Recent research shows that the pathogen causing Fusarium head blight on barley and wheat may be developing resistance to the triazole (DMI) fungicides, in particular, the active ingredient tebuconazole. Before I discuss this research, I believe it is important to provide you a very brief overview of some of the high points of fungicide resistance in plant pathogens. I will be providing a more in depth, “short course” on this blog in the near future that will cover fungicide resistance management in field and vegetable crops, so stay tuned.
When fungicides of the same mode of action are applied repeatedly to fields, the fungal population can develop resistance or insensitivity to that particular fungicide mode of action. For example, if a fungus develops resistance to azoxystrobin, it will also be resistant to other fungicides in the strobilurin class (group 11) of fungicides. This is because the fungicide kills most of the pathogen population, but a small subset of the fungal population has mutations that allow them to survive and cause disease in the presence of the fungicide. Therefore, if multiple sprays of fungicides with the same mode of action are used, these resistant individuals will continue to grow, reproduce, and cause disease. Over time, the population may contain a large proportion of resistant individuals, resulting in fungicide failure (Figure 1). This is basically what has been occurring with antibiotics and human pathogenic bacteria- the more people use and abuse antibiotics (ex-take them when not needed) the more the populations of bacteria will have the chance to develop resistance to the antibiotic. The result, as you may be aware, is that we are now dealing with high frequencies of antibiotic resistance. Some people even predict that current antibiotics will not work in the near future.
The potential for fungicide failure is one reason growers should rotate between fungicide modes of action if making multiple fungicide sprays during the growing season. In vegetables, growers often tank mix or alternate with multi-site, protective fungicides (ex-chlorothalonil), but this is not common in field crops. Growers may also tank mix fungicides of different modes of action. Unfortunately for field crops, we have a very limited spectrum of fungicide modes of action that we use. This makes rotating fungicides difficult. There are several premix fungicides available, which often contain fungicides of different modes of action (typically a group 11-Headline type active with a group 3-Tilt type active ingredient).
Figure 1. An example of the development of fungicide resistance in a fungal pathogen population. A) The fungicide containing a particular mode of action (X) is applied to a field containing a population of fungal pathogens. Most of the fungi sensitive to the active ingredient, but a small number are not. B) The insensitive or resistant individuals survive, grow, reproduce, and continue to cause disease. C) The field is continually sprayed with the same fungicide or fungicides of the same mode of action (X). D) Because the pathogen population was resistant to fungicides in the X mode of action, they continue to grow, develop, and cause disease.
Several pathogens have developed fungicide resistance in recent years. For example, the pathogen that causes Frogeye leafspot of soybean has developed resistance to strobilurin fungicides (ex-Quadris, Headline, and premixes) in the Midwest and portions of the South. In Indiana the pathogen causing Gummy Stem Blight of cucurbits has developed resistance to boscalid (group 7), azoxystrobin, and pyraclostrobin (group 11). Many are familiar with fungicide resistance to mefanoxam in the late blight pathogen, and resistance to the triazoles (group 3) has been reported in several plant pathogenic fungi.
So what’s the deal with Fusarium head blight?
Fungicide use is recommended as part of an IPM program to manage Fusarium head blight. Tebuconazole has been used in many states to suppress this disease since the 1990′s. Currently, the most effective fungicides use metaconazole or a prothioconazole, or a combination of prothioconazole and tebuconazole. All of these fungicides belong to the DMI (group 3) class of fungicides. Products containing these active ingredients can suppress bleaching and production of mycotoxins when applied around flowering. Mycotoxins are harmful to humans and some animals and can be a cause for rejection or downgrading of grain at mills. The Fusarium head blight pathogen produces mycotoxins after infecting the head. Triazole fungicides appear to be the best at penetrating tissues and stopping fungal growth and mycotoxin production.
So if all the fungicides contain group 3 active ingredients, why are they not equally effective? Some researchers speculate that variability in performance may be due to differences in populations of the Fusarium head blight pathogen to specific active ingredients. Others believe that the variability is due to differences in how the active ingredients target the fungus, as the triazoles in general differ greatly in spectrum of use. Our friends to the North at Cornell conducted a survey of Fusarium head blight pathogens in New York that was recently published online at Plant Disease (link below). Fifty pathogenic fungal isolates were tested for sensitivity to tebuconazole and metaconazole. Sure enough, the group identified one isolate that was resistant to tebuconazole. The interesting part of this story is that the area where the isolate was collected was not an area where tebuconazole use is prevalent and therefore it is unlikely that the isolate evolved resistance as I previously explained. It’s possible that the isolate originated from another region where there is more frequent use of tebuconazole. The authors suggest that a more likely source of the resistant isolate is simply due to the high level of genetic diversity in the pathogen. Thus, if you look at Figure 1, there may be more of the black dots in part A. This isn’t the first time this pathogen has developed resistance to tebuconazole, but it is the first time we’ve seen it in the United States. Past reports of the Fusarium head blight pathogen resistant to tebuconazole have come from Europe, Asia, and South America.
The study showed that isolates were more sensitive to metaconazole than tebuconazole, and the resistant isolate remained sensitive to metaconazole. This led the researchers to conclude that in the case of the triazoles, using different active ingredients within the triazole group may be used as a fungicide resistance management strategy for Fusarium head blight. Thus, tank mixes or pre mixes containing multiple triazole fungicides may help slow the risk of fungicide resistance development in the Fusarium head blight pathogen. More extensive surveys are needed to better describe fungicide resistance in the Fusarium head blight pathogen throughout the region.
Link to Reference: http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-10-13-1051-RE