Bacterial leaf scorch

What is bacterial leaf scorch?

Bacterial leaf scorch is caused by the bacterium Xylella fastidiosa. It lives in the xylem, the tree's water-carrying tissue, and clogs it as it multiplies. Sap-feeding insects such as leafhoppers (sharpshooters), treehoppers and spittlebugs pick it up while feeding and carry it to other plants.

Infected trees leaf out normally in spring. By mid to late summer, leaf edges turn brown while tissue near the veins stays green. Symptoms usually start on one branch and come back every year, spreading through the crown. Tennessee Extension says this can take 5 to 10 years on large trees. Growth slows and branches die back, and Kentucky Extension says the whole tree eventually dies.

Because the scorch looks like drought, transplant shock or other wilt diseases, eye diagnosis is unreliable. Kentucky Extension says a lab tests leaves and petioles (leaf stalks) of scorched leaves. There is no cure. Illinois Extension says easing root stress is the only way to extend an infected tree's life, and Kentucky advises planting a replacement before the sick tree has to come out.

Plants it infects: pin oak, northern red oak, scarlet oak, American elm, American sycamore, red maple, sugar maple, mulberry, sweetgum, hackberry, ginkgo, catalpa.

Symptoms and signs

• Leaf edges turning brown in mid to late summer while the area near the veins stays green
• A yellow or reddish-brown band along the inner edge of the scorched area on some trees
• Symptoms starting on one branch, often on lower, inner leaves
• The same branches scorching again each year, with more of the crown involved over time
• Scorched leaves that drop early or stay attached
• Twig and branch dieback and slower growth after several years

Bacterial leaf scorch identification: what to look for

• Symptoms: Marginal browning of leaves in mid to late summer, with green tissue remaining near the veins. Kentucky says the scorch can be brown, reddish-brown or olive green depending on the host.
• Symptoms: A yellow or reddish-brown band may run along the inner edge of the scorched tissue (Kentucky). UMD says a yellow halo shows on some species.
• Damage: Recurring scorch on more branches each year, then reduced growth and dieback. Water sprouts may grow at the base or along the trunk.
• Timing: Symptoms appear mid to late summer. Kentucky says samples are best collected in late summer or early autumn.

Disease cycle

The bacterium lives in the xylem of infected trees and many other plants, including shrubs, grasses and weeds. Xylem-feeding insects pick it up while feeding. Kentucky Extension says it multiplies inside the insect, which can keep spreading it until it molts or dies. Insects then infect healthy trees as they feed. Kentucky says tree-to-tree spread does not appear to be fast, and the disease usually builds slowly over several years.

When to look: Look in mid to late summer and early fall, when scorch shows up.

Bacterial leaf scorch look-alikes

A few things get mixed up with it:

• Drought or environmental scorch: University of Maryland says drought scorch is more uniform and starts on upper branches and younger leaves. Bacterial leaf scorch recurs on the same branches each year.
• Oak wilt (Bretziella fagacearum): Purdue notes bacterial leaf scorch leaves can look like oak wilt. Oak wilt can kill a red oak in one season, while bacterial leaf scorch builds over years. A lab test tells them apart.
• Other vascular wilts and root decay: Kentucky lists vascular wilts, root decay, drought and transplant shock as look-alikes. Only a lab test confirms bacterial leaf scorch.

How to manage bacterial leaf scorch

Cultural control

• Water during hot, dry weather and mulch the whole root zone to cut stress (Kentucky).
• Prune out infected and dead branches. UMD says prompt pruning can help trees persist.
• Fertilize and adjust soil pH only as a soil test calls for, and avoid overfeeding (Kentucky).
• Plant a non-susceptible replacement tree early, before the infected tree has to come out.
• Remove trees with extensive dieback and scorch (Illinois), or once they become hazards.

Chemical control

There is no cure. Controlling the insect carriers is impractical because they are active most of the season. Some arborists inject antibiotics into the trunk, which Tennessee Extension says only suppresses symptoms and must be repeated each year in late May or early June. Kentucky Extension does not recommend these injections because they lack long-term proof and the wounds can let in decay. Use only products labeled for this use; the label is the law. Any injection must be done by an ISA Certified Arborist with a pesticide applicator license (see pesticide applicator license guide).

Use only products labeled for this disease and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• When replacing trees, choose species that are not hosts (Kentucky Extension).
• Keep susceptible trees watered and mulched in hot, dry weather.
• Reduce root stress, which Illinois Extension says is the only way to extend an infected tree's life.

When to call a pro

Call an ISA Certified Arborist if the same branches scorch every summer. They can collect leaf samples for a lab test, check for hazard limbs, and help plan a replacement planting.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

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