This article by Renee Stern appeared in “The Grower,” March 1999.
he fight against crop diseases is moving to the micro-level, with growers enlisting bacteria, fungi, and viruses to target damaging pathogens.The use of biological controls against diseases began in the 1950s, but is only now gathering momentum, with about 40 products registered against specific targets, said Walt Mahaffee, research plant pathologist, USDA-Agriculture Research Service, Corvallis, Oregon.
Even more controls for row crops and tree fruit are in the works, either still in research labs or awaiting regulatory approval.
These microorganisms require careful use, and won’t solve every disease problem, Mahaffee said, but they often bring side benefits of healthier plants and higher yields.
“I’ve seen plants that were completely down to the ground because of cucumber mosaic virus,” he said. In adjacent rows were healthy plants treated with biological controls; the virus was present, but inactive, on those plants.
But biological controls aren’t a cure-all, he said. “We’re never going to replace chemicals,” Mahaffee said. “They’ll always have a place, just as biologicals will always have a place.”
HOW BIOCONTROLS WORK
“It’s not going to get you out of trouble,” Mahaffee said. He also warned against settling into “a false sense of security” after applying biological controls. High disease pressure may overwhelm their protection, he said.
Most products on the market are offered as either seed treatments or soil amendments, although methods can range from foliar sprays to root dips. The biggest category targets soilborne fungi, an area where traditional approaches have run into difficulties, he said.
Foliar sprays of fungicides are easy, but Mahaffee said “biocontrol agents let us do something that we wouldn’t otherwise be able to do,” and attack the problem in the ground. Many chemical seed treatments are short-lived, lasting two to three weeks, while the biocontrols may remain throughout the growing season.
Postharvest applications to combat diseases that erupt during storage are another area where biological controls offer hope, particularly as consumers grow more wary of chemical residues, he said.
The latest approach combines various organisms so that a complete microbial community locks out pathogens. A yeast may grow best on the plant’s fruit, while the bacterium prefers young leaves, and a fungus colonizes older leaves.
“It’s ecosystem management in a micro way,” he said.
In some cases the biological control works simply by beating out the pathogen for a home. In others, the combination attacks its target on all fronts. “No one product is a silver bullet,” he said. “You’re putting a living organism on a living organism. They interact, and there’s some instability.”
Biocontrol products function primarily as a protective shield, and are applied before a disease gets a foothold in the field.
SPECIAL HANDLING
“You can’t throw them up on the barn shelf” and ignore heat and cold that might kill the organisms, he said.
Chemical residues in spray tanks also may affect them, as will some pesticides applied to the crop during the season.
Some products must be applied at specific times -- in the evening, for instance, so that they find enough leaf wetness to accommodate growth. And growers may have to experiment with microclimates in their fields and orchards to determine where a product works best or not at all, he said.
Although the management requirements may seem onerous, growers who already have an integrated pest management program will find biological controls fit in smoothly, he said.
The payoff is often more than simply disease control. Healthier growth, and more of it, is a side benefit, perhaps because the biocontrols also suppress minor organisms that suck energy from the plant, he said.
Other products seem to induce resistance, activating a systemic response, much as antibodies function in humans.
Cotton growers have averaged a 10 percent yield increase after using Kodiak (Gustafson Inc.) at a cost of 60 cents per acre. “That’s cost-effective even if they don’t get those results every year,” Mahaffee said.
Kodiak also is used on beans with reported yield increases of several hundred pounds, he said.
A product may be registered for crops as varied as beans and cotton yet target a narrow range of pathogens. Galltrol-A (AgBioChem Inc.), for instance, is used only against crown gall.
A list of biological controls available commercially can be found on the Internet, at:
Using biological controls requires attention to detail. For one, they’re more sensitive to handling problems.
www.barc.usda.gov/psi/bpdl/bioprod.htm.
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