Just as doctors give children immunizations to protect against diseases, farmers and researchers are discovering ways to help plants to fight infection.
In induced resistance, the plant's own defense mechanisms are triggered to fight disease. The bullets firing the mechanisms can be biological, chemical or genetic.
Interest in induced resistance has sprouted recently because of increasing resistance to fungicides, a call for less fungicide use and environmental concerns.
Ray Hammerschmidt started looking at plants producing their own defense against pathogens nearly 30 years ago as an undergraduate. Today, the Michigan State University plant pathologist has researched induced resistance in cucumbers, onions, potatoes and soybeans.
In 1994, he began studying Actigard, a plant activator by Syngenta, for white mold control in soybeans. (Actigard is presently not labeled for soybeans.) The product stimulates the plant to resist infection by mimicking the effect of salicylic acid. Salicylic acid is a natural resistance inducer and also the active ingredient in aspirin.
Hammerschmidt has also researched Cobra herbicide by Valent for white mold control. “Both Cobra and Actigard reduced white mold severity, but didn't always give a big increase in yields,” he says. “White mold is a very aggressive disease, so I'm not surprised we didn't get 100% control or a big increase in yields. We did demonstrate the potential for induced resistance in plants was there.”
Actigard is labeled for leafy vegetables, tomatoes and tobacco. He doesn't expect it to be registered for soybeans soon because several applications would be needed to control white mold.
Hammerschmidt thinks farmers are open to induced resistance concepts. In the early '90s, Ohio farmers observed fewer white mold problems when Cobra was applied on soybeans (see story below). Other products also produced results. In 2000, the plant activator Messenger, by Eden Biosciences, was labeled for sweet corn, tomatoes, peppers and other crops (check label for crops and locations). It contains a bacterial protein called harpin, which induces resistance and also stimulates plant growth.
Compounds being researched to trigger plant defenses include inorganic phosphates, plant extracts, fungi, yeasts, jasmonic acid, chitosan and oxalic acid (the poisonous substance in rhubarb leaves).
“Induced resistance has the potential to be an additional tool farmers can use to reduce fungicides,” Hammerschmidt says.
Yet it isn't a cure-all — other disease management practices are required. For example, a short-term protectant fungicide should be used with Actigard because it takes several days to induce resistance. Also, researchers find a dramatic reduction in diseases using induced resistance, but not complete elimination.
Presently, there aren't any such products labeled for field corn. “Several biologicals and chemicals have been demonstrated or claim to induce disease resistance in corn,” he says about the worldwide research.
Even though research offers hope, the outcome won't always be predictable. Ohio State researchers have been studying induced resistance for 15 years, examining how long the resistance lasts and whether the response is localized or systemic throughout the plant.
For example, plant pathologist Terry Graham has tested Cobra, which contains lactofen. Lactofen induces plants to produce isoflavones to fight disease. In some cases, Cobra produced systemic (whole-plant) immunity. But the response depended on weather, soybean variety and disease pressure, Graham points out.
Iowa State University's X.B. Yang has also tested Cobra for resistance against white mold. Cobra burns soybean leaves and can have yield penalties in the absence of white mold or where white mold risk is low, reports the extension plant pathologist. Also, yield hits are more likely to happen in low-fertility fields or in Northern short-season (Group I) soybeans, where white mold is likely to occur.
In theory, induced resistance should not be disease-specific, Yang explains. His research shows Cobra reduced Sudden Death Syndrome (SDS) fungus. How-ever, induced resistance only lasts several weeks and SDS persists throughout the season. Cobra must also be applied right before an SDS fungal infection occurs. And that varies in different regions and from year to year.
Even so, triggering natural defenses should reduce the need for fungicide, thereby lowering its environmental impact, points out Ohio State's Graham. The ideal way to use induced resistance would be through genetic improvement, not necessarily through genetic engineering, but through classical breeding, too, Graham adds. “Even with chemically induced immunity, current results suggest that the chemicals may be active at much lower levels than traditional chemical applications.”
Ohio farmer Bill Lehmkuhl became interested in induced resistance after farmers across the state started boosting yields using Cobra for white mold control.
He was also concerned about possible increases in white mold in narrow row Roundup Ready soybeans. So Lehmkuhl, who farms 1,500 acres near Minster, and owns Precision Agri Services, put out test plots.
He didn't see any signs of white mold, yet the yield increase ranged from 3 to 5 bu/acre, with a 4.8-bu/acre average. Soybeans, in maturity ranges 3.3 to 3.8, were drilled in replicated plots with medium to heavy clay soils. They were seeded in both poor- and high-fertility soils from 1998 to 2001.
When soybeans are between the fourth and fifth trifoliate (6-8" tall) stage, he applies 6 oz of Cobra with 26 oz of Roundup and crop oil concentrate at 1 pint/100 gallons of water. Lehmkuhl plans to continue using Cobra because the application is easy, inexpensive and effective.
For more information visit: Actigard (www.syngentacropprotection.com); Cobra (www.Valent.com); Eden Biosciences (www.Edenbio.com); or Scottish Crop Research Institute (www.scri.sari.ac.uk/TiPP/default.htm). The Scottish site contains a table of materials claimed or proved to be resistance-inducers.