Morphological and biochemical characterization of strains of Xanthomonas axonopodis pv. vignicola isolated from cowpea grown in three agroecological zones in Nigeria

Abstract

Cowpea bacterial blight (Xanthomonas axonopodis pv vignicola) is a devastating disease of cowpea in Nigeria. Accurate identification is the first step for an effective management option. Thus, X. axonopodis pv. vignicola isolated from cowpea (Vigna unguinculata (L.) Walp) grown in different Agro-ecological zones in Nigeria were characterized using morphological characteristics, biochemical and antibiotic sensitivity tests. Pot experiments were conducted on two susceptible lines of cowpea (IT90k – 76) and (IT84s – 2246 – 4) in the screen house. In vivo infectivity studies were carried out to determine the effect of each of the
X. axonopodis pv. vignicola isolate groups on the cowpea. All isolates from each of the agroecological zones were classified into Group 1 (light yellow) and Group 2 (brownish yellow) based on their morphological presentation on nutrient agar medium. Results from four of the biochemical test showed differences between the two morphologically different groups. Group 2 isolates showed a large zone of inhibition to a Lincomycin hydrochloride while isolates from Group 1 were not affected by the action of the same antibiotic. Blight symptom was only observed when the two types of isolates were combined and inoculated simultaneously. This study showed that there are two morpho-types of X. axonopodis pv. vignicola and their synergetic infectivity results in blight symptoms expression in cowpea.