Bacterial leaf scorch

Xylella fastidiosa

also called Xylella leaf scorch

Shade trees browning at the leaf margins from midsummer, a little worse every year, until whole limbs die. Incurable, slow, and often mistaken for drought.

Bacterium No cure Spread by insectsSpread on tools & hands Spreads to neighbors

What you'll see

Leaf margins browning from midsummer, with a narrow yellow or reddish band between the dead edge and the green center — the band is the diagnostic detail.
The scorch appearing on the oldest leaves first and on one branch or one side of the tree.
Early leaf drop from the scorched branches, but a normal flush the following spring.
The same branches worse each year, and new branches joining them, over five to ten years.
Branch dieback beginning at the tips, and eventually the loss of whole limbs.

Why it happens

A bacterium that lives in the water-conducting vessels of the tree and slowly blocks them, so the leaf margins die of thirst while the roots are wet. It is moved from tree to tree by sap-feeding leafhoppers and spittlebugs, and on grafting tools. The same organism causes Pierce disease in grapes and olive quick decline in the Mediterranean.

Widespread in the eastern and central United States on street and garden trees. Drought years make the symptoms far more obvious, which is why it is so often written off as drought stress and only recognized when the pattern repeats.

Treating it

There is no cure. Manage the tree for as long as it is safe and plan for its replacement.
Water deeply in drought and mulch the root zone; the tree cannot afford any further water stress.
Prune out dead and dying limbs for safety, disinfecting the saw between cuts.
Do not fertilize heavily to force growth the tree cannot support.
Have an arborist confirm it with a laboratory test before you act; drought scorch, root damage and anthracnose all look similar.

Keeping it away

Plant a mixture of species rather than a row of oaks, elms or sycamores.
Choose less susceptible trees where the disease is common locally — ginkgo, honeylocust, katsura, hornbeam.
Keep new trees well watered and mulched through their establishment years.
Disinfect pruning tools between trees.
Remove a tree that is more than half dead; it is both a hazard and a reservoir for the insects to feed on.

Who gets it

Pin, red and white oaks, elm, sycamore, sweetgum, red maple, mulberry and grape.