What does it really mean for your management practices?

The term ‘‘disease resistance” is not a new one in the world of turf management. There are many references in literature, some over 40 years old, citing a “diminished effect”, or a “lack of control” that can be linked directly to the introduction of “biochemically selective” products that attacked a specific action site in the fungal pathogen. These were “penetrant” products that were absorbed into the plant and exhibited a specific ‘mode of action” to control the disease.

In my last article, “Points to Consider for Fairway Dollar Spot Control”, I led you through some ground work regarding “FRAC” codes as a resistance management tool and introduced you to some current research about resistance in Dollar Spot populations. In response to this article, I have received many emails and phone calls commenting on the message and information presented. I previously stated “A disappointing level of control does not mean that 100% of your population is tolerant. Most superintendents stop using a product when it stops delivering the results they want. For example, current research is indicating a rough 70/30 split in resistant Dollar spot populations exposed to Benzimidazoles, not 100%. The next important questions to ask are: Is the change competitive? How long does it last? How do I manage it?”

As it stands today, most pathologists would agree that these “biochemically selective” or “single site” mode of action products have a set number of uses, or population exposures, before genetic mutation magnifies in the population. As indicated above the first effect we see is the loss of economic control, however if we continued to use only that product, we could expect to totally change the population without hope of returning to a sensitive population. Knowing this, why would anyone continue to use the same product? We have numerous examples of this total change in the medical and insect worlds, but so far, not in the turf world. Why? I believe we can attribute this to three things: 1) we stop using products as they stop providing us the level of control we paid for, 2) we have developed disease management programs that change products (and hopefully the Mode of Action), and 3) some of those products we change to are “biochemically diverse” in their Mode of Action. I have often said, “Three things happen when we make an application of a pesticide: we kill the target, we miss the target, or we made the target sick. As we worry about the part of the population exposed but not killed, these “multi-site” mode of action products often correct for a multitude of product use sins in the disease population. Their use can indiscriminately reduce the mutations created by exposure to the single site products and reduce the opportunity for magnification as the population grows. Once again please remember we are delaying resistance by means of population diversity management. We could have a greater discussion regarding the presence of a “native” population, but that is a topic for another article.

Having said this, I want to correct a misnomer about combination fungicide products. Many superintendents that I talk with believe that if they have resistance to one FRAC group present in the combination product, that the other component of the mixed product is “doing all the work”. There is little evidence to support this idea, and much evidence to support just the opposite, that combinations can play a vital role in resistance management. Research presented from 2002 to 2005 by University of Wisconsin, Virginia Tech, Ohio State University, Rutgers University, and Dr. Randy Kane indicate that although resistance to an individual chemistry may be present to a level of unacceptable results, it does not mean that both of the products in the combination are not participating in disease management. There was still evidence of an “additive effect” with the two mode of action combination products. In these studies, there were varied levels of FRAC 1 and 2 resistance in the Dollar Spot population, however Cleary’s Spectro™ (Chlorothalonil + Thiophanate-methyl) and 26/36 Fungicide® (Iprodione + Thiophanate-methyl) still provided excellent levels of disease control over the entire course of the studies.

In closing, I want to remind you that there is no excuse for not knowing the mode of action of each product in your disease management program. This is the first step to delay the onset of that lack of control called resistance at your individual location. The second step in this delay is the mixing or rotation of modes of action in your management programs. Cleary’s development of Spectro and 26/36 Fungicides follows in the concepts set forth over 30 years ago by Dr. Paul Sartoretto, and are still valid today. And finally, research continues to support the validity of combination products like Spectro and 26/36 Fungicide for the disease management and performance you require in today’s turf markets. I hope this information can help each of you work smarter and not harder. You can count on Cleary.