Off the Record: Debunking three myths about microorganisms on putting greens

By |  June 24, 2024 0 Comments

There continues to be a steady stream of advertisements and secondhand stories abound about the increased use of microorganism products for putting greens. We should compare these biological products to proven management practices to see where they fit when dealing with disease or other turfgrass problems.

Photo: Mike Kenna, Ph. D.

Mike Kenna, Ph. D.

Twenty-five years ago, the USGA researched microorganisms found within putting greens. I summarized those results to deal with three common myths about the fate of microorganisms in greens.

Myth No. 1:

A product is needed because microorganisms do not exist in the harsh environment of a sand rootzone.

In the 1990s, a two-year research project at Clemson University by Horace Skipper, Ph.D., investigated the number and diversity of microorganisms found in amended sand bentgrass greens at Charlotte (N.C.) Country Club.

They combined the average of eight sampling periods for six microorganism categories and a total of all reported microbes. The results used a log scale rather than a linear scale, as linear graphs would be impossible to draw.

For example, the average gram-positive bacteria found was 100,000 per gram of soil. The bar would have to extend 25,000 inches or 0.4 miles for a linear scale. For most soil bacteria categories, one million to 10 million colony-forming units occur in a single gram of soil.

More importantly, there were 10 billion colony-forming units in a single gram of soil. That amount represents only a fraction of the soil microbe species that scientists can easily culture and identify at the time.

Myth No. 2:

Fumigation will kill all the beneficial microorganisms in putting greens.

Methyl bromide was the primary method to fumigate putting greens and fairways, particularly in the Southern U.S. Monica Elliott, Ph.D., at the University of Florida, demonstrated that for every soil bacteria category, except the fluorescent pseudomonads, microorganism levels were equal to, or greater, than the pre-fumigation levels and untreated control 23 days after treatment.

Clemson University and the University of Florida repeated a similar study on bermudagrass greens and produced similar results. All six categories of bacteria, including the fluorescent pseudomonads, reached levels of one to 100 million colony-forming units per gram of soil in less than two years after fumigation. Fumigation does not sterilize the soil. I would expect similar results for the various pesticide products we use today on putting greens.

Myth No. 3:

Repeated use of fungicides will kill all beneficial soil microbes in a putting green.

At Cornell University, Gary Harman, Ph.D., conducted a three­ year study comparing an untreated control with repeated applications of eight fungicides. The products included Daconil, Chipco, Subdue, Banner, Bayleton, Prostar and Sentinel.

There were no significant decreases for the soil microbes measured during each of the two-month sampling periods for the years of the study.

Don’t be fooled by false advertising or sales pitches. Research indicates that microorganisms establish quickly and thrive in high-sand root­ zones because they grow in association with plant roots. They have evolved over millions of years to have this close relationship with plants, not soil particles.

Second, fumigation does not kill all the beneficial microorganisms. Billions of microbes lie dormant in the soil and come awake in the presence of actively growing roots, providing them a home.

Lastly, fungicide applications have an insignificant effect on the number of microorganisms. This finding is most likely due to how fungicides adsorb to turfgrass leaves and organic matter, making it difficult for them to move downward into the soil. Systemic fungicide products would have even less effect on soil microorganisms.

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About the Author: Mike Kenna, Ph.D.

Mike Kenna, Ph.D., is the retired director of research, USGA Green Section. Contact him at mpkenna@gmail.com.


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