Elm yellows: what the agencies record
What Forest Research, University of Illinois Extension and the Maryland Invasive Species Council say about elm yellows: the phytoplasma, the elm species it affects, the leafhoppers named as carriers, the symptoms, the smell test the US sources describe, the 2013 find in England and the notifiable status in the United Kingdom.
Checked against the sources at the bottom of this page on October 8, 2026. Rules, fees and pay change: the source has the last word.
This page reports what the official source says. It is not legal advice: ask the office named here. It lists what the sources record about elm yellows and does not diagnose any tree. It describes a reporting rule and import measures for the United Kingdom, so the exact wording is on the pages linked below. The list of other diseases is on the tree diseases page, and the reporting lines are in the United Kingdom report-a-pest guide.
At a glance
| Item | What the sources say |
|---|---|
| Pathogen | Candidatus Phytoplasma ulmi. Forest Research says phytoplasmas are specialised forms of bacteria. The disease is also called elm phloem necrosis, especially in North America. Maryland adds "witches' broom" as the old European name. |
| Hosts | Elms (Ulmus). The Maryland release names American elm and every hybrid and cultivar tested so far, plus winged, cedar, Chinese, September and red or slippery elm. Forest Research names English elm and says wych elm has not been confirmed as a host. |
| Where | Forest Research lists Italy, France, Germany, eastern states of the United States and southern Ontario. The Maryland release says the eastern half of the United States and central and southern Europe. |
| Status in the United Kingdom | Not present, and notifiable. Forest Research says it was found in 2013 on elm plants imported into England in 2012 and that surveys since have found no evidence it became established. |
| Report to | Great Britain: TreeAlert. Northern Ireland: TreeCheck. Trade premises: the plant health authority for the country. See the United Kingdom guide. |
How the sources say it spreads
Forest Research says the phytoplasma spreads by insects such as leafhoppers and by the movement of infected plants, and that spread by natural means is unlikely in winter when the trees have no leaves and the insects are not active. The Maryland release names the white banded leafhopper, Scaphoideus luteolus, and possibly other leafhoppers, feeding on sap through the growing season. It adds that outbreaks can kill nearly all native elms in an area but spread outwards slowly, and that new outbreaks further away are thought to come from leafhoppers carried by wind. Illinois Extension says the pathogen overwinters in dead and dying trees and is believed to be carried mainly in late summer and fall. On root grafts the two US sources disagree: Illinois Extension says the pathogen may also pass through them, while the Maryland release says there is no evidence for that idea.
Symptoms as the sources state them
- Leaves. Forest Research lists downward bending of leaves, yellowing, dwarfing, early leaf fall and early opening of buds. The Maryland release says symptoms usually appear by late summer, with rapid yellowing on a few branches or the whole tree, and leaves that wilt, shrivel and stay attached for several weeks.
- Twigs. Forest Research and the Maryland release both describe witches' brooms, dense clusters of twigs growing from one point. Maryland says they develop on red elm and on Eurasian species such as Chinese elm.
- Inner bark. The Maryland release says the inner bark of a diseased elm shows yellow to butterscotch with darker flecks, and Illinois Extension says the butterscotch colour shows when the phloem is exposed to air. Both describe a faint wintergreen scent from sealed samples of some species. They say red or slippery elm smells more like maple.
- Timing. Illinois Extension says leaf symptoms can show at any time in summer and that by then the damage inside is extensive. The Maryland release says symptoms take three to ten months to appear and that susceptible trees usually die within two years of showing them. Illinois Extension gives up to two years on red or slippery elm. Forest Research says that in Europe elm species tend to be much less affected than American elm.
Look-alikes and testing
Forest Research and Illinois Extension both say elm yellows is sometimes confused with Dutch elm disease, which has its own page: Dutch elm disease. Forest Research says trees with Dutch elm disease die back and die rapidly, while elm yellows can cause symptoms that do not end in death of the tree. The Maryland release says Dutch elm disease leaves brown streaks in the wood, unlike the colour of the inner bark here. Forest Research says other diseases cause similar symptoms on elm, so laboratory testing is needed to confirm infection.
What the agencies publish for control and regulation
Forest Research says the only known treatment is destroying infected trees before the disease reaches healthy ones, and refers to a contingency plan. Illinois Extension says there are no chemical controls and lists immediate removal of the tree and stump as one of the more effective ways to slow spread. The Maryland release says no chemical control or preventive technique is proven, and that insecticides used against the bark beetles that carry Dutch elm disease may also reduce leafhopper numbers. On regulation, Forest Research says the United Kingdom held European Union protected zone status for the phytoplasma from 2017, so imported elm plants needed a plant passport showing they came from a free place, and that since 2014 pending landings of elm plants must be notified to the plant health authorities. For moving wood in general, see moving wood and firewood. Who may apply pesticides is covered on the pesticide licences page.
Credit
The Forest Research material was read under the Open Government Licence v3.0 and is restated here in our own words.