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Breaking the resistance chain
Pesticides don’t change the pest, only the population structure. Knowing the true story can save
superintendents time and money.
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A superintendent has a wide range of cultural and chemical weaponry from which to select when faced with managing pests. When circumstances increase pest pressure beyond what cultural practices can control, superintendents often turn to chemical controls — sometimes as a last resort. It’s important that this last line of defense be highly effective or there’s nothing between the pest and dead turf.
Chemical manufacturers are in the business of providing customers with effective chemistries. It’s in both the manufacturers’ and customers’ best interests for these chemistries to be successful. If they’re abused or not used properly, the development of a resistant pest population begins, and whole classes of chemistries can lose their effectiveness in a relatively short period of time.
When discussing resistance management, there is a notion among superintendents that the repeated use of a pesticide causes the pest to mutate and become resistant to the pesticide being applied. This is a simple and understandable misconception. But subscribing to this idea or not knowing any better costs superintendents time and money as they attempt to control their pest challenges. Furthermore, this line of thinking ultimately reduces overall product efficacy across the industry.
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Fungicide resistance didn’t become a major problem until the development of site-specific fungicides. GCM file photo |
What is resistance?
Resistance is not the result of pests mutating and becoming “resistant” from exposure to a particular pesticide. Real resistance means that a small part of the pest population has an inherited trait that leaves it unaffected by a specific chemical mode of action. Resistant strains of a pest occur naturally at an extremely low frequency. As this resistant population reproduces and passes these inheritable traits from one generation to the next, the pesticide will no longer provide control as the resistant, or “survivor,” population becomes the majority. Repeated fungicide use increases the frequency of resistant individuals as they reproduce and the genetically strong individuals survive.
Let’s look at a simplistic example to illustrate our point. Suppose our killing options were to drown, choke, poison or shoot our subject population of pests. Let’s call those killing options “modes of action.” For the purpose of this example, assume we use drowning as our only mode of action. Sooner or later we would eventually encounter one individual in this population we are trying to control that would be resistant to drowning. Perhaps that individual inherited a unique trait that allowed it to hold its breath for an extremely long period of time — long enough to survive drowning.
If that “resistant” individual is allowed to reproduce and pass along its inheritable trait of survival, eventually a shift occurs in the population. Over time, that shift turns into a population of individuals that are no longer effectively killed by drowning. In order to control the resistant population we would now need to employ a different mode of action — say shooting or poisoning — to which the population is not resistant.
How does resistance develop?
Fungicide resistance wasn’t a major concern until the introduction of site-specific fungicides. The benefits of these fungicides are systemic and curative properties that affect a specific metabolic pathway with reduced environmental impact.
Site-specific fungicides introduce a chemical that selectively kills the pathogen by altering or blocking a critical metabolic process so that the organism can’t complete its life cycle. Over a relatively short period of time — from hours to days — the organism dies.
Resistance development is much easier with these fungicides because small genetic changes in fungi overcome fungicides targeted at one enzyme or biochemical pathway. The organism develops an alternative to the critical pathway the fungicide blocks in the essential metabolic process. If the fungicide is blocking pathway A and the organism has pathway B available, it can overcome the effects of the introduced chemical.
More and more of the industry’s newly developed fungicides are single-site mode of action products. Depending on the product and pest population being challenged, the length of time it takes for a resistant population to develop can be as short as one to two years. Populations also ebb and flow from one year to the next depending on environmental factors.
When turf professionals apply a fungicide on a green to control dollar spot for the first time, for example, the majority of the fungus population is killed. Naturally, the fungicide misses some of the susceptible population of individuals with the initial application. They may have survived because they’re located deep in the soil or in a resting stage not affected by the chemical. Selected resistant individuals are not affected by the fungicide application. As those survivors begin to slowly reproduce, that population’s inherited resistance is passed along to subsequent generations.
The golf course is a dynamic environment. Conditions that can increase or decrease resistant strain reproduction rates are ever-changing. For example, there may have been a specific micro-organism in the soil that was holding the resistant population in check naturally. Removal of the natural check because of a change in the environment may allow the resistant population to reproduce quickly and start to gain a foothold. Eventually, the resistant group gets large enough where it directly infects the plant and causes disease symptoms.
As the resistant population increases, a superintendent will notice that he or she needs increased rates or more frequent product applications to control the pathogen. The superintendent may apply 5 ounces of a fungicide per acre and achieve 14 days of control one year while the following year may spray 5 ounces and observe only 10 days of control. The next year may shift to a 7-ounce application with seven days of control.
When the resistant population persists, the fungicide loses efficacy and can no longer be used. To re-shift the population, we need to whittle down the resistant strain to decrease reproduction and increase the susceptible population.
Real resistance management strategies
The economy of pest management is predicated on having effective products to use. If you don’t have effective products, you’re going to be paying more money to apply existing products at higher rates more frequently, or as a worst-case scenario, apply a chemical and see no result.
The population of a pest is either resistant or susceptible to the chemical being applied. Reversing the resistant population takes about the same amount of time it took to build the population up. To keep the population susceptible to current pest control products, superintendents need to rotate chemistries. Using our earlier example, rather than focusing on one mode of action such as drowning to kill the population, the approach needs to incorporate a variety of tactics including choking, drowning, poisoning and shooting.
Make the most of your resistance management program by incorporating the following strategies:
1. Use labeled rates and application intervals. Rate ranges are tied to environmental conditions and pest pressure. If you have high pest pressure with ideal weather for development, use the highest labeled rates. You want to be able to control the highest number of the pest population with an application to limit the resistant population. Apply the chemistries at the highest rate and within the proper application interval on the label.
2. Focus on good spray coverage. This is common sense but if you aren’t achieving even spray coverage you’ll need to look into proper nozzle technology and appropriate adjuvants to maximize coverage.
3. Limit the number of strobilurin applications. As a classic chemistry class, strobilurins have a high potential for resistance.
4. Rotate different modes of action in your spray program. Once resistance to a fungicide develops, all fungicides with the same mode of action are affected. Introduce unique modes of action affecting different metabolic pathways. In some cases it may be limiting mitochondrial respiration. In others it may be blocking a specific protein or enzyme needed for the fungi to develop to the next growth stage.
5. Adhere to the label and Fungicide Resistance Action Committee (FRAC) recommendations. Labels can be up to 16 pages, which is a lot of information to digest. All chemical manufacturers are beginning to list the FRAC mode of action group on product labels to help with resistance management efforts. The FRAC numbers make it easy for anyone to understand how to rotate among different group numbers.
Looking to the future
Superintendents value sustainability, including preserving business practices for future generations. One of the ways to accomplish this in our industry is to be good stewards of chemistries and keep them viable as lasting chemical control options for the next group of superintendents.
Superintendents are constantly challenged to manage resources — personnel, finances and customers — to help deliver the best product to the customer. A part of delivering the ideal golfing environment is incorporating a comprehensive pest management program. To manage pests successfully, chemistries need to be stewarded to minimize resistant pest populations and therefore maintain product efficacy.
Chemical manufacturers are constantly thinking about resistance management and the discovery of new modes of action. In fact, as new chemistries are developed, they’re put in the most challenging environments to observe how resistant populations might react to them. They’re also being evaluated based on how they interact with other products in regard to resistance management. Labels are written to try to incorporate as much of that knowledge as possible to help stave off the development of resistance.
The manufacturer’s commitment to developing unique modes of action and communicating rotation recommendations, along with the superintendent’s knowledge of how real resistance works, will help achieve true resistance management.
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