Chalara

Chalara is the historical name for the asexual stage of Hymenoscyphus fraxineus, a fungal pathogen that causes ash dieback in ash trees. The disease is commonly called Chalara dieback of ash. The currently accepted name for the fungal species is Hymenoscyphus fraxineus; Chalara fraxinea was formerly used to describe the asexual morph.

The pathogen primarily affects ash species, especially Fraxinus excelsior, but can infect other Fraxinus species under some conditions. It was first detected in Europe in the early 1990s, with Poland among the earliest reported findings, and subsequently spread to many European countries and parts of western Asia. The introduction is linked to movement of plant material and wood products, and the disease has been reinforced by favorable climate in some regions.

Infection begins in the growing season, as the fungus colonizes leaf tissue and causes necrotic lesions on leaves, petioles, and shoots. This leads to progressive shoot and canopy dieback, reduced vigor, and, in many cases, tree mortality, particularly among young plantings. The pathogen overwinters in fallen leaves and other debris; in spring, fruiting bodies produce spores that can infect new growth. The disease can spread via windborne spores and through the movement of infected nursery stock, wood, or contaminated soil.

Management focuses on prevention, surveillance, and sanitation, as there is no widely effective chemical cure. Practices include monitoring vulnerable stands, removing or destroying infected trees and debris, restricting movement of ash material from infected areas, and implementing biosecurity measures in nurseries. Breeding and selection for resistant or tolerant ash genotypes are ongoing, offering long-term alternatives to mitigate impacts. The disease represents a significant ecological and economic threat to European ash populations and associated ecosystems.