For at least 15 years, application of strobilurin fungicides azoxystrobin and trifloxystrobin has been the major control method for sheath blight. In 2010 and 2011, in an area near Mowata, sheath blight control with these fungicides was very poor, even after multiple applications.
Several factors were examined, including sources of fungicides, application timing and methods, additives, water quality and unusual weather patterns. None of this information could explain the consistently poor fungicide performance.
Isolated R. solani collected from these fields and tested for sensitivity to azoxystrobin, the active ingredient in Quadris and one of two active ingredients in Quilt fungicides showed that they were at least 10 times more tolerant of azoxystrobin than isolates collected both before strobilurin fungicides were commercially available.
Azoxystrobin-resistant isolates were also resistant to trifloxystrobin, the active ingredient of Gem and one of two active ingredients of Stratego fungicides
The consensus had been that fungicide resistance in R. solani would not develop or would be slow to develop because traditionally, only one fungicide application was made per season. Also, R. solani reproduces asexually, and populations may therefore be less genetically diverse than those of a sexually reproducing pathogen.
Unfortunately, because of increased fungicide use after the 2006 epidemic of narrow brown leaf spot (Cercospora janseana) on rice and the new threat of Asian soybean rust, R. solani populations were increasingly challenged by strobilurin fungicides.
Resistance quickly developed and spread thereafter, causing rice farmers in this area to lose a major tool for sheath blight control. Indications are that the strobilurin resistant R. solani continues to spread into new areas in south central Louisiana.
A Section 18 application for fluxapyroxad (Sercadis, a SDHI fungicide), which has a different mode of action than the strobilurins, was applied for in 2012. Sercadis is very effective against both the resistant and wild types of R. solani and received a full federal label in 2014.
Soon after the makers of flutolanil (Elegia), which has the same mode of activity as Sercadis, also increased the use rate, making it more effective against sheath blight.
These compounds have been widely used in rice along with similar fungicides on soybeans for aerial blight (the same pathogen) for the last five years in resistance areas. In 2016, several sheath blight “nests” in a rice field in Acadia Parish were detected where Sercadis was applied.
Herbicide Resistance Info
Samples were collected, the pathogen isolated, and tested for resistance to Sercadis. The fungus was found to be resistant to Sercadis and cross resistant to Elegia. It also had the resistance to the strobilurins. At this time further testing of additional isolates is being conducted, but the distribution and severity of this new resistant pathogen is unknown.
To avoid fungicide resistance only use fungicides when needed, use full label rates and rotate the mode of action (don’t keep using the same fungicide over and over).
A new fungicide maybe available in 2017-2018 called Amistar Top. One of its components has a different mode of action than the strobilurins or the SDHI fungicides and is effective against sheath blight.
Just like the other fungicides, if we use it exclusively, resistance is likely to develop.