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SICK SYCAMORES

written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA

Have you noticed how sick some area sycamore trees are looking? That’s because they’ve been hit hard by sycamore anthracnose, also known as sycamore blight. Sycamore anthracnose is a fungus disease that attacks developing buds, shoots, twigs and leaves. This year it’s particularly severe because the conditions were just right in early spring. The ‘just right’ conditions for an infection were cool (temperatures below 55 degrees) wet weather about the time that the buds started to swell and expand.

Perhaps you’ve also noticed that some sycamores are not as severely affected as others. Microclimates may be responsible in some cases, but different species of sycamores also vary in their susceptibility to the disease. The native sycamores, the western sycamore (Plantanus racemosa) and the American plane tree (Plantanus occidentalis) are highly susceptible. The non-natives, the Oriental plane tree (Plantanus orientalis) and the London plane tree (Plantanus acerifolia) are less affected, but are not immune to anthracnose.

Because the disease is a severe problem in areas of the country that repeatedly have cool, wet springs, several cultivars of London plane tree have been selected for resistance to the disease. Today anyone who plants a sycamore and doesn’t want to deal with repeated severe ‘blight’ infections should look for ‘Bloodgood,’ ‘Columbia,’ and ‘Liberty’ cultivars.

We know the right conditions for infection, but how does the disease start? Like other fungus diseases, infections come from fungal spores. These spores are produced in cankers on the twigs and branches of trees that were infected in the past. Old infected leaves are also a source of spores. The spores are spread by wind and water. When the conditions are right, an infection can easily occur. The spores will attack buds, shoots, or leaves, depending on the stage of growth when the infection occurs. If it happens early, just as the buds and twigs start to expand, a tree will look almost dead and will initially develop few if any leaves.

If infection happens later, then just leaves may become infected. The leaf spots caused by an infection are brown angular areas associated with the midrib or main veins in the leaf. The area first looks brown and water soaked. These spots later turn tan and dry. Severely infected leaves may drop from the tree.

Despite the obvious ugly, denuded appearance of the infected sycamores, there’s really not much to worry about. Trees in our region seem to survive these occasional severe infections. By summer, new shoots and leaves will develop and the trees won’t look so bare. Twig die-back is also a result of infection and causes bushy clusters of twig growth, giving the tree a ‘witch’s broom’ appearance. However, this is more of a cosmetic problem and is mostly evident only in the fall and winter when the tree is bare.

Fungicides can be applied for control of the disease just when the buds start to swell and again 10 days afterwards. However, it’s difficult to get this timing just right and to get good coverage over the entire crown of a large tree. There are fungicide injections that can be tried, but these must be done over a two year period by a trained arborist. Unless your sycamore is a specimen tree, it’s probably not cost effective to invest in spraying or injection treatments. While it may render the trees a bit ugly, sycamore anthracnose will not kill our area sycamores.

Published: 5/20/2006 11:18 AM

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