When selecting a foliar fungicide, producers typically had two choices: triazoles and strobilurins. Before making a decision, you had to weigh the pros and cons of the two chemistry groups.
Time passed, and eventually premixes of the two classes of fungicides became the norm, making those decisions much easier. Two modes of action with a broad spectrum helped fight resistance concerns, extended the effective application window and increased the fungicide’s effectiveness.
Over the past few years, foliar fungicide choices have been enhanced again with products containing not only strobilurins and triazoles but also the SDHI (succinate dehydrogenase inhibitors) class of chemistry. Examples include Trivapro from Syngenta and Priaxor from BASF. Unlike a strobilurin-triazole combination, adding SDHIs to the mix makes fungicide selection a little more complex. I believe complexity is a more than fair price to pay for added efficacy, but it does mean that you need to consider the options more carefully.
Like strobilurins, SDHI are respiration inhibitors, but they are commonly mischaracterized as having unique modes of action. Rather, they have different “sites of action” affecting mitochondrial respiration that prevents spore germination and mycelial growth in plant pathogens. Because strobilurins and SDHIs share the same mode of action, there is concern that cross resistance will develop between the two fungicide classes. That’s why we recommend using them with an additional mode of action, such as triazoles.
While premixes with SDHIs are fairly new, the SDHIs have been used in specialty crops since the 1970s. However, they’ve had limited uses in row-crop production until recently. Their application in corn, soybeans and wheat has been limited because, as a standalone product, SDHIs are effective against a very narrow range of diseases compared to strobilurins or triazoles.
So if SDHIs are not a separate mode of action to the strobilurins and have a more limited spectrum of disease control, why adopt them? Depending on the particular active ingredient, benefits of SDHIs include extended residual activity, control beyond that of other active ingredients and synergistic performance with strobilurins. Good examples of additional control from adding an SDHI is extended residual activity against rusts such as stripe rust in wheat and southern rust in corn when using Trivapro. Trivapro is basically Quilt XL, a strobilurin and triazole premix plus Solatenol, an SDHI. Though Solatenol is pretty narrowly focused on rusts, it has a higher efficacy and length of control on that family of diseases than Quilt XL alone.
Since strobilurins and SDHIs share a mode of action but have different sites of action, they attack mitochondrial respiration and the production of adenosine triphosphate (ATP) in targeted diseases at two spots, creating some synergy. Think of it this way. If ATP production were a road that had to cross several creeks, using both an SDHI and a strobilurin would be like taking out two bridges instead of just one.
With the added benefits that these new three-way fungicide mixes offer, it’s important to understand what diseases you most commonly face to ensure that you get the most out of the product. If the proper SDHI is not matched to your driver disease, the additional benefit won’t be realized. It would be like adding a grass herbicide to help with waterhemp control. It may not hurt control, but it won’t enhance it. Are you in an area where southern rust is a perennial problem, or is northern corn leaf blight or grey leaf spot a bigger issue? What diseases are favored by this year’s weather conditions?
It will be less of a one-size-fits-all approach when selecting fungicides going forward, but, if managed appropriately, the benefits of the additional chemistry bring rewards.