New Ophiostoma species associated with Dryocoetes himalayensis and decline of Juglans regia in Czechia

Abstract A new ophiostomatoid fungus involved in the decline of walnuts was discovered in Czechia. The fungus was isolated from necrotic wood surrounding the larval galleries of Dryocoetes himalayensis , the alien bark beetle attacking walnut trees. This insect species is rather unique within hardwood-associated bark beetle species for its aggressiveness and ability to attack apparently healthy trees. The isolated fungus, associated with an exotic insect vector is described as a new species, Ophiostoma juglandis sp. nov., based on its unique morphology and phylogenetic position. The pathogenicity tests confirmed the aggressiveness of the species to both Juglans regia and J. nigra plants, manifested by formation of extensive bleeding and necrotic lesions. The symbiotic relationship between O. juglandis and D. himalayensis is expected due to massive presence of the fungal spores on surface of adult beetles in pits forming the elytral striae. In the case of hardwood-associated ophiostomatoid fungal species, phytopathogenicity is very rarely observed. The described species is exceptional in this ability, which corresponds to the aggressive behaviour of its vector. The role of the fungus in walnut tree mortality caused by D. himalayensis is discussed.