We’re all familiar with the various fungicide classes available for disease control on turf, with the demethylation inhibitor (DMI) and strobilurin (or QoI) classes amongst the most prominent. Once an effective active ingredient is discovered in a particular class, it is usually common practice for the R&D teams at the various chemical manufacturers to develop several additional fungicides in that same class until it is seemingly exhausted (DMIs are a prime example). From the looks of it, it appears that the next wave of fungicides brought to the turf market will come largely from the succinate dehydrogenase inhibitor (SDHI) class. In general SDHI fungicides work by disrupting energy production in the fungal cell, and while this class is not entirely new (Emerald, for example, is an SDHI), the ‘next generation’ SDHI’s now coming to market appear to have a much broader range of efficacy than Emerald did. The new SDHI currently on the market from BASF is fluxapyroxad, which on its own is known as Xzemplar and is known as Lexicon when mixed with pyraclostrobin (i.e. Insignia). The other SDHI active ingredient nearing market arrival from Syngenta is penthiopyrad, known as Velista. Both of these products show good to excellent dollar spot control while also providing control of anthracnose, brown patch, snow molds, and various other diseases.
While both products will make excellent additions to your disease control arsenal, both (and any new SDHIs that arrive in future years) should also be used with caution. A recent study conducted on strawberries by Amiri et al. and published in the April issue of Plant Disease found that not only did resistance develop rapidly in gray mold (Botrytis cinerea) populations never previously exposed to either fluxapyroxad or penthiopyrad, but that cross-resistance existed with boscalid (i.e. Emerald). Worse yet, the researchers found there was no fitness cost to the pathogen for having the SDHI resistance mutations, suggesting that the resistant pathogens will remain present in the population even in the absence of SDHI fungicides.
This is certainly concerning for managers who are just beginning to use these products, but things aren’t as bleak as they might appear. First, resistance to the SDHI fungicides is quantitative in nature, meaning that efficacy gradually declines over time rather than an abrupt loss of control seen with other fungicides such as thiophanate-methyl. Second, Botrytis cinerea is an organism that generally becomes resistant to fungicides very quickly, and it’s likely that turfgrass pathogens won’t develop resistance quite so fast. Third, both BASF and Syngenta are well aware of the resistance concerns with these products and have provided language to manage resistance right on the fungicide label (making it legally binding). Proper rotation of fungicide classes and inclusion of multi-site inhibitor fungicides such as chlorothalonil and fluazinam will likely significantly reduce and/or delay the onset of resistance at your facility. So incorporate the new SDHIs into your program, but don’t forget the resistance lessons we learned from the DMIs and QoIs.