EFFECT OF CROPPING HISTORY ON SOIL POPULATIONS OF BIPOLARIS-SOROKINIANA AND COMMON ROOT-ROT OF WHEAT

Six winter and four summer crops, two pasture legumes and two pasture grasses, were grown in rotation with wheat in soil naturally colonized with Bipolaris sorokiniana. Soil populations of B. sorokiniana under the different rotations were measured at the beginning of winter and of summer between 1982 and 1986. Populations increased under wheat, barley and triticale, remained static under oats, safflower and chickpea, and declined under buffel grass, cocksfoot, lucerne, mung bean, snail medic, sorghum, sunflower and White French millet. Populations also declined under a fallow, and propagules of the fungus were still present after 4 years. Where populations had declined owing to cropping history, they were effectively restored to their previous levels after one wheat crop. Severity of common root rot in wheat caused by B. sorokiniana, was reduced from that occurring with continuous wheat when wheat was planted after safflower, sorghum, mung bean, snail medic, lucerne and buffel grass. Disease levels in wheat following barley, triticale, oats, chickpea, millet, sunflower, cocksfoot or a fallow were not significantly different from that of continuous wheat. When a second wheat crop was planted, severity of common root rot was high in all treatments, irrespective of the previous cropping history. The implications of these findings in developing control strategies for common root rot by crop rotation are discussed.