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Pathogenic and physiological variability among Macrophomina phaseolina isolates associated with soybean in Ohio

Terry Niblack: The Ohio State University

<div>The fungus that causes charcoal rot, <em>Macrophomina phaseolina</em>, is responsible for significant yield loss in soybean worldwide and can also affect corn. A recent study revealed the widespread distribution of <em>M. phaseolina</em> in both soybean and corn fields in Ohio. The objectives of this study were to evaluate the physiological and pathogenic variability of these <em>M. phaseolina</em> isolates as an initial step to understanding its population structure. A total of 200 <em>M. phaseolina</em> isolates were obtained from soil collected in soybean and corn fields throughout Ohio and used in physiological and pathogenicity studies. For the physiological assay, pure isolates were grown under different temperatures (15, 20, 25, 30, 35 and 40 ºC) on potato dextrose agar (PDA) and area under the growing progress curve (AUGPC) were obtained for each isolate. A soybean cut-stem inoculation technique was used to obtain the area under the disease progress curve (AUDPC) to assess the pathogenicity and aggressiveness of <em>M. phaseolina</em> isolates. Overall, larger variation in AUGPC between isolates was observed at lower temperatures than at temperatures reported as optimal (30 – 35 ºC) for <em>M. phaseolina</em> growth. All isolates were pathogenic to soybean; however, isolates displayed significant variation in aggressiveness. A subset of isolates was selected based on phenotypic data and geographic location for further genotypic and population genetic analyses.</div>