The primary hosts for fungi of this genus are coniferous trees, particularly spruce, pine, and fir species. The pathogen typically gains entry into the plant through root wounds or root grafting, after which the mycelium colonizes the wood, causing a specific type of white pocket rot that degrades the structural integrity of the tree base.

Symptoms are often latent, manifesting only in advanced stages of decay. External signs include chlorosis or thinning of the needles, premature needle drop, and the presence of conks or fruiting bodies at the base of the tree or on exposed roots. These fruiting bodies are typically funnel-shaped or fan-shaped with a tough, leathery texture.

The development of the fungus is favored by high soil moisture and physical damage to the root system. Spread occurs through both airborne spores and root-to-root contact, allowing the fungus to move between trees in dense stands. This often results in the formation of infection centers, leading to group mortality within forest ecosystems.

Management strategies focus on silvicultural practices, including the sanitary removal of infected trees and the extraction of stumps to prevent pathogen transmission through root contacts. While chemical control is rarely used in forestry due to ecological considerations, effective forest hygiene and monitoring are essential for minimizing the economic and biological impact of the disease.