Using Biological Control Strategies for Turf Part III: Weeds
By Nick Christians, Iowa State University
Grounds Maintenance 34(3):28-32. March 1999.
After World War II, a series of synthetic herbicides became available to the turf market. Among them were 2,4-D and MCPP, highly effective products for the selective control of broadleaf weeds at low levels of active ingredient. Manufacturers have added a variety of herbicides to the list over the years, and today we have an arsenal of pre- and post-emergence materials to address most weed problems turf managers face.
In the 1970s, environmental activists were publicly raising the first concerns over the possible health effects of lawn herbicides, resulting in the removal of effective broadleaf product silvex in 1979. The 1980s brought additional pressure to reduce the use of herbicides and pesticides of all types on lawns, and by the early 1990s, activists and some local governments were making serious attempts to ban their use in some urban areas.
Justified or not, these environmental concerns have brought new research emphasis to develop “natural” substitutes for the synthetic herbicides that currently predominate in the industry. The public is receptive to this effort, and a market appears to be ready if effective substitutes are available.
While the industry has had reasonably good success in developing natural substitutes for insecticides and fungicides, the development of natural products with herbicidal properties has been more difficult. These substitutes fall into three categories, including herbicidal soaps that provide a contact burn to foliage, corn gluten meal (a naturally occurring plant protein) and biological controls.
Herbicidal soaps are commercially available from Verdant Brands Inc. (Bloomington, Minn.) under the “Safer” label. Superfast Weed and Grass Killer is the Safer soap for use on lawn weeds. Herbicidal soaps work by disrupting the outer cuticle on the leaf’s surface, which causes the plant to dehydrate. While herbicidal soaps do not really fall into the category of “natural,” they are acceptable to many people as a substitute for synthetic herbicides. They can be effective, but they are not selective so you must use them for spot treatments only. They also provide contact activity only and will not translocate to underground plant parts. Thus, they do not control perennial weeds. Although herbicidal soaps can be difficult to use, those consumers who choose not to use synthetic herbicides readily accept them as a substitute.
Corn gluten meal
The idea of using corn gluten meal as a natural herbicide began in the mid-1980s with an unrelated project on which I was working at Iowa State University. The original research involved the use of corn meal to grow a fungal organism for study in a newly established golf green. During this study, I observed that the corn meal had an inhibitory effect on the germination of grasses. After further work, I demonstrated that some type of naturally occurring compound existed in the protein fraction of corn&emdash;the gluten meal&emdash;that had an inhibitory effect on the root formation of germinating seeds. Corn gluten meal contains 10 percent nitrogen by weight and makes an excellent fertilizer for plants with a well-established root system. It is a pre-emergence material only and has no post-emergence effects on weeds that are already established.
Considering its characteristics, it wasn’t hard to see that corn gluten meal could be a natural weed-and-feed material for turf&emdash;inhibiting the establishment of germinating weeds while acting as a fertilizer for the turf. We patented corn gluten meal in 1991 as a natural pre-emergence herbicide for use on all crop areas. Numerous distributors presently market it nationwide under a variety of trade names (a complete list of sources is available via the internet at http://www.hort.iastate.edu/gluten/cframe.html).
Consumer acceptance of corn gluten meal as a natural herbicide has been good in the turf market. To date, most of its use has been on home lawns, but professional use has been increasing. Like any natural product, it has some disadvantages. You should time its application in the 4- to 6-week period before target-weed germination, which means that you must have a good knowledge of weeds and their life cycles. Use in the first year generally results in a reduction of 50 to 60 percent of the target weeds and 2 to 3 years are necessary to match the results of synthetic pre-emergence herbicides. The product is also more expensive than synthetic weed-and-feed materials. The target market for the product is the growing number of people who refuse to use synthetic pesticides and fertilizers but still want to do something about their weed problem and are willing to pay the higher price.
Further work in the 1990s has resulted in the isolation and identification of six organic compounds from corn gluten meal that have the ability to inhibit root formation of plants at the time of germination. These compounds are patented, as is a sprayable form of corn gluten meal. However, neither the sprayable material nor the active components are presently on the market.
The third approach to controlling weeds without synthetic herbicides is to identify biological agents such as viruses, bacteria and fungi that selectively kill the target weed without killing the turf. The odds of finding such organisms with both the selectivity and the survivability for commercial use are small. However, some promising research may eventually lead to successful products. The organism that has received the greatest attention in the turf industry is Xanthomonas campestris pv. poannua. This bacterium selectively controls annual bluegrass (Poa annua), a serious weed in close-mowed turf such as on golf courses. The main limitations of the product are its short shelf life and that it appears to work better on annual types of Poa annua than on the more abundant perennial types. However, researchers have identified more active strains of the bacterium and Eco Soil Systems Inc. (San Diego) expects to receive EPA registration for its Xanthomonas product in about a year (see “Xanthomonas registration pending,” page XX).
What the future will bring in the line of natural herbicides is anyone’s guess. However, it is clear that the public’s demand for “natural” products for use on their lawns will result in an increase in research to develop commercially viable alternatives.
Biologicals have some potential for commercial use in the turf industry, but the problem of keeping them alive on the shelf and viable after application will likely prevent their widespread use in the foreseeable future.
The most promising area of research is in the isolation, identification and development of naturally occurring compounds from plants and other sources that selectively control weeds. The active compounds isolated from corn gluten meal are just the beginning of this type of work. For years, scientists have recognized the phenomenon of allelopathy, by which one plant exudes a chemical that restricts competition from competing plants. What has changed recently is the technology available to isolate and identify these compounds. This was a difficult process only a few years ago, but it is a relatively easy process today. Once identified, it is possible to extract these compounds from natural sources and reproduce them for commercial use.
One word of caution, however. “Natural” does not always mean “safer.” Many natural materials are toxic to humans. The emphasis must be on isolating biologically active compounds from existing food-based materials that we already know are safe.
Dr. Nick Christians is professor of horticulture with a specialization in turfgrass science at Iowa State University (Ames, Iowa).
XANTHOMONAS REGISTRATION PENDING
Xanthomonas campestris is a bacterium that infects Poa annua plants. The bacteria multiply within the host plants and produce a substance&emdash;xanthan gum&emdash;that plugs the water-conducting tissue (xylem) of the infected plants. Subsequent water stress causes wilt and eventual death. Researchers have worked with Xanthomonas for years and, finally, a commercial product is nearing the market. Eco Soil Systems Inc. (San Diego) plans to market a Xanthomonas product when it receives EPA registration, probably sometime in 2000.
The unique qualities of such a product require certain techniques for successful application and control. To infect the Poa plants, these bacteria require fresh wounds through which they can enter. Thus, you must mow immediately after application to create openings for the freshly applied Xanthomonas to infect. Although Eco Soil doesn’t recommend tank mixing with other pesticides or fertilizers, the bacteria seem to be tolerant of most fertilizers and pesticides in turf. One exception is Aliette, which is lethal to Xanthomonas.
Applying the product weekly for 4 consecutive weeks results in good control, according to Eco Soil’s Tom Vrabel. Regarding the observation that Xanthomonas is less effective against perennial Poa annua types, Vrabel notes that the growth habit of the perennial plants makes them inherently tougher to kill completely. Rooted stolons and multiple growing points dictate that a single infection site would not be adequate to take out the entire plant. However, they are no less susceptible to infection than annual types, and the application program Eco Soil is developing is actually geared toward the perennial types. Ultimately, Vrabel says, Xanthomonas does work on the perennial Poas, just not as quickly as it does on annual types.
Vrabel notes, however, that simply asking how well a product kills its target doesn’t reveal the whole story. He explains, “This bacterium is very selective. It infects annual bluegrass but doesn’t even touch the bent. The same cannot be said for PGRs and herbicides. Existing chemical treatments [for Poa annua] are a lot like chemotherapy&emdash;you’re always on the edge of keeping the patient alive. With Xanthomonas, the bent remains healthy and vigorous, so it’s able to compete better. You give the bent a chance to compete in [closely mowed greens] that actually favor the Poa.” Thus, concludes Vrabel, even Poa that survives treatment will continue to struggle against a vigorous stand of bentgrass. He notes that athletic-field managers report similar success with Xanthomonas.
Eco Soil expects EPA to grant registration for its Xanthomonas product in about a year. Until then, Eco Soil is testing it on golf courses around the United States under an experimental-use permit issued by EPA. According to Vrabel, superintendents have reported positive results.
The strip of Kentucky bluegrass turf in the middle of this photo has been treated with corn gluten meal for the past 5 years.
Dr. Nick Christians
A bacterium&emdash;Xanthomonas campestris&emdash;that infects Poa annua is a promising possibility for biological control of this serious weed.
AgrEvo Environmental Health
These foxtail seedlings show the effects of treatment with corn gluten meal (right) compared to an untreated plant (left).