Sycamore anthracnose

Apiognomonia veneta

also called Plane anthracnose

A sycamore or plane that leafs out and then drops everything in May, with brown killed along the veins. It refoliates by July and is rarely in real trouble.

Fungus Disfiguring Airborne sporesRain splash Spreads to neighbors

What you'll see

Buds and new shoots killed outright in early spring, so the tree looks frosted or dead in patches.
Brown, irregular blotches on expanding leaves following the veins rather than making round spots.
Heavy leaf drop in late spring, often a second one, with the ground under the tree carpeted in green-brown leaves.
Small cankers on the twigs, and a witches-broom of crowded shoots at the ends of branches after several years.
A second flush of clean leaves in early summer, which is the normal course of it.

Why it happens

A fungus that overwinters in twig cankers and infects the emerging buds and leaves in spring. It is a close relative of the anthracnose that spots beans and tomatoes but a quite different disease in practice — a tree disease with a twig phase, driven entirely by the weather in the two or three weeks around bud break.

A cool wet spring, and particularly a run of days below 13 °C while the leaves are unfolding. A warm dry April gives a clean tree. American sycamore is hit hardest; London plane much less; Oriental plane hardly at all.

Treating it

Do nothing in summer; the tree has already refoliated or is about to.
Rake up and remove the fallen leaves and twigs, which carry the fungus over.
Prune out dead twigs and small cankers in dry weather during the dormant season.
Water and mulch a young tree that has been defoliated twice in one year, which is a real drain on it.
Thin the crown of a tree that has built up a broom of congested twiggy growth.

Keeping it away

Plant London plane or a resistant cultivar such as Bloodgood or Columbia rather than American sycamore.
Give the tree an open site with air movement rather than a damp enclosed corner.
Keep the litter cleared each autumn under a small tree.
Feed and water a young tree well so it can afford two flushes of leaves in a bad year.
Do not reach for a fungicide on a garden tree; the spray timing is at bud break, the coverage on a large tree is impossible, and the disease does not warrant it.

Who gets it

American sycamore above all, then London plane, Oriental plane, and to a lesser degree ash, maple, oak and dogwood with their own species of the fungus.