Researchers at Cornell University recently discovered an isolate of Fusarium graminearum (the organism which causes Fusarium head blight (FHB)) with greatly reduced sensitivity to tebuconazole. Tebuconazole is the active ingredient (A.I.) in fungicides such as Folicur and one of the A.I.s in Prosaro. These fungicides are routinely used to control both leaf diseases and also for FHB suppression.
The researchers conducted a study to examine the sensitivity of 50 isolates of Fusarium to tebuconazole and another A.I , metconazole (the A. I. in Caramba). They found one isolate out of these 50 (designated TEB-R) so have such a reduced sensitivity to tebuconazole, they deemed it tebuconazole-resistant.
How has a tebuconazole resistant isolate of FHB arisen?
It's important to remember that fungicides themselves do not cause the mutations giving rise to the genetic variation mediating resistance. When fungicide applications are made over a period of time across a landscape, they create an environment where resistant isolates now have a competitive advantage over susceptible isolates. It's a classic case of survival of the fittest. This results in selection for resistant isolates at a much higher frequency in the population.
Interestingly, until recently, New York has been one of the few states where tebuconazole has not been widely used to suppress FHB. However, seed treatments containing tebuconazole have been commonly used throughout the state.
Is the discovery of a fungicide -resistant isolate surprising?
In a word, no. It is not uncommon to find fungicide resistant isolates occurring at low frequencies in natural populations of Fusarium, even before exposure to fungicides. Fusarium graminaerum is known for its wide genetic variability. Once surveys are conducted to measure sensitivity of isolates to fungicides, we would expect to find some with resistance to tebuconazole or other fungicides.
Should producers be changing management practices in light of this discovery?
These findings don't mean that we should drop fungicides from our control programs, but it does mean we need to be smart about our fungicide use. Deploying resistant varieties in combination with fungicides where different modes of action are used in rotation or in tank mixes all help to prevent causing a huge population shift in the fungus towards resistant isolates resulting in loss of efficacy of A.I.s. It is worth remembering that this isolate, although resistant to tebuconazole, was still susceptible to other A.I.s. In the absence of the selection pressure caused by tebuconazole, this isolate of Fusarium seemed to have no competitive advantage over other tebuconazole susceptible isolates.
So what is the impact of this finding?
Should we stop using tebuconazole to control fungal diseases? This discovery highlights the need for careful monitoring of fungal populations of FHB and other disease-causing fungi and establishing baseline sensitivities to commonly used A. I.s in these fungicides. This will allow an early warning if populations are shifting towards an increased number of isolates with reduced sensitivity or resistance. Until we are able to gain a broader picture of what is happening in our fungal populations, integrated management strategies utilizing the best resistant wheat and barley varieties available and judicious use of fungicides ( including tebuconazole) are still the best approach to control this devastating disease. Indeed, in many other wheat growing regions of the world where fungicide resistance is a problem for some diseases, A.I. management (rotation of A.I.s) is already practiced as a matter of routine across all cropping systems. The issue of fungicide resistance should garner as much as attention as the currently recognized problems with herbicide and insecticide resistance in the U.S.
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Reference: P. Spolti et al. 2014 Triazole sensitivity in a contemporary population of Fusarium graminearum from New York wheat and competitiveness of a tebuconazole resistant isolate. Plant Disease.Vol 98: p 607-613