Epidemiological Aspects of Cowpea Bacterial Blight

Abstract

Cowpea bacterial blight, caused by Xanthomonas axonopodis pv. vignicola (Xav), can severely reduce the yields of cowpea in tropical regions. Despite its destructiveness, research on the epidemiology of bacterial blight is scarce in Brazil. The objectives of this study were to quantify the effect of the Xav inoculum concentration and of the duration of leaf wetness periods on disease severity, in addition to assessing bacterial transmissibility by seed and to identify indicator plants that might be used for simple diagnostic tests. In experiments conducted in greenhouse and field conditions, cowpea plants were inoculated with 105, 106 and 107 CFU (colony forming units) of Xav/mL, and seeds harvested from field plots were used to determine the transmissibility of the bacteria. Under greenhouse conditions, plants were subjected to leaf wetness duration periods of 0, 4, 8, 12, 16, 20 and 24 h after inoculation. In another experiment, soybean, common bean and wild poinsettia were inoculated with Xav. It was observed that plants inoculated with a concentration of 107 UFC/mL and subjected to 12 h of leaf wetness had higher disease severity. There was transmission of Xav to the seeds in all inoculum concentrations used. Hypersensitivity reaction was observed in wild poinsettia less than 24 h after inoculation.