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Understanding tree diseases

Tree diseases are difficult to understand. If you are a regular reader of this column, then you may remember me talking about the disease triangle and its importance to whether or not a disease will occur.

While it’s true that tree diseases are contagious, the disease will only affect trees of the same species.

Verticillium wilt of maples does not affect oak trees. In fact, verticillium wilt only affects trees in the maple family.

A related misconception is that powdery mildew can develop on branches and trunks and kill a tree.

Powdery mildew is a cosmetic disease and does not usually kill any plant. It may get on tree leaves, but it’s never a concern in my book.

Powdery mildew may get on a branch or stem, but I have never seen powdery mildew get on a trunk. This disease just doesn’t work that way.

Then there’s the question of chlorosis. Chlorosis is a nutrient deficiency (say of nitrogen, iron or manganese). It’s very hard to diagnose a nutrient deficiency without a tissue sample.

Chlorosis can be caused by a lot of things; it has to do with the plant not being able to get the necessary nutrients.

As far as having flowers being underneath a diseased tree, the disease will probably not spread to these healthy plants. As mentioned earlier, a disease that affects a tree won’t likely affect your flowers or other landscape plants nearby.

Diseases are host specific. They survive on specific plants. Diseases are not generalists.

That said, I mentioned verticillium wilt affects maples and plants in the maple family. Therefore, impatiens are susceptible to verticillium wilt.

However, the pathogen that causes this disease on your maple tree is not the same pathogen that causes the disease on your impatiens planted under this maple tree.

Diseases are a challenge and sometimes difficult to understand. You can count on me to teach you about them and how to manage them.

Pamela Corle-Bennett is the state master gardener volunteer coordinator and horticulture educator for Ohio State University Extension. Contact her by email at

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