Linda F. Benedict, Robertson, Clark, Schneider, Raymond W., Price, III, Paul P, Purvis, Myra, Padgett, Guy B.
Paul P. Price III, Myra A. Purvis, Clark L. Robertson, Raymond W. Schneider and G. Boyd Padgett
Cercospora leaf blight is a fungal disease caused by Cercospora kikuchii and is the predominant foliar soybean disease in Louisiana. Early symptoms usually appear during pod fill in the upper portion of plants. Purplish lesions begin to appear on petioles (Photo 1), causing leaves to exhibit a bronzing or leathery appearance. As the disease progresses, stem lesions increase in size and occurrence, and leaves exhibit reddish purple, angular and irregular lesions (Photo 2). The pathogen has been noted to produce spores profusely within lesions, creating an ashy appearance (Photo 3). Lesions may grow together, resulting in premature defoliation, which reduces yields (Photo 4).
Since 1991, fungicide use has increased in Louisiana soybeans with estimates by LSU AgCenter scientists ranging from 40 percent to 75 percent of planted acres. This increase may be attributed to increased yield potential, increased soybean prices, application conveniences, industry encouragement and increased disease awareness brought on by the discovery of soybean rust in 2004. Strobilurin fungicides such as Quadris (azoxystrobin) and Headline (pyraclostrobin) or benzimidazole fungicides such as Topsin M (thiophanate-methyl) were historically recommended for management of Cercospora leaf blight and are currently in use. Recently, triazole fungicides such as Topguard (flutriafol) and Domark (tetraconazole) have been recommended for disease management. According to plant disease management reports and results from research station trials conducted over the past 15 years, strobilurin and benzimidazole fungicide efficacy on Cercospora leaf blight has decreased in Louisiana and across the Southeast.
Since a decrease in fungicide efficacy was observed and fungicide resistance has been documented in other Cercospora species affecting other crops, fungicide resistance was suspected in C. kikuchii. Fortunately, a collection of 176 isolates of the pathogen, collected in 2000 prior to heavy fungicide use, has been maintained in the LSU AgCenter Department of Plant Pathology and Crop Physiology. To determine if resistance had occurred, isolates were collected from research stations and producer fields in 27 soybean-producing parishes across Louisiana in 2011 (160 total) and 2012 (80 total) and compared to the baseline population from 2000.
In the laboratory, isolate sensitivity was assessed by conducting growth tests on fungicide-amended media. Selected fungicides used in the experiments were technical formulations of azoxystrobin (Quadris), pyraclostrobin (Headline), trifloxystrobin (Gem), flutriafol (Topguard), propiconazole (Tilt), tetraconazole (Domark) and thiophanate-methyl (Topsin M). The effective concentration to inhibit 50 percent growth (called the EC50 value) for each isolate was calculated and used as a measure of sensitivity (low values = sensitive; high values = resistant). In the case of thiophanate-methyl, discriminatory doses (growth/no growth) were used to test for resistance.
Median baseline EC50 values (calculated for the 2000 collection) for azoxystrobin, pyraclostrobin and trifloxystrobin were low, indicating that isolates from that time period were sensitive to strobilurin fungicides (Figure 1). In 2011 and 2012, median EC50 values for azoxystrobin, pyraclostrobin and trifloxystrobin were much higher, indicating resistance to strobilurin fungicides (Figure 1). Resistance to strobilurin fungicides appears to be widespread in Louisiana, with 21 of 27 parishes testing positive in this study (see map). Results also indicated the majority (85 percent) of the pathogen population appear to be composed of strobilurin-resistant individuals (Figure 2). Cross-resistance (resistance to the same chemistry type) to all three fungicides was observed in the 2011 and 2012 populations, indicating that replacing one strobilurin with another would be ineffective.
When comparing baseline triazole sensitivity to values for the 2011 and 2012 samples, significant shifts toward less sensitivity were detected in isolates exposed to flutriafol and propiconazole (Figure 3). Conversely, isolates from 2011 and 2012 were more sensitive to tetraconazole than the baseline population (Figure 3). Outliers toward less triazole sensitivity were detected in 2012 for all three triazole fungicides, which indicates a possible shift towards less sensitivity. Populations require further monitoring to confirm this shift. Crosssensitivity was observed among isolates exposed to all three triazole fungicides, indicating a need for alternating chemistry types.
Benzimidazole resistance was detected at 23 percent in 2000, 45 percent in 2011 and 36 percent in 2012 populations, indicating that this type of resistance occurred long ago and has remained stable in the pathogen population (Figure 2). Benzimidazole resistance also is widespread, with resistant isolates identified in 19 of 27 parishes (see map). Isolates exhibiting multiple resistance (resistance to two or more chemistry types) to strobilurin and benzimidazole fungicides also were detected in 15 of 27 parishes. Ninety-eight percent of benzimidazoleresistant isolates also were resistant to strobilurin fungicides, which was unexpected because of differences in the modes of action of the fungicides. Further investigation is required to determine the correlation between the two resistance mechanisms.
Results from this research confirm strobilurin and benzimidazole resistance in the Cercospora leaf blight pathogen. Because of the highly specific modes of action and selection pressure induced by strobilurins and benzimidazoles, resistance is not uncommon in areas where these fungicide classes are used regularly. Once this resistance occurs, it is persistent in pathogen populations; therefore, producers should not apply these fungicides for management of Cercospora leaf blight in areas where resistance has been documented. These fungicide classes, however, may still be effective on other soybean diseases. Furthermore, proper management practices should be followed to extend the effectiveness of available products. These include avoiding fungicide applications if they are not necessary, using the proper nozzles and spray volumes to ensure adequate coverage, not applying the same fungicide classes in succession, alternating chemistries in subsequent applications, and using tank mixes or pre-mixes with multiple modes of action.
Acknowledgment: The Louisiana Soybean and Grain Research and Promotion Board for funding this project.
Paul P. Price III is an assistant professor and Myra A. Purvis is a research associate at the Macon Ridge Research Station in Winnsboro, La. Clark L. Robertson is a research associate and Raymond W. Schneider is a professor in the Department of Plant Pathology and Crop Physiology. G. Boyd Padgett is a professor at the Dean Lee Research and Extension Center, Alexandria, La.
(This article was published in the 2014 winter issue of Louisiana Agriculture magazine.)