Occurrence of leaf spot on cowpea (Vigna unguiculata) caused by Xanthomonas axonopodis pv. vignicola in Mozambique

In June 2004, angular, necrotic, pustuliform leaf spots, surrounded by a thin water-soaked halo, were observed on cowpea (Vigna unguiculata) plants (Fig. 1), cultivated in the Umbeluzi area (Maputo, Mozambique), with an incidence of 70-90%.

Yellow, circular and raised bacterial colonies were consistently isolated on nutrient agar from diseased leaves. Six selected representative bacterial strains and the reference strain LMG 8752 of Xanthomonas axonopodis pv. vignicola were subjected to identification tests. All strains were gram negative, had oxidative but not fermentative metabolism and a mucoid growth on yeast dextrose chalk agar. All hydrolysed esculin and casein, produced acid from arabinose, and hydrogen sulphide from cysteine. When the first trifoliate leaves of 3-week-old cowpea plants (four plants for each strain) were infiltrated with 10⁶ CFU per ml bacterial suspensions, small water-soaked spots started to appear 6-7 days after inoculation. Bacteria were re-isolated from inoculated plants and found to be identical to the bacterial isolates used for inoculation. With the primers used for Xanthomonad identification in PCR analysis (Leite et al., 1994), RST2/RST3 generated a 840 bp amplification product from all bacterial strains, whereas RST9/RST10 produced no amplification products. Furthermore primers X4c-X4e, specific for the detection of the cowpea pathogen X. axonopodis pv. phaseoli (Audy et al., 1994), did not generate any amplification products. On the basis of these results we can conclude that the bacterial strains isolated from cowpea plants belong to X. axonopodis pv. vignicola. Characterisation of the bacterial strains carried out using the Biolog system demonstrated that our bacterial strains grew on the 19 substrates used by all the 55 X. axonopodis pv. vignicola strains characterised by Khatri-Chhetri et al. (2003). However, we found metabolic profiles different from those observed by these authors, when 10 variably used carbon sources were considered (results not shown). While X. axonopodis pv. vignicola strains from Mozambique have been characterised by Khatri-Chhetri et al. (2003), this is the first definitive report of the disease cowpea leaf spot caused by Xanthomonas axonopodis pv. vignicola in this country.