Epiphytic Survival of Xanthomonas axonopodis pv. phaseoli (E. F. SM)

Abstract

Common bacterial blight, caused by Xanthomonas axonopodis pv phaseoli, is a major disease in beans (Phaseolus vulgaris) throughout the world. Field and greenhouse experiments were conducted to determine epiphytic X. phaseoli populations of bean varieties and nonhost plants. Leaves of three bean varieties grown in the field were sampled for X. phaseoli populations at two-week intervals from two weeks after crop emergence while stems, flowers and pods were assessed for the pathogen at eight weeks after crop emergence. Weeds and maize growing in the vicinity of the bean fields were also assessed for the pathogen. Epiphytic pathogen populations of up to 3.75 x 104 cfu/ml were detected on bean varieties (Mkuzi, Cranberry and Michigan C20), while on Zea mays, Oxalis latifolia, Cyperus rotundas, Bidens pilosa, and Amaranthus hybridus, the pathogen population was between 1.02 x 104 and 2.14 x 104 cfu/ml. Leaves of Amaranthus hybridus and Zea mays hosted pathogenic strains of X. phaseoli, but those of Oxalis latifolia, Bidens pilosa and Setaria spp had nonpathogenic xanthomonads. Both plant age and bean variety had no effect on pathogen population, and bacterial populations in the nonhost plants were not significantly different from each other (p=0.345). The study confirmed the existence of X. phaseoli on nonhost plants and on the stems, flowers and pods of bean varieties. The epiphytic bacteria have potential to act as inocula reservoir during and between growing seasons. The nonhost plants must be controlled as part of common bacterial blight management.