Sycamore Anthracnose: Why Spring Leaves Turn Brown and Curl
Brown, shriveled new leaves and dieback on the youngest twigs are the classic signs of sycamore anthracnose — the single most common problem on American Sycamore. It's a fungal disease that flares in cool, wet springs and looks alarming, but rarely kills an established tree. Here's how to recognize it and break its cycle.
Sycamore anthracnose (the usual culprit)
What's happening
The fungus Apiognomonia veneta infects expanding spring leaves and tender shoots, killing tissue along the veins. Young leaves emerge already browned, curled, and distorted; whole shoots may blacken and die back, often making the tree look as if it were frost-bitten. Repeated infection over the years produces the gnarled, crooked twig growth common on older sycamores.
How to confirm
Damage appears on the newest spring growth, not older leaves, and follows a cool, wet spring. Brown blotches spread along leaf veins rather than between them, and small twigs at the branch tips die back. Look for tiny dark fruiting bodies on the underside of infected leaves and on fallen leaf litter from last year.
How to fix it
There's no spray cure once infection takes hold. Prune out the dead, blackened twigs back to healthy wood with clean cuts to improve airflow and remove fungal reservoirs. A vigorous, well-watered tree usually pushes out a healthy second flush of leaves by early summer once the weather warms and dries.
Prevent it
Rake up and remove (don't compost) all fallen leaves and twigs each autumn, since the fungus overwinters there. Thin the canopy to improve air circulation, and keep the tree healthy with deep watering. For prized young trees, a preventive fungicide applied at bud break in chronically wet regions can help.
Late frost damage (the look-alike)
What's happening
A hard frost after the buds have opened can kill tender new leaves outright, blackening and curling them in a way that closely mimics anthracnose. The two often strike in the same cool springs, which adds to the confusion.
How to confirm
Damage appears suddenly across the whole tree right after a cold snap, rather than spreading along leaf veins over days. Check the forecast: a recorded freeze in the days before symptoms appeared points to frost, not fungus.
How to fix it
Nothing to do but wait — the tree will leaf out again from secondary buds within a few weeks as the weather warms. Avoid pruning until you can see which wood is truly dead.
Prevent it
You can't prevent weather, but a healthy, unstressed tree recovers from a late frost far more readily than one already weakened.
Powdery mildew on later growth
What's happening
After the spring flush, a separate fungus can coat leaves in a white, powdery film, distorting and curling them. This affects mid-to-late-season leaves rather than the earliest spring growth.
How to confirm
Look for a dusty white coating on the leaf surface, usually in late summer and on growth in humid, shaded, poorly circulated spots. Affected leaves curl and may yellow but lack the vein-following brown blotches of anthracnose.
How to fix it
Prune to open up the canopy for better airflow and sunlight. Remove the worst-affected leaves. Most established sycamores tolerate powdery mildew without lasting harm.
Prevent it
Site and prune for good air circulation, and avoid the soft, succulent growth that heavy nitrogen feeding encourages.
When to worry (and when not to)
Anthracnose looks dramatic but is rarely fatal — a healthy American Sycamore re-leafs and shrugs it off most years. Worry only when the same tree is severely defoliated several years running, when large branches (not just twig tips) are dying back, or when the tree fails to push a healthy second flush by midsummer. Persistent decline points to a stressed or aging tree that may need an arborist's eye; a single bad spring almost never does lasting damage.