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Confirmation of Widespread Occurrence of Frogeye Leaf Spot Resistance to QoI Fungicides

It is common knowledge that frogeye leaf spot (FLS) disease (caused by the fungus Cercospora sojina) is one of the most serious that affects soybeans. In fact, disease surveys over the last 5 years estimate it to be the most serious foliar disease of soybeans in the Midsouth. Thus, loss of any control measure such as foliar fungicides that have been used to manage this disease is/will be a serious blow to its management.

A recent article in Plant Health Progress (Vol. 19, No. 4, p. 205, 2018) titled “Widespread Occurrence of Quinone Outside Inhibitor Fungicide-Resistant Isolates of Cercospora sojina, Causal Agent of Frogeye Leaf Spot of Soybean, in the United States” paints a convincing picture of just how serious the occurrence of C. sojina resistance to this class of fungicides is for soybean producers in the US.

The above article is authored by 29 plant pathology scientists/specialists in 17 US soybean-producing states. The article contains results gleaned from the collection of FLS isolates in 14 of those states to determine resistance to quinone outside inhibitor (QoI) fungicides. The collections of these isolates occurred between 2010 and 2017. Major findings from those collections follow.

•   Over the last decade or more, QoI fungicides were used on a large number of soybean acres in the US, and their application had been an effective method for managing FLS.

•   The objectives of this research were to document unpublished observations of QoI fungicide-resistant C. sojina in the US, and to discuss the implications of these and previously reported observations on US soybean production and management of FLS.

•   Determination of QoI fungicide resistance was done by determining the effective concentration at which 50% conidial germination was inhibited by using a discriminatory dose assay or by using a molecular assay, or in some cases, by using both methods (when multiple methods were used, all methods were in agreement every time).

•   QoI fungicide-resistant C. sojina isolates were detected in all 14 of the surveyed states and represent a total of 240 counties or parishes. These counties/parishes represent approximately 13% of the harvested soybean acreage in the US in 2017.

•   This research determined that QoI fungicide-resistant isolates of C. sojina are widespread throughout US soybean-producing states. It can be surmised from these results that areas within these states that have not been confirmed likely have QoI fungicide-resistant isolates as well.

•   These results indicate that FLS management has become more complex, especially since results from previous research indicate that QoI fungicide-resistant C. sojina isolates may be more aggressive in causing FLS on soybean.

•   These results also indicate that when a foliar fungicide application is warranted, fungicide products that contain active ingredients from chemistry classes other than the QoI class should be applied for FLS management. Fungicides in the demethylation inhibitor (FRAC code 3) and methyl benzimidazole carbamate (FRAC code 1) classes have been reported to reduce FLS severity compared with nontreated controls [Click here to access a White Paper (Table 2) for a list of products with these chemistries, and their efficacies against FLS].

Results reported in the above article are not new as far as determining that fungicide-resistant C. sojina exists (C. sojina resistance to QoI fungicides was first reported in Tennessee in 2010) . Rather, they confirm that this resistance is now widespread and must be reckoned with posthaste to prevent future serious soybean yield losses in the US.

This is another example of just how dangerous it is to depend on a pesticide of only one class or mode of action to continually control a targeted pest. It further iterates the absolute necessity of using all available control measures–e.g., varietal resistance, crop rotation, rotation of pesticide chemistries, using efficacious pesticides from different chemical classes–to manage the myriad pests that afflict soybeans.

Final notes. 1) A small percentage of soybean producers taking the necessary action to control pest resistance to pesticides will not get the job done. Recommended remedial actions must be taken industry-wide to prevent or mediate the certain forthcoming resistance to pesticides (fungicides, insecticides, and herbicides) that are overused. 2) The documented continuous development of pest and weed resistance to chemical pesticides further underlines the importance of an industry-wide (both public and private) effort to identify genetic resistance to the myriad pests that afflict soybeans, and the identification and adoption of cultural practices that can be used to thwart the yield-reducing effect of these pests and weeds.

Composed by Larry G. Heatherly, Jan. 2019,