Studies on Pyricularia oryzae Cav. in Sierra Leone: Morphological and Physiological Variability of Some Isolates

The morphological and physiological variability of six isolatesof Pyricularia oryzae, the causal organism of rice blast disease,were investigated. The rate of growth, colony characters, andtime of sporulation were found to vary with the different isolatesthough not by appreciable amounts. Each of the isolates showedconsistently better growth on Takahashi's B medium than on Czapek-Dox'smedium although the growth trend was the same in both media.The colony characters developed by each isolate are not dependenton the medium on which it is growing—a pointer to thefact that such characters may be genetically controlled. Germinationwas faster in distilled water than in 2 per cent agar. All theisolates produced appresoria in vivo and in vitro; those producedin vivo were, however, considerably larger than those producedin vitro. On the basis of appresorial types, the isolates werefound to fall into two physiological races—smooth-walledand rough-walled. Each isolate produced consistently only oneappresorial type in vitro from the apical or basal cell of theconidium. The utilization of carbon and nitrogen compounds variedfrom one isolate to the other, carbon compounds being generallybetter utilized than nitrogen compounds. Pyricularia oryzaecan metabolize a wide range of carbon compounds. However, mannose,sucrose, glucose, fructose, and maltose proved to be most suitablecarbon sources. The variations in the utilization of the variouscarbon and nitrogen compounds seem to reflect inherent biochemicaland physiological differences among the isolates.