A Simple Technique for Inducing Sporulation of Bipolaris oryzae (Breda de Haan) Shoemaker Caused Brown Spot of Rice

Abstract

Brown spot of rice caused by the fungus Bipolaris oryzae (Breda de Haan) Shoemaker used to be considered as a minor disease in Thailand. Nowadays, it becomes a major disease of rice, due to climate change and cultivation practices. Spore morphologies are a major character for fungal taxonomy and spore inoculation is the most effective method for screening of varietal resistance to brown leaf spot. However, several research articles have reported that B. oryzae failed to sporulation on artificial and natural media. The spore inducing method for B. oryzae needs selective media with complicated environments. Therefore, it is necessary to develop a new and simple method to induce sporulation of B. oryzae. The aim of this research was to develop a new and simple method for inducing sporulation of B. oryzae for further study of morphology and screening rice varieties for resistance to brown spot disease. Two media, commercial rabbit food agar (CRFA) and potato dextrose agar (PDA) with simple and optimum environmental conditions (room temperature) were tested for inducing sporulation of five isolates of B. oryzae. The result revealed that PDA induced spore production 6-12 times higher than CRFA. The simple and optimum environmental condition for spore induction was the cycle of 12 hours of fluorescent light at 25±1 oC followed by 12 hours of complete darkness at 28±1 oC for five days. Then first hot scraping treatment of the mycelium culture surface with red-hot bent needle in four directions to form a fine square grid pattern was performed following by incubation at the simple and optimum environmental condition for three to four days. If the spore were not inducing at first hot scraping, then another hot scraping needed to be repeated. We found that two times hot scraping could induce spore in all isolates.