Biological control of turfgrass diseases

Today's technology offers much promise.

Eric B. Nelson, Ph.D.

Dollar spot (Sclerotinia homoeocarpa) is suppressed by applications of the fungusTrichoderma harzianum as shown by the healthy stand of creeping bentgrass on the right.

Traditional turfgrass management relies heavily on fungicides for disease control. But in the past 10 years, biological control of turfgrass diseases has developed into an accepted and viable technology with tremendous promise.

Golf course superintendents may prefer biological control over fungicide treatments for a number of reasons:

• Biological control may be good for public relations by demonstrating to golfers that course managers are doing their part to help the environment.
• Biological control may augment conventional fungicide treatments and, at the same time, reduce the environmental load of fungicides.
• Finally, a growing number of superintendents view biological control as an effective, long-term, sustainable solution for maintaining turfgrass health.

To be sure, some superintendents remain reluctant to adopt biological strategies because they fear product failure or they lack the confidence to handle living microorganisms. Biological disease control has strengths and weaknesses that a superintendent should consider before adopting or rejecting the technology.

Two approaches
Two biological control approaches have proved effective against turf diseases. With either approach, the goal is to increase the populations and activity of disease-suppressive microbes (31, 32).

Microbial inoculants
The goal of microbial inoculant applications is to temporarily and dramatically increase populations of disease-suppressive microbes in turfgrass soils and on turfgrass plants. Inoculants require repeated applications to maintain populations at levels acceptable for control.

Organic amendments
When combined with sound cultural management, organic amendments can effect a long-term and perhaps permanent change in the soil environment to favor disease-suppressive microbes, making disease management more sustainable. Some organic amendments not only introduce varied and diverse microbes into soils, but also provide food for microbes already present. However, to sustain this activity, periodic applications are required.

Whereas inoculants increase the populations of organisms foreign to the site of application, the use of organic amendments increases populations of both foreign organisms and those already present and best adapted to the particular site.

Inoculants for disease control
Microbial inoculants have been used in turfgrass management for such things as thatch reduction (27), fertilizer enhancement (42), insect control (57), weed control (66) and, increasingly, disease control. Many biological-control organisms have been described and tested in both laboratory and field studies.

Most studies with biological-control organisms have occurred in laboratories, primarily documenting control efficacy. Field evaluations have been rare. Few studies have focused on ecological relationships, biological-control mechanisms or formulation and delivery technologies, all of which are important in making biological control successful in the field.

Foliar diseases
Several field studies have demonstrated that foliar diseases such as dollar spot, brown patch and Typhula blight can potentially be controlled with bacterial and fungal inoculants. For example, in studies on putting greens of bentgrass and Poa annua, topdressings fortified with the bacterium Enterobacter cloacae significantly reduced dollar spot disease compared with untreated plots (33). Monthly inoculations with this bacterial preparation introduced populations of approximately 10 million to 1 billion cells per gram dry weight of thatch and provided up to 63 percent disease control. This level of control was as effective as iprodione or propiconazole in reducing dollar spot severity.

The dollar spot control provided by this bacterial inoculation was evident up to two months after application and was more effective in prevention than as a cure. In other dollar spot studies, weekly applications of the fungal agent Fusarium heterosporum to Penncross putting greens reduced dollar spot incidence nearly 80 percent (8).

Another well-studied microbial inoculant for foliar diseases of turfgrasses is Trichoderma harzianum Strain 1295-22 (12, 22, unpublished data). This fungus is in the newly registered granular product, BioTrek 22G. It is an effective control agent for dollar spot and brown patch diseases on creeping bentgrass. In field trials conducted over a 4-year period, monthly applications of Strain 1295-22 reduced initial dollar spot severity by as much as 71 percent and delayed disease development by up to 30 days.

One of the more intriguing properties of this organism is its ability to persist in the rhizosphere of creeping bentgrass. Monthly applications of T. harzianum have been effective in maintaining populations at levels of nearly 1 million cells per gram of thatch and soil. In some experiments, populations increased with each successive application (unpublished). However, if populations fall below 1 million cells per gram, biological control efficacy is lost (unpublished).

Many other agents have been tested for control of foliar diseases such as brown patch, Pythium blight, Southern blight, leaf spots and Typhula blight, all with varying degrees of success. In order to maintain consistent levels of foliar disease control, suitable formulations and delivery methods are required for microbial inoculants. The foliar environment, because of dramatic fluctuations in temperature, moisture and light, is rather inhospitable to high microbial populations and activity.

Root diseases
Among the greatest challenges in biological control is management of root diseases such as summer patch, take-all patch, necrotic ring spot, spring dead spot and Pythium root rot. These troublesome diseases are among the most destructive and difficult to control. Furthermore, turfgrass roots are not typically accessible to chemical or biological agents, making disease control erratic and unpredictable.

The impact of soil organisms on take-all patch severity is quite clear. Take-all patches recolonized by healthy grasses contained substantially higher soil populations of fluorescent Pseudomonas species than the surrounding diseased areas or disease-free turf (47, 48). Of all the Pseudomonas spp. isolated, 44 to 82 percent of these strains were antagonistic to Gaeumannomyces graminis var. avenae compared with only 12 to 24 percent in the adjacent disease-free area. Treatment of bentgrasses with Pseudomonas species and Phialophora species have been effective in reducing take-all severity (1, 59-62).

Promising biological control has been observed recently with introduced bacterial strains for control of summer patch on Kentucky bluegrass (17, 18, 55). Strains of Serratia marcescens and Xanthomonas maltophilia provided greater than 50 percent suppression of summer patch symptoms in greenhouse trials, but were not tested further in the field (18). Significant levels of control were observed only when populations were greater than 100 million cells per milliliter. Similarly, strains of Bacillus subtilis and Enterobacter cloacae were effective in suppressing summer patch symptoms in greenhouse experiments and also reduced summer patch symptoms as much as 53 percent in field trials (55).

Meanwhile, recent studies have demonstrated the efficacy of microbial inoculants for suppressing Pythium root rot symptoms. Weekly spray applications of T. harzianum (Strain 1295-22) significantly reduced foliar symptoms ofPythium root rot in both greenhouse and field experiments and enhanced turf quality (unpublished data). In other studies, strains of actinomycetes isolated from Pythium-suppressive composts were effective in reducing Pythium root rot symptoms on field-collected turf tested in the greenhouse (53).

From lab to field
In laboratory tests, biological controls are often as effective as fungicide treatments, but under field conditions, biological controls may be ineffective or inconsistent. One important factor is the populations of microbes in turfgrass soils. Numerous experiments reveal that populations of biological organisms must be maintained at high levels (usually greater than 1 million cells per gram of soil). If populations drop below this level, control efficacy is jeopardized (22, unpublished data).

• Applications must occur frequently.
• The inoculants' activity must be enhanced.
• Or, the inoculant must be formulated and applied in a way that promotes survival, persistence and activity.

Questions arise about the sustainability of maintaining high populations of inoculants and the potential environmental impacts of such applications. Clearly, the continuous application of inoculants is not a sustainable practice, but it may provide the only effective approach for reducing foliar disease epidemics and maintaining populations on turfgrass foliage since this habitat is generally less favorable for microbial activities than a soil environment. Selection of inoculants more adapted to specific turf-grass environments may facilitate the establishment of microorganisms on turfgrass surfaces and the maintenance of a stable population in these sites.

Compost in the root zone can suppress disease. Plants on the left were grown in 100 percent sand inoculated with the damping-off pathogen Pythium graminicola. Plants in the center were grown in 80 percent brewery sludge compost and also inoculated with the pathogen. Plants on the right were not inoculated wtih the pathogen.

Composts and disease control
The goal with organic amendments is to enhance the native populations of soil organisms at the expense of plant pathogenic fungi so disease severity can be reduced. Organic amendments have been used for centuries in soil management. Sludges, mulches, manures and composts are considered effective and renewable products for improving soil productivity and overall plant health. Mature plant tissue amendments and composts have been among the more effective in suppressing soilborne diseases.

Studies conducted over the past 10 to 15 years have clearly shown the potential for compost amendments to reduce the severity and incidence of a wide variety of turfgrass diseases. For example, monthly applications of topdressings composed of as little as 20 percent compost by volume and applied at rates of as little as 10 pounds per 1,000 square feet are effective against diseases on golf course turf (31). Diseases such as dollar spot, brown patch, Pythium root rot,Pythium blight, necrotic ring spot, red thread, and Typhula blight have been effectively reduced following topdressing applications of various composted materials. The levels of disease control vary widely, ranging from zero to 94 percent, depending on the disease, the compost feedstock and the manner and degree of composting.

Organic root-zone amendments can induce much higher and longer-lasting levels of disease suppression, particularly for root-infecting pathogens, than topdressings. Studies have shown that amending sand-based greens with either a municipal biosolids compost, a brewery sludge compost or a reed-sedge peat induces a high level of suppression of Pythium root rot disease. In our studies, these amendments provided complete control six months after incorporation and retained their suppressive properties for up to four years (31, 56).

Microbial basis of disease suppression
Some types and batches of compost are not suppressive to disease. Immature composts (1 to 3 months old) that are still undergoing thermophilic decomposition are not suppressive to Pythium and other pathogens. However, when allowed to age for two or three years, many composts become highly suppressive. These suppressive composts contain relatively high populations of heterotrophic bacteria, actinomycetes and fungi compared with nonsuppressive composts.

However, research has shown that, at least for Pythium species, disease control with compost amendments is dependent not only on the microbial properties of the amendment (5), but on the soil microbial responses to the amendment (31). This is particularly true for poultry composts where, although compost microbial populations and activity are relatively low, Pythium suppression may result from the direct enhancement of the activities of native soil microorganisms. It is not yet clear whether these same relationships are true for other turfgrass pathogens and diseases.

Without a doubt, composts provide some of the richest sources of disease-suppressive microorganisms in nature. In our experience, between 65 and 100 percent of all the microbes recovered from composts may have disease-suppressive properties (31, 53). Many of these microorganisms could potentially provide the basis for the development of microbial-based fungicides for turfgrasses or for microbial compost inoculants to make composts more predictably suppressive (9, 10).

Products
The numbers and types of biological products marketed for disease control are overwhelming. In many cases it is difficult to know which of these products should be taken seriously. They can be divided into four classes.

The first class of products are inoculants that have been registered with the Environmental Protection Agency in the same manner as chemical fungicides. Currently, there are only two. Binab T is a preparation of the fungi Trichoderma harzianum (ATCC 20476) and Trichoderma polysporum (ATCC 20475) available in Sweden and the United Kingdom. BioTrek 22G is a preparation ofT. harzianum (1295-22) available in the United States. Other products currently registered for other crops will likely see registrations on turfgrasses in the future. And other microorganisms currently in the registration process should be available within the next few years. By registering their products, manufacturers are able to legally make claims about specific diseases that can be controlled through proper use of the product.

The second class of products is difficult to assess. These are unregistered products that are marketed at least in part as disease-control materials. Hundreds of these products are available to superintendents with dozens of new ones appearing every month. Many of these products make explicit claims of disease control, some of which are thoroughly validated through research. Many of these products are likely to be effective biological disease-control materials, but are not widely known to the industry. In deciding whether to use these materials on the golf course, it would be wise to review the research and talk to those with experience in either testing or using the product.

Many other biologically based products are sold for a variety of turfgrass ailments, including turfgrass disease control. It is doubtful that many have ever been tested extensively in a scientific manner. These products rely primarily on marketing savvy and testimonials to validate their efficacy. This type of product should be viewed with skepticism and a considerable amount of caution. As a general rule, make sure that the salesperson can show you convincingly that the product has been adequately tested and shown to be effective. Further, any university research staff involved with the product should be contacted and asked to share their experience with the product.

A fourth group of products is represented primarily by the natural organic fertilizers, root enhancers, soil inoculants and organic soil amendments commonly used in turfgrass management. These materials are familiar to superintendents, but have not been sold traditionally as disease-control products. Among these products are effective, well-tested materials, as well as those for which no disease-control efficacy has been documented.

Furthermore, although products such as these may have a high degree of quality control as far as fertility or other parameters are concerned, there often is little or no quality control maintained over disease-suppressive properties. Again, it is wise to contact research scientists and respected superintendents for their experiences with any products you are considering.

The future
Whether we like it or not, we are quickly entering a biological age of turfgrass management where microbiological solutions are sought for biological problems. It is becoming more and more apparent that maintaining active microbial communities in turfgrass soils is a vital part of overall turfgrass health. Research to date clearly shows the potential to bring about disease control through microbial-based technologies.

Currently there are more questions about how to optimize this technology than we are able to answer. As we learn more through research, we should be able to develop better formulations and application technologies for inoculant-based products and develop a better understanding of how amendments affect soil organisms for optimized disease control.

Biological disease control strategies will require a shift in thinking about how diseases are combatted. Most superintendents are accustomed to handling, mixing and applying chemical fungicides. However, living organisms must be handled, applied and managed in different ways, requiring precautions to maintain viability. While you can walk away after making a fungicide application, biological strategies require more rigorous monitoring and management following application and often require integration of cultural practices that will enhance and sustain microbial activity.

Golf course turf represents one of the most intensively managed plant/soil systems known. Agronomically unrealistic mowing, high-sand root zones, ever-increasing amounts of traffic on putting greens and low nutrient inputs to maintain unnecessarily high green speeds have placed unprecedented stresses on turfgrass plants, making them highly susceptible to damage from diseases, some of which were previously considered relatively unimportant. Along with this have come increased fungicide use and a number of negative side effects and additional stresses. Each of these stresses trigger further problems requiring some sort of treatment.

As we become increasingly concerned with environmental contamination and a gradual degradation of soil and plant health, biological-based approaches to turfgrass management will provide additional tools for maintaining a more sustainable and healthier turfgrass ecosystem.

Literature cited

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Eric Nelson, Ph.D., is associate professor of plant pathology at Cornell University.