There’s no doubt about it, microbes—fungi, bacteria, viruses, and
others—can wreak havoc on plants, and people. Perhaps the best—or
worst—examples of this fact are the potato famines that occured in Ireland in the
1800s. To say that potatoes were a staple food for the Irish is an understatement; for
many, potatoes were the only food. During the famine of 1845-1847, over one million
people died of starvation, and millions more emigrated to the United States, mainland
Britain, and Australia—all because of a microbe named Phytophthora infestans.
This fungus causes the disease late blight of potatoes; in the summer of 1846, it wiped
out virtually the entire potato crop in Ireland.When
a microbe causes disease in an organism, it is called a pathogen (Gr. pathos =
suffering, genesis = beginning). Late blight of potatoes is just one of the many
plant diseases caused by microbes. While you may or may not have encountered this
particular disease, you’ve undoubtedly seen symptoms of disease in your garden. Have
you seen the powdery gray-white coating on your phlox, bee balm, or squash plants’
leaves? Powdery mildew is caused by the fungus Erysiphe cichoracearum. Have you
ever seen blackened twigs on your pear or apple trees? Fire blight, so named because the
twigs appear scorched, is caused by the bacterium Erwinia amylovora. And the
tobacco mosaic virus (TMV) causes disease symptoms in tomatoes, eggplant, peppers, and,
yes, tobacco. (The dried tobacco in cigarettes can harbor the virus, so smoking in the
garden can introduce TMV; another good reason to quit!) Fungi, bacteria, and viruses cause
most of the diseases you’re apt to come across in the garden, though there are other
categories of microbial pathogens.
To sum things up: Not all microbes cause harm, and not all
are harmless either.
Next, we’ll look at some plant-insect relationships.
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 Powdery Mildew
Blues
Are you concerned that the powdery mildew on your phlox will
spread to your beloved roses? It won’t.
Although the symptoms are similar—a powdery grey-white
coating on leaves—there are many different species (and subspecies) of powdery
mildew, and each attacks a different family of plants. So while the powdery mildew on your
cucumber plants may spread to your pumpkins, it won’t spread to your bee balm.
Powdery mildew is caused by the fungus Erysiphe. It
prefers cool nights and hot days, but requires a period of high humidity to spread. (Even
in dry regions, this condition is often met sometime during the night or early morning.)
Powdery mildew rarely kills plants, but it can make them unsightly, and severe infections
can weaken plants, making them vulnerable to other problems. |
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