Relationship between common bacterial blight severity and bean yield loss in pure stand and bean-maize intercropping systems

Field experiments were conducted in the 1999 and 2000 seasons to determine the relationship between common bacterial blight (CBB) ( caused by Xanthomonas campestris pv. phaseoli) and bean ( Phaseolus vulgaris) yield in different cropping systems. Different levels of disease severity were created using natural epidemics by spraying copper hydroxide ( Kocide 101) at 7-, 14- and 21-day intervals beginning at the first observation of CBB symptoms. The susceptible cultivar Red Wolaita was grown in broadcast, mixed and row bean - maize intercropping and pure stand. The bean growth stage at which the disease severity was correlated with seed yield varied among the cropping systems. In broadcast and mixed intercroppings, CBB appeared to be correlated with seed yield at later growth stages. In row intercropping, disease was correlated at preflowering to 50% flowering and in pure stand at 50% flowering to pod filling. In intercropping systems, both relative and predicted seed yield and 100-seed weight losses to CBB were, generally, less than in pure stand. Single-point regression models in 1999 predicted that for each per cent increase of CBB severity in broadcast and mixed intercropping, about 5.2 and 9.1 kg ha(1) seed yield loss, respectively, occurred at physiological maturity. In pure stand, for each percentage increase in CBB at an earlier growth stage, 73 days after planting (DAP) in 2000, a yield loss of 38.8 kg ha(1) occurred. Similarly, in row intercropping a 71.1 kg ha(1), seed yield loss was estimated at 52 DAP in 1999. Decreasing seed weight was correlated with yield loss to CBB. The type of cropping system influenced CBB severity and the amount of loss in yield attributable to CBB. Variation in CBB and its relation to yield between years was influenced by difference in seasonal weather between years.