Comparison of measurement methods for determining Macrophomina phaseolina isolate aggressiveness

Comparison of measurement methods for determining Macrophomina phaseolina isolate aggressiveness

Charcoal rot, caused by the fungus Macrophomina phaseolina, is an economically important sorghum stalk disease in the northern grains region of Australia. It is often associated with lodging and yield losses that usually occur during hot and dry conditions during the growing season. There are few available management strategies to minimise its effect, and so far, no resistance in sorghum has been reported. An effective charcoal rot resistance screening method requires both an aggressive isolate, representative of the pathogen population, and a repeatable inoculation method. The area under disease progress curve (AUDPC) has been used for identifying disease resistance and can be used in the selection of aggressive plant pathogen isolates for screening purposes. This study aimed to investigate if current methods of inoculation and measurements used to determine M. phaseolina isolate aggressiveness being used in Australia are effective. Two trials were conducted using 33 isolates from the northern grains region. The first trial used a single point assessment at 28 days after inoculation (DAI). The second used four weekly measures ending at 28 DAI to calculate AUDPC using a two-point method and all four timepoints for traditional AUDPC. In both trials, sorghum plants were inoculated by inserting M. phaseolina infested toothpicks into the stalk ~5 cm above the soil surface. Stalks were split and lesion length was measured. The single point method was unable to detect any difference in isolate aggressiveness. The two-point AUDPC method was not feasible due to some measurements not exceeding zero until the final reading, while the four-point AUDPC method showed significant differences at p > 0.05, but a Tukey’s post-hoc test was unable to determine any groupings. The current method of inoculating the lower stalk generated variable lesion lengths and should be re-evaluated to find methods that can generate more consistent results.