Using biological control strategies for turf, Part 2: Diseases
Turfgrass managers increasingly view biological control as a desirable alternative treatment because it can be a rational way to extend and augment the efficacy of fungicides and, at the same time, reduce the overall environmental load of pesticides. Most importantly, however, biological control is an effective, sustainable solution for maintaining turfgrass health.
The most common biological-control strategies involve either applying microbial inoculants or using organic amendments to encourage the activities of native pathogen-suppressive microorganisms. The goal with both of these strategies is to increase the populations and activity of disease-suppressive microbes associated with turfgrass plants.
Bioaugmentation with microbial inoculants Researchers have studied numerous microbial inoculants over the past decade-especially species of the bacteria Pseudomonas, Bacillus, Enterobacter, Streptomyces and the fungus Trichoderma-for control of several diseases, including brown patch, dollar spot, Pythium blight and root rot, summer patch, take-all patch, leaf spots and Typhula blight. Many inoculants still are in development, though two are available, and a few more could reach the market place in the next year or two.
* Maintaining populations. One of the more difficult obstacles to the successful use of microbial inoculants has been their inconsistent performance in the field. Many examples exist where biological control is as effective as fungicide applications in laboratory tests but inconsistent and unpredictable in field tests. Although we do not understand all the factors that contribute to variability in performance, it may be due do to variables influencing population level, activity and survival. For example, environmental factors such as temperature, moisture and ultraviolet light change dramatically within a turfgrass canopy, perhaps in ways detrimental to microbial growth and activity. Further, management practices such as grooming, irrigating, fertilizing and applying pest controls may also influence microbial growth and activity.
One of the primary factors contributing to reduced efficacy in the field is the failure to maintain adequate populations of the microbes in turfgrass soils. Numerous laboratory experiments show that populations of biocontrol organisms must stay at high levels (usually greater than 1 million cells per gram of soil). If populations drop below this level, control efficacy is in jeopardy. To overcome this, one or more things must happen: *Applications must be frequent *Applications must occur at times when environmental conditions strongly favor activity of the inoculant * The inoculant must be formulated in a way that favors its activity and survival.
Some people question the sustainability and environmental impacts of frequent applications of high levels of microbial inoculants. Nevertheless, frequent application of inoculants may provide the only effective approach for suppressing foliar diseases and maintaining populations on foliage, because this habitat is generally more unfavorable for microbial persistence than a soil environment. Selection of inoculants adapted specifically to the turfgrass environment may facilitate their activity and the maintenance of stable populations.
In addition to maintaining populations of introduced inoculants, application timing is also critical. Recent studies with various microbial inoculants show that applications made after sundown result in superior control compared to applications made during the daytime hours. Further, applications made on a daily basis generally result in more effective control than applications made on a weekly basis. Recent studies also suggest that for the most effective control with microbial inoculants, the volume of water with which you apply inoculants must be greater than 2 gallons per 1,000 square feet (rates similar to conventional fungicide applications).
Researchers must improve their understanding of these factors to develop better application techniques and formulations to increase performance consistency. From the turf manager's perspective, it's equally important to provide the agronomic conditions that favor turfgrass growth and development, because these are the same conditions that tend to promote biological control.
On February 11, the Environmental Protection Agency (EPA) granted Eco Soil Systems Inc. (San Diego) registration for their Pseudomonas aureofaciens (strain TX-1) bacterium, which they have branded "Spot-Less." Application is with Eco Soil's BioJect irrigation-injection system.
BioJect actually ferments TX-1 on-site and is part of the package approved by EPA. John Doyle, vice president of Eco Soil, explains, "What is unique about this approval is that this is the first time EPA has approved a biopesticide delivered in a method of on-site fermentation. In this case, BioJect ferments and dispenses biologicals through the irrigation system."
Key to approval was ensuring that BioJect posed no significant risk of applying some harmful microbe through the system. "What we documented to the EPA was essentially that what we put into the BioJect system is exactly what was coming out, via the irrigation system," says David Odelsen, vice president of research and development. "Specifically, we documented that when [TX-1] is inoculated and fermented (as an active ingredient) in a bioreactor system, TX-1 is the only organism that was produced and subsequently distributed through the system," Odelsen states. Eco Soil will lease the BioJect system to golf courses and provide maintenance and upgrades through Turf Partners Inc.
EPA granted approval after university research demonstrated that TX-1 is non-pathogenic to humans, plants and animals. It controls Anthracnose, dollar spot, Pythium, leaf spot, take-all patch, fairy ring, pink patch, gray leaf spot, Microdochium patch, summer patch and necrotic ring spot. TX-1 does not control diseases in the manner of traditional fungicides and does not entirely eliminate the need for them. However, it does provide significant suppression of the listed diseases. Therefore, it can extend spray intervals considerably or eliminate applications altogether by reducing disease activity below typical treatment thresholds.
TX-1 bacteria provide control by secreting phenazine carboxylic acid (PCA), a compound with significant anti-fungal properties. For ongoing control, frequent applications are necessary to maintain high population levels of the bacteria. The BioJect irrigation-injection system is a practical method of doing so, and its on-site production (fermentation) of TX-1 eliminates the shipping and storage problems inherent in using biological products.
Biostimulation with compost amendments Unlike microbial inoculants, the goal with compost amendments is to enhance the native populations of soil organisms. People have used composts in soil management for centuries and it has been an effective and sustainable means of improving productivity and overall plant health. Over the past 10 or 15 years, studies have clearly demonstrated the potential for composts to reduce the severity and incidence of many turfgrass diseases. For example, monthly topdressing applications of composts at rates as low as 10 pounds per 1,000 square feet are effective in suppressing diseases such as dollar spot, brown patch, Pythium root rot, Pythium blight, necrotic ringspot, red thread and Typhula blight. Levels of disease control can vary from 0 to 94 percent, depending on the target disease, the type of compost and the manner and degree to which the material is composted.
Root-zone compost amendments have the potential to provide much higher and longer-lasting disease suppression of root-infecting pathogens than do topdressing amendments. Studies have shown that amending sand-based greens with municipal-biosolids compost, brewery-sludge compost or an uncomposted reed-sedge peat induces a high level of suppression of Pythium root rot. These amendments have provided complete control for 6 months after incorporation and retained disease-suppressive properties for up to 4 years.
Composts affect diseases largely because of their impacts on soil microorganisms but also because of the microorganisms they contain. Suppressive composts generally are higher in microbial activity than non-suppressive composts. Usually, immature (1 to 3 months) composts that are still in an active state of decomposition are low in microbial activity and not disease-suppressive. However, when allowed to age for a suitable period (2 to 3 years), many composts become highly suppressive.
Biological-control products for turfgrass Today, turfgrass managers have numerous products available for managing diseases. The numbers and types of organic amendments and microbial inoculants currently on the market for disease control in turfgrasses are overwhelming. In many cases, it is difficult to know which of these products you should take seriously. I group biological-control products currently available to the turfgrass industry into four different classes: *EPA-registered inoculants *Unregistered inoculants for which documented levels of disease reduction are claimed *Unregistered inoculants for which disease reduction claims have not been validated * Products that suppress diseases but are marketed for other purposes. Such products include natural organic fertilizers and compost amendments, whereas many of the microbial inoculants fall into the first three categories.
The first class of products is registered in the same manner as chemical fungicides. Currently, Wilbur-Ellis' BioTrek 22G, a preparation of Trichoderma harzianum, is one of only two microbial-based fungicides registered for use on turfgrasses in the United States. The other is Eco Soil Systems' TX-1, a Pseudomonas aureofaciens strain which received federal registration in February (see "TX-1 receives registration," page XX).
Manufacturers do not have the right to make claims about the control of specific diseases without EPA registration as a biological fungicide. Products not registered with the EPA for turfgrass use-but with labels that claim control of specific diseases-cannot legally be sold or used for turfgrass applications. Nevertheless, many of these types of products are currently available to turfgrass managers. Some are registered on other crops but currently lack a turfgrass registration. Although EPA does not require efficacy data for registration, many states demand it before granting registration in that state. The registration process, therefore, ensures that manufacturers have conducted some efficacy testing of the product.
The second class of products is one of the more difficult groups to assess. Suppliers market these unregistered products, at least in part, for disease control. Although their labels do not normally list specific diseases, the wording frequently infers that the product will non-specifically reduce the incidence or severity of turfgrass diseases. By making such claims, regardless of how vague they may be, such products require EPA registration for legal use. Strictly speaking, then, their sale and use for disease control in turfgrass is a violation of the law both on the part of applicators and the suppliers that market them for disease control. Nevertheless, dozens of these products are available to golf-course superintendents, with many new ones appearing every year.
The third class of products includes a large group of biologically based materials sold for a variety of turfgrass ailments, including disease control. In many cases, the manufacturers have used little or no logic in the selection of the particular "active" microbial strains in the product and no logical development of appropriate application strategies. Although it is difficult to know how much testing has gone into the development of these products, it is doubtful that most manufacturers have ever scientifically tested them on turf for disease-control efficacy. These products rely primarily on shrewd marketing and testimonials to support sales. It is this group of products that poses the greatest risk to the future of biological control in turfgrasses. Failures with these products can instill deep-seated skepticism among turfgrass managers, making them reluctant to use any such products, including those whose manufacturers have taken the steps necessary to obtain efficacy data and EPA registration.
The fourth group of products includes a variety of natural-organic fertilizers, root enhancers, soil inoculants and organic amendments. Many of these materials have been available to the turfgrass industry for years. Most are not marketed for disease control but may have some disease-control efficacy. In some cases, these products may be well-tested, whereas others have no documented efficacy. Although some of these products may have a high degree of quality control for their fertility content, the manufacturers provide little or no quality control over disease-suppressive properties.
The future of biological disease control in turf Turfgrass management is entering an age where turf managers are seeking microbiological solutions for biological problems. It is increasingly apparent that maintaining active microbial communities in turfgrass soils is a vital part of overall turfgrass health. Studies on biological control clearly show the potential to affect disease control through both of the microbial-based technologies I've described. Currently, we have more questions than answers about how to optimize these technologies. Nevertheless, interest in the development and commercialization of biological-control products continues to grow. Sound, biological-based approaches to turfgrass management will provide additional tools for maintaining a sustainable, healthy turfgrass ecosystem.
Dr. Eric B. Nelson is professor of plant pathology at Cornell University (Ithaca, N.Y.).
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