Managing Resistance --Part I: Fungicides

Site-specific fungicides, better known as systemic fungicides, are vital tools for turfgrass managers. Their effectiveness and length of control are superior to that of contacts, so they have become important tools for managing disease in high-quality turfgrass. However, systemics have increased the problem of resistance because fungal pathogens more easily tolerate site-specific fungicides, which primarily act on just one physiological mechanism in the fungus. Contact fungicides, though they offer shorter and, in some respects, inferior control, typically affect several sites in the fungus. Therefore, it's more difficult for fungi to develop resistance to contacts. That's why the resistance problems we know of almost entirely involve systemic fungicides.

The most significant problem with resistance to systemic fungicides is with Sclerotinia homoeocarpa, the cause of dollar spot. The other pathogen with which resistance occurs frequently is Microdochium nivale, the cause of Microdochium patch. Resistance to systemic fungicides by Pythium aphanidermatum, a cause of pythium blight, and by Colletotrichum graminicola, the cause of foliar and crown-rotting anthracnose, also occur but much less frequently.

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The three major fungicide groups to which resistant strains of S. homoeocarpa have developed are the benzimidazoles, dicarboxamides and the demethylation inhibitors (DMIs). We have even found strains of S. homoeocarpa with resistance to all three chemistries, which beckons the question: How did strains with resistance to all three chemistries develop on the golf course? The answer is quite simple: The golf-course superintendent rotated all of the different chemistries into his fungicide program, just as he was told to do to prevent resistance from occurring. This would have worked fine had the individual strains of S. homoeocarpa been able to accumulate resistance to only one fungicide. However, we now know that strains of S. homoeocarpa can accumulate resistance genes to at least three different chemistries. Therefore, alternating systemic fungicides with different modes of action will not prevent resistance from developing. It will only produce strains of S. homoeocarpa resistant to whatever chemistries you put into the rotation.

I hope that we can eliminate from our vocabulary the phrase, "You should practice good fungicide-resistance management and rotate fungicides with different modes of action." Alternating different chemistries will only select for strains with multiple resistance to whatever chemistries you use in the rotation. You actually would be better off using one site-specific chemistry for the control of dollar spot until resistance develops, then switching to a product with a different mode of action.

For example, if it takes 5 years for S. homoeocarpa to develop resistance to fungicide X and 5 years to develop resistance to fungicide Y, then using fungicide X for 5 years and then switching to fungicide Y should give you 10 years of dollar-spot control. However, alternating fungicide X with fungicide Y will give you resistance to both fungicides, often in less than 10 years, because S. homoeocarpa strains exist in the population with resistance to both chemistries. You actually could lose several years of dollar-spot control that you could have had by using the two fungicides sequentially.

Other practices help extend the effective life of fungicides. These include scouting and only applying a systemic fungicide when dollar spot has reached a certain threshold level or alternating with a contact fungicide. These techniques delay the development of resistance by reducing the number of fungicide applications you make with the systemic fungicides. This increases the number of years it will take to reach the critical number of applications necessary for resistant strains to become dominant in the population (see table, page 18).

Conventional wisdom has been to alternate systemics with contact fungicides or combine a systemic fungicide with a contact fungicide. This way, as the thinking goes, the contact fungicide will eliminate resistant individuals that the systemic does not kill. The reason this doesn't work (in the way people think it does) is that the contact cannot differentiate between the resistant strain and the wild types-it works equally well on both. If it cannot tell them apart, how is it going to eliminate one and leave the other? In other words, the contact will not alter the ratio of resistant types to wild types. Conversely, the systemic fungicide can only eliminate the susceptible wild types from the population because the other strains are resistant to it. Thus, the proportion of resistant types in the population will increase due to selection by the systemic, while the contact remains essentially neutral in its effects on the composition of the population. Therefore, the benefit of including a contact fungicide is not to eliminate the resistant strain (which it will not do) but to reduce the number of systemic applications you make in any one year. This extends the number of years you can use the systemic fungicide, but it does not prevent the eventual development of resistance.

The fact that strains of S. homoeocarpa with resistance to three fungicide groups exist invalidates the theory that alternating different fungicide groups prevents resistance from occurring. You should use one class of chemistry to manage dollar spot until resistance occurs. Do this in conjunction with good integrated-disease-management practices and the use of contact fungicides to prolong the useful life of the systemic fungicide. Remember, contact fungicides will not eliminate resistant strains; they merely extend the life of site-specific fungicides by delaying the time it takes to reach the critical number of applications necessary for resistant strains to become dominant.

Dr. Joseph Vargas is professor of plant pathology at Michigan State Univeristy (East Lansing, Mich.).

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