Comparison of Different Culture Media and Different Spawn Substrata to Increase Inoculum of <i>Cercospora sojina</i> and <i>Septoria glycines</i>

Frogeye leaf spot and Septoria brown spot, caused by Cercospora sojina and Septoria glycines, respectively, are economically important foliar diseases of soybean (Glycine max L.). Artificial inoculation is important for fungicide efficacy trials when natural infection does not occur. Several culture media and light regimes were evaluated to enhance C. sojina sporulation. PDA, PGM, SSLB, DV8, and DV8 with filter paper were tested under two light regimes (12 h photoperiod, and 24 h light) at 25°C for 14 days. The best light regime × media interactions in conidia production per mm 2 showed statistical similarity, such as SSLB in dark/light (24.72), DV8 in dark/light (24.28), DV8 with filter paper in dark/light (16.96), PGM in dark/light (12.20), and DV8 in light (10.53). Sorghum, proso millet, and long grain rice were tested as a source of spawn grain to increase the inoculum of C. sojina and S. glycines using culture plug and conidia suspension as inoculation methods. Jars were incubated at 25°C (12 h photoperiod) upside down, and the colonization percentage was visually assessed at 14 and 28 days after inoculation (DAI). Sorghum grain, in combination with conidia suspension, was the most effective substrate at 14 DAI for both pathogens. At 28 DAI, sorghum and rice were the most effective substrates for both pathogens. DV8 or PGM media can be used to enhance C. sojina conidia production at 25°C (12 h photoperiod), and sorghum as a grain spawn substrate to increase inoculum production for both C. sojina and S. glycines.