Temporal progression of bean common bacterial blight (Xanthomonas campestris pv. phaseoli) in sole and intercropping systems

The effects of four planting patterns of bean (Phaseolus vulgaris) (bean only, maize-bean (MB), sorghum-bean (SB), and maize-bean-sorghum (MBS)) and four cropping systems (sole cropping, row, mixed, and broadcast intercropping) on the temporal epidemics of bean common bacterial blight (CBB) caused by Xanthomonas campestris pv. phaseoli were studied. The experiments were conducted during two consecutive spring and summer seasons in 1999 and 2000 in replicated field experiments. The Gompertz model described disease progress curves better than the logistic model. Intercropping delayed epidemic onset, lowered disease incidence and severity, and reduced the disease progress rate. The type of cropping system and planting pattern affected CBB incidence and severity at initial, final and overall assessments and also affected the rate of disease development. Statistical significance of treatment interactions based on disease assessments was found for incidence in all four experiments and for severity in three experiments. A slower disease progress rate and lower incidence and severity occurred on beans planted with maize or sorghum in row, mixed and broadcast intercropping than on bean planted alone. Incidence was reduced 36% and severity 20% in intercropping compared to sole cropping. The built-in disease delay and the slowing of the disease progress rate could provide protection for beans from severe CBB epidemics in intercropped systems. Variation between years appeared to be related to relative humidity (RH).