Grain spawn preparation
Sorghum (Sorghum bicolor L.) grain was used for spawn preparation. For this purpose, about 3.6 kg of sorghum grain was half-cooked, excess water drained off and allowed to cool down to room temperature. It was then spread uniformly over a surface sterilized (70 % ethanol) plastic sheet until optimum moisture (51–54 %) is attained. The grain was then mixed with 10 % wheat bran, and 2 % gypsum (calcium sulfate) as nutrient supplement and the pH was adjusted to 9 (Romero 2007). The mixture was then filled into sterilized one-liter bottles to two-third capacity, plugged and sterilized in an autoclave (at 121 °C, 1.5 p.s.i, for 1 h). Sterilized bottles were then allowed to cool and aseptically inoculated with a piece (5 mm × 5 mm) of mycelia culture (14 days old). The bottles were subsequently incubated at 24 ± 3 °C for 14 days until the mycelia fully invade the grains. After 15 days, the grain spawn was ready to use.