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We all know when flu season comes around and what we can do to protect ourselves against likely infection. We also know the familiar scratchy throat that warns of an oncoming illness.

But how do we know when our trees are most susceptible to disease and when they’re trying to tell us they’re sick?

Every infection has its own symptoms and opportune season, but two of the most common diseases for trees in Michigan are particularly fatal if not nipped in the bud: Oak Wilt and Needle Cast.

Oak Wilt occurs primarily in red oaks between late spring and mid-fall, as infected trees begin to shed their leaves from the top down.

Leaves will often have a marked separation of color, with browning around the edges toward the center, or will fall prematurely while still green. The disease itself is caused by a fungus that obstructs the tree’s water passageways, affecting the outermost limbs first—which explains the downward direction of the wilt throughout the tree. Trees usually contract Oak Wilt from the root systems of other infected trees, often within a wide radius. Once manifesting symptoms, a red oak can die within several weeks if not properly treated.

Needle Cast, on the other hand, targets spruce trees and works from the inside out.

A spruce with Needle Cast will show a browning and eventual loss of its needles in patches, usually beginning with the lower limbs. Trees are most susceptible during periods of intense humidity, when the fungus is thriving, although there are a number of factors can make a tree vulnerable to infection.

In general, a tree that has been drastically or incorrectly pruned is more likely to contract a disease either from other trees or from pests, so it’s best to prune conservatively and to do so in the spring before most diseases are in full swing. Mulching can also help to ensure that a tree has enough moisture to maintain its immune system against potential infection. And as with your own health, it is wise to keep an eye out for changes in your trees’ appearance. Catching an infection early on could be the difference between a simple cure and a removal.

If you notice unhealthy signs in your trees, your best bet is to contact an ISA Certified Arborist to assess the condition and help you decide whether treatment is a worthwhile investment. In some cases it may be more economical to consider removal, depending on how far the infection has progressed.

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