Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
1/4/99
Contacts:
Mark Gleason, Extension Plant Pathology, (515) 294-0579
Elaine Edwards, Extension Communication Systems, (515)
294-5168
Yard and Garden Column for the Week Beginning Jan. 8
Sick Plants Take Aspirin
By Mark Gleason
Extension plant pathologist
Iowa State University
"Sick plants take aspirin" sounds like a supermarket headline, right up there with "Face of Elvis appears in sidewalk" and "Aliens borrowed my brain." But there's a grain of truth here. Plants don't get headaches (no heads), but they can get sick. And salicylic acid, the active ingredients in aspirin, turns out to be one of Mother Nature's most potent weapons against plant diseases.
Unlike humans and other animals, plants lack an immune system to make antibodies against invaders. But plants have their own bag of tricks. The first line of defense is a tough overcoat of wax and cellulose. This armor is enough to discourage all but the baddest fungi, bacteria, and viruses.
The next trick is the "scorched leaf" defense. When an invader breaches the outer walls, cells in the neighborhood of the invasion up and die. The invader, surrounded by a suddenly wilted salad bar, is often stopped in its tracks. But not always.
As the defender cells are falling on their tiny swords, a distress signal goes out to the rest of the plant. This Mayday message is like a chemical relay race, with a sequence of participants. The chemical that often jump-starts the process is salicylic acid. At the end of the race, plant cells start cranking out large amounts of chemicals with names like chitinase and glucanase. The whole plant is now ready to repel all sorts of microbial invaders, not just the ones that provoked the reaction. This warning system goes by the name of "Systemic Acquired Resistance," or SAR for short.
The catch is that plants sometimes can't set up their SAR "force field" quickly enough to stop the Evil Empire (fungi, bacteria, etc.) If you're a farmer or gardener, this is when you see spotted leaves, rotted fruit, and wilted plants.
Since SAR was discovered two decades ago, plant scientists have searched for ways to fine-tune it. Why not just grind up some aspirin and spray it on the plants? It turns out that salicylic acid is actually poisonous to plants when applied this way. So chemical companies tried to make a similar chemical, but without the toxic properties.
Novartis Crop Protection recently began testing Actigard ®, a synthetic form of salicylic acid, for disease control on a wide variety of crops. Like any new tool, it takes a while to learn how to use it. Control of some diseases, such as bacterial spot on tomatoes, has been excellent. When Actigard ® was applied to tomato seedlings in a greenhouse, however, the plants became stunted. At Iowa State last summer, I tested it for control of anthracnose, a fungal disease of strawberries; it seemed to suppress the disease a bit, but not as well as conventional fungicides.
Experiments like these are beginning to clarify the "do's" and "don'ts" for Actigard ®, and it's likely to reach the market soon. Other products using SAR are likely to appear within the next few years.
Many chapters of the SAR story remain untold. For example, researchers are feverishly seeking to identify all the chemicals involved in "signal transduction"&emdash;the SAR relay race. But research on SAR is accelerating, so understanding of the process is likely to keep pace.
Why the excitement about SAR-inducing chemicals? They have the potential to control, or help control, major diseases on many crops&emdash;without the health and environmental hazards of conventional pesticides. By turbo-charging a plant's natural defenses, SAR inducers may be pointing the way toward a more environmentally friendly agriculture.
ml: isugarden
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