Getting the Bugs Out
The dream of biological control with microorganisms was to find a “super bug” that could be applied to the turf that would push all of the other microorganisms out of the way to establish itself and, by either competition or through the production of antifungal compounds, prevent turfgrass diseases from ever being a problem again. While hope springs eternal, it was foolish to think that one organism could survive in all of the different micro-climates that exist on any one golf course, not to mention all the different golf courses. What we found was that for even “super bugs” to be effective in controlling foliar turfgrass diseases, they have to be applied frequently.
Biological control with microorganisms is based on two concepts: competition and antimicrobial activity.
Most fungal pathogens of turf remain dormant until some nutrient stimulant causes them to germinate. It is this survival mechanism that prevents them from germinating until the conditions are ideal for infection. When large numbers of beneficial microorganisms, such as bacteria, are present, they can quickly utilize the nutrients that are introduced. Thus removing from the system the nutrient stimulus that would normally cause the fungal pathogen to germinate and infect. Mycorrhizal fungi act in a similar manner when they cover the root of a plant, preventing exudates from leaking out of the root to trigger the germination of soil-borne pathogens, like the Pythium root-rot fungus or the take-all patch fungus.
Some microorganisms can use pathogenic fungi as a food source. Both beneficial bacteria and fungi are capable of doing this. Beneficial fungi like Tricoderma attach themselves to the hyphae of the pathogenic fungi, attacking the fungus similar to the way the turfgrass pathogen would attack a plant. Beneficial bacteria, on the other hand, produce chemicals that create holes in the cell wall of hyphae of pathogenic fungi, allowing the bacteria to feed on the cytoplasm leaked from the pathogenic fungi hyphai cells.
- Antimicrobial activity
Most microorganisms produce some chemical that helps them hold on to the little space that they are growing in. Some of these compounds have very broad-spectrum activity against many fungi and bacteria.
After searching for many years, my laboratory found one such organism: a bacterium called Pseudomonas auerofaciens type TX-1. The chemical it produced was phenazine carboxcylic acid (PCA). In petri plate studies, TX-1 inhibited the growth of dollar spot (Sclerotinia homoeocarpa), brown patch (Rhizoctonia solani), Anthracnose (Colletotrichum graminicola), pink snow mold or Microdochium patch (Microdochium nivale), gray leaf spot (Pyricularia gresie) and even Pythium blight (Pythium aphanadermatum), which is normally only controlled by a separate class of chemicals from the others. We had great hopes that we had found the “super bug” that turf managers could apply like a fungicide once a week or every other week to control the most important turfgrass pathogens. We used high rates: 2×10
|Treatment||Rate||Interval||Avg.# of spots|
|Autoclaved broth||volume as below||3 days/week||33.5 A|
|No treatment||------||------||30.0 A|
||3 days/week||22.3 AB|
|Sterile broth||volume as below||3 days/week||21.5 AB|
||3 days/week||12.5 B|
|Daconil Ultrex*||0.5 oz/1000 sq ft||14 days||1.5 C|
|Treatment||Rate||Interval||Avg.# of spots|
||5 days/week||8 A|
|Daconil Ultrex*||3.8 oz/1000 sq ft||14 days||26.2 B|
||5 days/week||41.4 B|
It became apparent that if TX-1 was going to work as a biological control agent against dollar spot, more frequent applications were going to have to be made. We next tried to apply TX-1 three times a week at both the 2×10
We then began applying the bacteria at both rates 5 days a week. The 2×10
It became obvious that the two limiting factors in the successful biological control of dollar spot with TX-1 were the effects of drying and the ultraviolet light on the bacterium. These two factors can kill bacteria very quickly. It's curious that biological products for disease control make no mention of these factors on the effectiveness of their products and leave you to believe that you can apply the microbes anytime of the day. While the single application of the microbes will not have any significant effect on the disease, at least you might get one night's benefit from them if you applied them close to dark.
USING IRRIGATION APPLICATION
While this study may have demonstrated how to make biological control with microorganisms work, it was not very practical. Under normal circumstance, you are not going to spray your golf course five days a week with anything. Secondly, for this biological control to work, you would have to have an onsite fermenter that brewed up the bacteria every day, not to mention determine the amount of bacteria you were actually applying. The only way it could be done from a practical point of view is to have the bacteria brewed up in an on-site fermenter and applied through the irrigation system. Such a fermenter was developed, called a BioJect, which did brew up and inject the bacteria through the irrigation system. There were some outstanding cases of success, but there were many more failures. Many factors lead to these failures, including the fermenters being too small or not brewing up adequate numbers of TX-1. Poor-quality irrigation water and poorly designed irrigation systems for the application of microbes also contributed to the failure of the system. Some of the failure is also attributable to the users who refused to turn on the irrigation systems during rainy periods for fear of criticism from the golf course membership for over watering and making the course too wet. However, if biological control of foliar turfgrass pathogens is ever going to become a reality, it is going to be through the use of an on-site fermenter and an irrigation system designed to apply the bacteria in an efficient manner.
J.M. Vargas Jr. is professor of plant pathology at Michigan State University (East Lansing, Mich.).
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