Garden Tips: Look up! The sycamores have blight

All you need to do is look up to see that many of the big old sycamores in our area are in trouble.

Sycamore blight, more correctly called sycamore anthracnose, is the cause. Before we talk more about this disease and care of infected trees.

What is a blight?

A plant blight is defined as a disease, usually fungal or bacterial, that causes the rapid decline and death of various plant parts, including leaves, flowers, stems, and buds.

For a blight or other plant disease to become a problem, there needs to be a causal pathogen, a susceptible host and environmental conditions favorable to disease infection and development.

When they all come together at the same time, you get the “perfect storm” leading to a plant disease.

Sycamore blight is caused by a fungus. It is a common disease on sycamores and plane trees, especially on western sycamore and the American plane tree.

Sycamore blight typically occurs when the prevailing daily temperature is about 55 degrees when the buds first start to open and during the two weeks following bud break as young leaves develop.

Infections are initiated by spores that are produced in cankers on already diseased trees. The spores are spread to new growth by wind and rain.

Sycamore blight tends to be most severe when spring weather is wet. Seeing how sad many local sycamore trees appear right now, our spring weather must have been perfect for sycamore blight.

Sycamore blight can infect and kill young leaves as soon as the buds open or infect them a little later causing brown blotches on the leaves. These blotches often grow together, killing the leaves and resulting in leaf drop.

The disease also causes twig and branch dieback and the development of the spore producing cankers that allow the disease to persist year after year.

The dieback of twigs results in the development of witches’ brooms at branch tips and along branches. Witches’ brooms are a proliferation of twigs at one spot that look somewhat like brooms and are very evident on the almost bare local sycamores.

It is difficult to control sycamore blight on large, mature trees. Cultural control includes pruning out infected twigs and branches when practical, raking and removing fallen leaves and providing the tree with sufficient water and fertilizer to keep it healthy.

There are protective fungicidal sprays that can be applied right as the buds start to open and again 10 days later, but getting good coverage over a massive mature tree is difficult and costly.

Tree injections with certain fungicides have proven effective, but both injections and spray applications on large trees must be done by licensed commercial applicators.

If anyone intends to plant a sycamore tree, they should consider planting a cultivar that is reportedly resistant to the disease, such as Bloodgood, Columbia and Liberty.

Will the area’s sycamores survive?

Yes, the presence of many witches’ brooms on older sycamores indicates that these trees have survived severe infections in the past and probably will again this time.

A second set of leaves will develop by the time summer arrives and make us forget the trees are infected, but the blight can easily erupt in coming years whenever springtime conditions are perfect.

For more detailed information on sycamore blight refer to the PNW Disease Handbook at https://pnwhandbooks.org/plantdisease/host-disease/sycamore-platanus-spp-anthracnose.

This story was originally published May 12, 2017 at 11:52 AM with the headline "Garden Tips: Look up! The sycamores have blight."