The management of turfgrass diseases has become one of the more challenging and cost intensive aspects of turfgrass maintenance and culture. More money per acre is spent annually for disease control in turfgrasses than on any other commodity.
Increasing concern has arisen about fungicide dependency of turfgrass and the environmental consequences of repeated and sometimes unnecessary fungicide application. Biological controls are the practice of deploying micro-organisms individually or in mixtures to either reduce the activities of pathogens or enhance the tolerance of plants to disease.
Most turfgrass managers are familiar with the negative aspects of soil micro-organisms because many are pathogenic and can damage a turfgrass stand. However, in addition to these turfgrass pathogens, the soil harbors a variety of micro-organisms that improve plant health.
These soil bacteria and fungi are responsible for increasing the availability of plant nutrients, forming symbiotic associations with turfgrass roots, and producing substances stimulatory to plant growth. They also protect plants against infection from pathogenic fungi. Biological control may be achieved either through the application of introduced disease-suppressive microbes, or though the manipulation of native microbes present in soils and on plant parts.
Cultural management techniques such as core aeration, verticutting, or the application of lime may reduce disease development by altering the soil and thatch microbial communities, which pathogens require to survive. These cultural practices may indirectly affect disease severity by changing the environment to favor disease-suppressive micro-organisms. Similarly, the application of well-decomposed sources of organic matter to turf provide substrates on which disease-suppressive soil micro-organisms can grow. At the same time, this introduces some of the necessary populations of micro-organisms that reduce diseases from pathogenic fungi. Reducing pathogen inoculum in soil, and protecting plant surfaces from infection is the goal.
Biological control through the microbial destruction of pathogen spores or the prevention of spore formation serve as a means of reducing pathogen inoculum in soil. Antibiotic-producing micro-organisms displace pathogens in decaying plant residues, such as thatch, and also reduce their populations in soil. Many non-pathogenic soil micro-organisms are able to effectively colonize above ground as well as below ground to protect these tissues from becoming infected by pathogens.
Some root and crown colonizing soil micro-organisms also can induce natural defense mechanisms in plants, rendering them more tolerant to disease. Soil bacteria can also compete more effectively than pathogens for essential nutrients and other growth factors, thereby reducing pathogen spore germination, growth and plant infection.
Many disease suppressive microbes prefer to live in decaying organic matter, which they use as a food source and protective habitat. Therefore, some level of organic matter is usually necessary to promote effective biological control of turfgrass diseases. Diversity of disease suppressive microbes is quite low in putting greens that are low in organic matter. As a result, natural biological processes do not operate optimally in them. This is one reason that some diseases, particularly root diseases, can be so damaging on high sand-content greens. In order for biological control strategies to work effectively, disease suppressive microbes must be compatible with other turfgrass management inputs.
Biological control strategies must be employed primarily to control the pathogen, but maintain the associated suppressive micro-flora at the same time. Biological control agents differ fundamentally from chemical fungicides in that they must grow and proliferate to be effective. Effective disease-suppressive microbes must be able to establish and survive in turfgrass ecosystems, and remain active in controlling pathogens during periods favorable for plant infection. The most important factors in determining how well these microbes establish and grow are the environmental conditions, particularly temperature, organic matter content and pH. The use of microbial fungicides and disease suppressive microbes has become a more viable option for turfgrass management than it was in the past. Each microbial community makes an important contribution to the nature of the composted material.
Golf greens and tees are typically top-dressed several times a season with a mixture of sand and peat or soil. The use of top-dressing amended with disease suppressive inoculum, composts, and organic fertilizers are broadly accepted by turfgrass managers as an attractive disease control alternative. Substantial reductions in fungicide use have accompanied the adoption of these strategies.
The use of composts, organic fertilizers, and broad spectrum micro biologic inoculum for turfgrass disease control is economically and technologically practical. In some instance, they can provide levels of control as good as that attained with fungicides. Effective disease suppressive microbes must be able to establish and survive in turfgrass ecosystems for biological control to occur. The interaction of these microbes with other soil micro-organisms, and the various soil and plant factors affecting optimum biological control activity, are important in developing control strategies with compost-based materials.
Performance of micro-organisms efficacy can rival the control provided by fungicides, plus provide the added advantage of residual control, which may last weeks, months, or perhaps even years. The benefits of biological controls, will ultimately change the way in which disease control is approached.