Report of Biscogniauxia nummularia as pathogenic agent of charcoal canker disease of beech (Fagus orientalis) from Iran

 Beech trees (Oriental beech: Fagus orientalis Lipsky.), native to Hyrcanian forests of Iran, are considered as one of the most valuable forest tree species in the north of the Iran. Biscogniauxia nummularia (Bull.) Kuntze is an endophytic fungus which acts as a pathogenic fungus under favorable conditions and causes strip-cankers (commonly known as charcoal canker) in Fagus species. The disease occurs in tree hosts affected by stress. High temperatures and summer droughts are favorable conditions for causing and distributing this disease in the forest.

During the field surveys across the beech forests of Sangdeh region in Mazandaran province, beech trees with severe symptoms of charcoal canker were observed. The most visible symptoms of the disease in the form of round spots to black stripes on the trunk and branches, as well as dieback of the beech tree branches in the area. Several samples were collected from infected parts of oriental beech trees and transferred to the plant pathology laboratory. Microscopic slides of the fungal stroma were prepared from infected tree bark and subjected to microscopic observation based on valid identification keys. Furthermore, the samples were cultured on Potato Dextrose Agar (PDA) culture medium and the pathogenic agent was identified after purification by hyphal tip and single spore techniques, using microscopic examination and morphological characteristics.

Results showed that Biscogniauxia nummularia is the agent of disease, which causes severe damage to beech trees and has already been identified as cause of beech charcoal canker in many European countries. This fungal species has some special morphological characteristics, including applanate stroma, ostioles are slightly papillate to nearly at the same level as the stromatal surface, ascus apical apparatus blue in iodine and Melzer's reagents, so it is separated from other species. This is the first report of identification of B. nummularia as the causal agent of beech canker disease from Iran. This fungus spends most of its life cycle as an endophyte, and under unfavorable environmental conditions, especially drought stress. B. nummularia takes advantage to alter host physiology, invading host tissue and causing black bark cankers on trunk and branches, known as strip cankers, and wood decay in mature trees.  

The direct relationship between the consequences of climate change, especially water and temperature stress in the host and the development of forest pathogens in the case of several important and key pathogenic fungi, including B. nummularia, has been reported by various researchers in the world, and it has been determined that environmental stress plays an important role in the development of this disease on beech trees in different parts of the world. Meanwhile, as one of the requirements of disease management, it is important to identify the phenotypic and genotypic characteristics of the populations of this fungus in Iran and to identify the distribution and frequency of the disease, which can be very useful in choosing its management strategies. On the other hand, due to the prediction of progressive adverse climate changes in the future, which are an important factor in the development of the disease, continuous monitoring of current changes in beech natural habitats should be included in forest ecosystem monitoring programs to predict future changes, possible ecosystem consequences, and as a vital part of sustainable forest management.