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The effects of common root rot (CRR) caused by Bipolaris sorokiniana on grain yield, number of tillers, number of grains and grain weight of wheat plants were determined in four field experiments. Sites with different soil populations of B. sorokiniana were selected and inoculum of the fungus added to some plots. Disease and yield measurements were made on eight cultivars and lines differing in susceptibility to CRR. Timgalen, Songlen and Hartog were susceptible whereas Kite, 1008 C16, 1 4 1 4 and ISWYN 32 were partially resistant to CRR.Grain yield, tiller and grain number, but not grain weight decreased as disease severity increased. Diseased plants had lower tiller numbers than healthy ones and as a consequence a reduced number of grains and grain yield per plant.Five methods were compared for estimating yield loss caused by the disease. Polynomial regression equations for each cultivar between yield and disease rating of sub-crown internodes or multiple regression equations between yield and disease parameters of sub-crown internodes or tiller bases were established. A third method involved the projection of yield losses from one cultivar to other cultivars and in a fourth method yield losses were estimated from actual yields. In addition, an equation Yield loss (%) = 3 46 + 0.23 disease severity) (%) was established in one experiment and used as a fifth method in the other experiments.Yield losses estimated by methods 1 and 2 were similar and higher than those from the other methods. In areas where disease severity is high, methods 1 and 5 appear to be the most suitable for determining yield losses. Losses in a susceptible cultivar, Timgalen, varied between 13.9 and 23.9% whereas those in a partially resistant cultivar, 1008 C16, varied between 6.8 and 13.6%.

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.

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