Dutch elm disease

Caused by fungus Ophiostoma ulmi

Elms and lindens are infected by Ophiostoma ulmi, which has small, black, rounded fruiting bodies that grow under peeling tree bark or in passages made by wood-boring insects. The mycelium of the fungus develops in the bark and vascular tissue of the tree during the next stage of fungus development (conidial stage). This stage of development of the fungus is called Graphium ulmi, therefore the disease is also often called graphiosis. The tree withers and loses its leaves first, then the branches dry out and the tree dies. Fungus spores are mainly carried by the wind and small beetles – bark beetles (Scolytus sp.). Tree decline is also cyclical because these beetles reproduce in large numbers every 4–6 years.