Dutch elm disease—Ophiostoma =Ceratocystis spp.
Dutch elm disease (DED) is caused by introduced vascular wilt fungi, Ophiostoma (=Ceratocystis) ulmi and O. novo-ulmi.
Dutch elm disease initially causes foliage to yellow and wilt, usually first in one portion of the canopy. Leaves then turn brown, curl, and die but remain on branches. Cutting off bark reveals blackish to brown streaks in infected wood. Discoloration appears as dark, concentric rings when infected branches are cut in cross-section.
Do not confuse DED symptoms with the chewing damage of elm leaf beetle, which also causes leaf browning; viewed from some distance the damage looks superficially similar. Elm leaf beetle chews holes through leaves and skeletonizes leaf surfaces, which can be distinguished by inspecting foliage or dropped leaves close-up.
Dutch elm disease is spread by elm bark beetles, Scolytus spp. Bark beetle adults emerge from dead or dying trees or from elm logs infected with the fungus and carry spores that infect healthy elms when the adults feed in the crotch of young twigs.
People inadvertently spread this pathogen by moving elms or elm logs infested with beetles or fungi to disease-free areas. Tools used to cut, inject, or prune infected elms spread the fungi to other elms. The fungi also spread from infected elms through natural root grafts to nearby elms.
Dutch elm disease has killed several thousand elms in California, mostly in the San Francisco Bay Area and around Sacramento. Millions of elms have been killed in midwestern and eastern states since 1930.
American and European elms are highly susceptible to the disease, which originally is from Asia. Plant elms that are resistant to Dutch elm disease and elm leaf beetle. Some of these trees also resist phloem necrosis (elm yellows), a phytoplasma not yet reported in California. Alternatively, plant hackberry (Celtis spp.), hornbeam (Carpinus spp.), or other species that resemble elms but are not attacked by elm pests.
Most elm species and hybrids are adapted to summer rainfall. Maintain tree vigor by providing adequate summer irrigation in areas with summer drought.
Report suspected Dutch elm disease to the county agricultural commissioner. Remove infected elms immediately to eliminate them as a source of the fungi, which otherwise will spread to nearby elms. Digging a trench around infected trees deep enough to cut the roots may prevent the fungi from spreading by natural root grafts to nearby elms.
Pruning. Promptly prune off and dispose of dead and dying elm limbs as they commonly are infested with the fungus-spreading elm bark beetles and can pose a limb-drop (failure) hazard. Promptly pruning off recently diseased limbs may be a management method in areas where quarantine regulations do not require the removal of the entire tree. This "therapeutic pruning" can be effective only if done promptly during the first season when disease symptoms appear on a tree.
For therapeutic pruning to potentially be effective, the trees must be otherwise healthy and vigorous. Symptoms must be limited to one or a few limbs, and at least 10 feet of healthy wood, free of visible disease streaking, must separate the infected wood from the pruning point on the main trunk.
Be aware that unless tools are thoroughly cleaned and sterilized after contacting infected tissue, DED and other vascular wilt pathogens can be mechanically spread by unsterilized tools that contact internal parts of trees, such as tools used to prune trees or inject them with chemicals.
Avoid unnecessary pruning. Properly prune elms when needed, especially when trees are young and preferably during late fall and winter. Bury, chip, or (where permitted) burn freshly cut elm wood. Alternatively, seal elm logs tightly under clear plastic in a sunny location through the warm season and for at least 7 months, after which they are no longer suitable for beetle breeding.
Fungicides. No fungicides have been demonstrated to be effective against Dutch elm disease in California. Based on use in the eastern United States, injection of certain systemic fungicides by a competent, professional applicator may prevent DED and help some recently infected trees to survive if properly treated while less than 5 to 10% of their canopy is symptomatic.
Consider fungicide treatment only if Dutch elm disease has been discovered infecting nearby elms. Do not use fungicide alone; it has limited effectiveness, and only when combined with proper pruning and tree care.
If the main trunk or many limbs show symptoms, promptly remove the dying tree to reduce pathogen spread to nearby elms. The prompt detection and removal of dead and dying elms is the most effective method for managing the disease. See Dutch elm disease by the American Phytopathological Society and the U.S. Forest Service DED website for more information.