Ash yellows

Candidatus Phytoplasma fraxini

A slow decline of white and green ash: short growth, thin pale leaves, and tufts of witches-broom shoots low on the trunk. No cure, and often mistaken for drought.

Phytoplasma No cure Spread by insects Spreads to neighbors

What you'll see

Growth slowing year on year, with the annual shoot extension shortening to a few centimeters.
Leaves smaller, thinner, paler green and often tinged yellow, and dropping early.
Tufts of many spindly shoots — witches-brooms — appearing on the trunk or the main roots, which is the most reliable sign.
Branch dieback beginning at the top of the crown and working down over several years.
Cracks in the bark of the lower trunk, and a tree that dies over three to ten years.

Why it happens

A phytoplasma — a wall-less bacterium living in the food-conducting tissue — carried between trees by sap-feeding leafhoppers. It disorganizes the tree hormone signalling, which is why the growth is short and the brooms appear. It is a relative of the aster yellows that greens the flowers of coneflowers and carrots.

White and green ash across eastern and central North America, and lilac. Drought years make the decline much more obvious. In much of the range it is now overshadowed by the emerald ash borer, which kills the same trees faster.

Treating it

There is no cure and no treatment. Confirm the diagnosis, then plan.
Water and mulch to keep the tree comfortable for as long as it has left.
Remove dead limbs for safety as the crown thins.
Have the tree assessed by an arborist; a declining ash is a hazard tree long before it is fully dead.
Rule out emerald ash borer first, which produces D-shaped exit holes and serpentine galleries the yellows does not.

Keeping it away

Plant something other than ash. This is the honest advice in a landscape that now has both the yellows and the borer.
Keep existing ash well watered and mulched so decline is slower and more obvious when it starts.
Do not propagate from a tree showing witches-brooms.
Manage the leafhopper reservoir in rough grass around a valued tree, though this only slows things.
Watch lilacs for the same brooming and stunting, which is often the first sign in a garden.

Who gets it

White, green and velvet ash, and lilac, which carries the same phytoplasma.

Lilac