Crater disease is seen as severely stunted patches in wheat crops on the Springbok Flats, Transvaal, South Africa. Roots of stunted plants bear pronounced bead-like swellings composed of hyphae of Rhizoctonia solani. The root cortex in the beads is obscured or destroyed, the root xylem vessels are brown and occluded by gel-like material, and roots break easily at these points when retrieved from soil.

In glasshouse studies, typical root symptoms of crater disease occurred on wheat grown in undisturbed blocks of soil from inside craters, but not in loose soil from inside craters or in blocks of soil from outside craters. Root symptoms were reproduced by inoculation of wheat, barley, oats, rye and maize with isolates of R. solani from crater sites. Shoot growth was significantly reduced by inoculation. No other isolate of R. solani, including some from barley stunt disorder in Scotland, produced ‘root beads’ in glasshouse tests. Crater isolates did not anastomose with other R. solani isolates.

It is suggested that R. solani causes crater disease in conditions in which root growth is impeded by soil structure. The implications of this for other Rhizoctonia stunting diseases of cereals are discussed.

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