Ophiostoma ulmi

Ophiostoma ulmi is a species of fungus in the family Ophiostomataceae. It is one of the causative agents of Dutch elm disease. It was first described under the name Graphium ulmi, and later transferred to the genus Ophiostoma. Dutch elm disease originated in Europe in the early 1900’s. Elm trees were once an ecologically valuable tree that dominated mixed broadleaf forests, floodplains, and low areas near rivers and streams. They were planted in urban settings because of their aesthetic appeal and their ability to provide shade due to their V like shape. An outbreak of Dutch elm disease in the 1920s and again in the 1970s was responsible for the death of more than 40 million American elm trees. Ophiostoma ulmi was the first known cause of Dutch elm disease . Since its discovery in 1910, new forms of the fungus, specifically Ophiostoma novo-ulmi, have emerged and appear to be more resistant to control measures and more aggressive in their infection. Host Range and Symptoms: Ophiostoma ulmi has a relatively narrow host range as it infects only elm trees (Ulmus spp.) and Zelkova carpinifolia. Habitat preferences of elms play a large part in determining their susceptibility as a host for Dutch elm disease. For example, of the three native European elm species (Ulmus glabra Huds., Ulmus laevis Pall. and Ulmus minor), all are susceptible to infection by O. ulmi, but Ulmus glabra has a much smaller chance of being inoculated than Ulmus minor. This is because the insect vector prefers the warm humid habitat of Ulmus minor to the cold hemiboreal habitat of Ulmus glabra. For this reason, Ulmus minor has been almost completely eliminated by the disease.  In North America, Ulmus americana, U. thomasii, U. alata, U. serotina and U. rubra are listed as highly susceptible to Dutch Elm disease, while U. crassifolia is less threatened. Ophiostoma ulmi causes symptoms commonly associated with most vascular wilts. Trees that have been infected by a vector will exhibit symptoms of leaf wilting and yellowing on branches and twigs that have been colonized by the Scolytid beetle. These symptoms are most often apparent from July into the autumn months. Trees that have contracted the disease via root grafts will often proceed much more quickly because the whole tree is compromised at once. Diagnosis of this disease is usually done by examining the xylem tissue of twigs and branches of the trees. Symptoms of brown streaking that runs in the direction of the grain of the wood, and tylosis formation by the tree as a reaction to the fungal infection are characteristic of this disease. Breeding efforts began as early as the 1920’s to try to combat this disease, and some crosses bred from resistant Asian species of elm and susceptible European species have shown a decrease in susceptibility to the pathogen. However, with the introduction of Ophiostoma novo-ulmi many of these resistant species struggle to survive. Environment: