What attacks ash: pests and diseases the sources name
A register of the insect, the fungus, the mite and the phytoplasma that Canadian, British and Wisconsin sources tie to ash: which ash species each page names, where it is recorded, the season each source mentions, and where the full profile is.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page reports what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. Each row names its source, all read on October 9, 2026. Where this site has a full profile, the row links it, and the regulated status tables are on the linked pages and are not repeated here. Ash trees are covered species by species in tree species in the agency records, including white ash, green ash, black ash, pumpkin ash and Oregon ash.
The register
| Agent | What the sources record |
|---|---|
| Emerald ash borer, Agrilus planipennis | Kind: Beetle Hosts: The Canadian Food Inspection Agency says it attacks and kills all species of ash in the genus Fraxinus. Where and status: The agency lists it as present and regulated in set areas of Ontario, Quebec, New Brunswick, Manitoba, Nova Scotia and British Columbia. The University of Wisconsin-Madison says it was first found in southeastern Wisconsin in 2008 and that as of 2024 all 72 counties have confirmed infestations. Season: No season given on the pages read. The agency says infested trees in North America generally die after 2 to 3 years, and heavily infested ones have died after 1 year. |
| Ash dieback, Hymenoscyphus fraxineus | Kind: Fungus Hosts: Forest Research says it is especially destructive of common ash, Fraxinus excelsior, and also names narrow-leaved ash as susceptible. It says scientists believe all of the more than 60 ash species are susceptible, with varying tolerance. Where and status: Forest Research lists it as present in the United Kingdom and reportable, widely present in continental Europe and in Ireland. Season: Blackening and wilting of leaves and shoots in mid to late summer, July to September, which Forest Research gives as the best months to survey for foliage symptoms. |
| Ash flower gall | Kind: Eriophyid mite Hosts: The University of Wisconsin-Madison says the mite makes male ash flowers grow into green, tumor-like galls half an inch to an inch across. It does not name an ash species. Where and status: The page gives no regulated status. It says the gall does not harm the health of a tree, though in some years the tree looks unsightly and rarely the weight of the gall strains a branch. Season: The mites spend winter under the buds and begin feeding and starting gall growth early in spring. In late summer the galls turn dark brown and woody and can stay on the tree for more than one season. |
| Ash yellows, Candidatus Phytoplasma fraxini | Kind: Phytoplasma, a bacteria-like organism Hosts: The University of Wisconsin-Madison says it affects ash trees of all ages and that white ash is particularly susceptible. The same organism causes lilac witches'-broom. Where and status: The page says it likely occurs wherever ash is grown and has been reported widely in the United States and southern Canada. It gives no regulated status. Season: No season given. Symptoms usually occur within three years of infection, and leafhoppers are thought to be the main way the organism moves between trees. |
How the sources describe the effects
The agents in the table do very different things. Forest Research says ash dieback can kill young and coppiced ash quite quickly, while older trees can resist for some time until prolonged exposure or another pest or pathogen, and it names Armillaria, finally weakens them. The University of Wisconsin-Madison describes ash yellows as a chronic disease: infected trees grow slowly, sometimes at half the rate of healthy ones, with smaller, thinner, lighter green leaves, tufted branches called brooming in many cases, and crown dieback that can continue until the whole crown is dead. Its flower gall page sits at the other end, a cosmetic problem in most years.
The Wisconsin page on emerald ash borer lists canopy thinning from the top of the tree, sprouts along the trunk, bark splitting and woodpecker damage as general stress symptoms that other diseases and insects can also cause. It lists D-shaped exit holes 3/16 inch wide, S-shaped larval tunnels and metallic green adult beetles as specific signs.
Profiles on this site
The two regulated agents have full profiles. See the emerald ash borer profile for life stages, hosts and spread, and the ash dieback profile for symptoms, look-alikes and how it spreads. Ash flower gall and ash yellows have only the rows above, because the pages read give too few separate facts for a page each. Forest Research's own species entry for common ash is on the common ash page.
Who the sources say to report to
The Canadian Food Inspection Agency asks that emerald ash borer detections outside regulated areas be reported immediately through its online report form. Forest Research asks that sightings of ash dieback in Great Britain be reported through TreeAlert, and in Northern Ireland through TreeCheck, both of which need clear close-up photographs. The flower gall and ash yellows pages name no report line. The Wisconsin ash yellows page names the University of Wisconsin Plant Disease Diagnostics Clinic as the contact for testing. The reporting offices for each country are on the report a pest in Canada and report a pest in the United Kingdom pages.
Sources
- Canadian Food Inspection Agency, emerald ash borer page (modified July 27, 2026), read October 9, 2026
- Forest Research, ash dieback (Hymenoscyphus fraxineus) page, read October 9, 2026
- University of Wisconsin-Madison Division of Extension, Ash flower gall (revised August 6, 2012), read October 9, 2026
- University of Wisconsin-Madison Division of Extension, Is my ash tree worth treating for emerald ash borer?, read October 9, 2026
- University of Wisconsin-Madison Plant Disease Diagnostics Clinic, Ash yellows (revised February 28, 2024), read October 9, 2026