Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
11/20/98
Yard and Garden Column for the Week Beginning Nov. 27
Contacts:
Mark Gleason, Extension Plant Pathology, (515) 294-0579
Elaine Edwards, Extension Communication Systems, (515)
294-5168
The Secret Sex Lives of Fungi
By Mark Gleason
Extension Plant Pathologist
Iowa State University Extension
Let's take a moment for a frank discussion about "sex" and "fungi." Probably you've never had occasion to use both of these words in the same sentence. To most people, sex doesn't suggest fungi or vice versa. But a few people obsess about the intertwining of sex and fungi. These people are called… plant pathologists.
Why do plant pathologists focus on microbial frolics? Because sex, as practiced by fungi, is one of the most potent threats to our ability to suppress crop diseases.
Down at the cell level, sex looks the same in fungi and people. Two sex cells, each carrying genetic information from one parent, merge together. The offspring of this merger bring a new blend of genes into the world. For people, shuffling the genetic deck creates unique combinations of everything from eye color to toenail thickness. Fungi lack eyes and toenails, but sex-induced changes are just as profound.
One fungal trait that can change through sexual deck-shuffling is resistance to fungicides. Imagine that the sexual offspring in a potato field include a few fungi (let's call them Arnold, Bruce and Stephen) that can resist fungicides. All the other fungi are still vulnerable to the poison. Once the fungicide spray goes on, almost all the fungi die… but Arnold, Bruce and Stephen survive. In a few weeks, these three tough guys have multiplied into zillions of clones, all fungicide-resistant. The next fungicide spray has little effect, and the one after that has no effect. The potato crop turns brown and dies. Arnold and his moldy buddies have won.
This scary scenario plays out all too often in the real world. Fungi are amazingly versatile when it comes to reproduction. They can manage sex when needed, but they also can make babies by cloning themselves. Cloning, also called asexual reproduction, turns out to be a fast, efficient way to multiply. So by turning sex on and off like a faucet, fungi can shuffle the genetic deck, then crank up the copy machine to "warp speed."
Speaking of potatoes, a fungal disease called late blight caused the Great Famine in Ireland about 150 years ago. It was one of the first diseases to be successfully controlled with fungicides. Late blight was held in check in much of the world until just a few years ago. Suddenly, new "races" (genetic combinations) of the late blight fungus appeared, which were much more aggressive and harder to control with standard fungicides. Potato growers now spend much more money for fungicides, but still worry about the "new" late blight.
The Dutch elm disease story has a sex angle, too. Dutch elm disease is caused by a fungus that massacred most of the American elms in Iowa cities and towns during the 1960s and '70s. During the 1990s, a second wave of the disease&endash;powered by the appearance of a fiercer form of the Dutch elm disease fungus&endash;has wiped out many of the surviving trees.
With their quick-change sex strategies, disease-causing fungi can be formidable opponents. But farmers and plant pathologists are continually counterattacking. One strategy is risk reduction. This is "wellness" for crop plants. The basic idea is simple; modify the growing environment so it's less favorable for the fungus&endash;whether by changes in crop rotation, tillage, fertilization, plant spacing, weather monitoring, biological controls or other tactics. The goal is to reduce the need for fungicide spraying, since the fewer sprays applied, the less likely it becomes for resistant fungi to take over the field.
Clever deployment of fungicides also can delay or prevent resistance problems. Farmers will often change the type of fungicide they apply every time they spray, so that fungi resistant to one fungicide may fall prey to another type of fungicide. This zigzag strategy tends to keep the fungi off-balance. The agrichemical companies also now recommend alternation of fungicide types as a way of suppressing resistance problems.
The war between farmers and plant diseases is an ancient one, with battles continually won or lost on both sides. Despite our sophisticated control techniques, it's hard not to admire the versatility and trickiness of the humble fungus.
ml: isugarden
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