Fungicides are an important and effective tool for management of fungal diseases of crops including soybean. Unfortunately, over time fungal pathogens have the potential to develop resistance to specific fungicide modes-of-action. Mutations conferring resistance to fungicides are relatively rare, but multiple applications of the same fungicide chemistry impose selection pressure on pathogen populations and increase the frequency of those mutations over relatively short periods of time. Thus, specific fungicide chemistries have the potential to lose their effectiveness over time. Fungicide resistant isolates of Cercospora sojina, the causal agent of frogeye leaf spot in soybean, have recently been confirmed throughout the southeast including in North Carolina in 2013 and Virginia in 2014. In 2014, a small preliminary survey was conducted to determine if fungicide resistance is occurring in Virginia populations of frogeye leaf spot. Four fields were tested, and two of those fields had fungicide resistant strains. The resistance is specific to strobilurin (QoI, FRAC group 11) fungicides, which are highly effective for control of fungal foliar diseases but to which resistance can rapidly occur within fungal populations. Effective foliar disease management requires appropriate fungicide chemistry selection based on the specific pathogens present and their sensitivity to different fungicide modes of action.
Additional isolates of the frogeye leaf spot pathogen from throughout Virginia need to be collected and tested for fungicide resistance so that appropriate fungicide recommendations can be made. Other states have already initiated fungicide resistance monitoring programs for the causal agent of frogeye leaf spot (C. sojina), and we will implement a similar program in Virginia. Soybean leaves with symptoms of frogeye leaf spot will be collected throughout the 2015 growing season, the fungus will be isolated, and isolates will be tested for resistance to strobilurin (QoI) fungicides. If fungicide resistance is widespread in Virginia, recommendations for foliar fungicides and/or cultivar selection may need to be modified. If incidence of fungicide resistance is low, we will continue to monitor fungal populations and assess the risk of fungicide control failures on a year-by-year basis.
In order to implement an effective fungicide resistance monitoring program, we are requesting that leaf samples from soybean fields with symptoms of frogeye leaf spot (see pictures above) be submitted to the Tidewater AREC for testing. For more information on submitting samples, please contact Dr. Hillary Mehl (757) 657-6450 ext. 423 or hlmehl@vt.edu.