GB KHATRI-CHHETRI1 and K WYDRA2 and K Rudolph11Institute of Plant Pathology and Plant Protection of the University, Gottingen, Germany; 2International Institute of Tropical Agriculture, PHMD, Cotonou, Benin

Background and objectives
Cowpea bacterial blight caused by Xanthomonas campestris pv. vignicola (Xcv) is common worldwide, whereas cowpea bacterial pustule is reported only from Nigeria and West Africa where mixed symptoms of blight and pustules are often observed. The pathovar vignaeunguiculatae proposed for pustule causing bacteria [1] has not been accepted internationally. For both bacterial variants differences in aggressiveness have been reported, but the existence of races has not been clearly established [2]. It is still an open question, whether blight and pustule causing bacteria belong to the same pathovar vignicoia. Therefore, the objectives of this study were to characterize blight and pustule causing strains by analysing their capability to metabolize 95 organic substrates.

Materials and methods
Fifty-five strains of Xcv (36 from blight, 13 from pustules, 6 reference strains) originating from 11 countries, were studied for their metabolization pattern of 95 carbon sources using the Biolog GN Microplate System.

Results and conclusions
The strains varied considerably in utilization of the carbon sources. Twenty-seven substrates were used by all the strains, 48 not by any strain and 20 were used by some strains and not by the others. Although the strains isolated from pustules differed to some extent from strains isolated from blight in the metabolization of N-acetyl-D-glucosamine, D-galactose, cis-aconitic acid, malonic acid and succinamic acid, no specific difference could be observed. Only two pustule strains originating from Mozambique differed greatly in the utilization of gentiobiose, lactulose, D-melibiose, D,L-lactic acid, glycerol and D,L-a-glycerol phosphate. No considerable difference was observed between reference strains and fresh isolates.

A specific difference could be observed between strains according to geographic origin. Dextrin, glycogen and succinamic acid were not used by strains from Benin and Thailand, but were used by all the others, whereas N-acety]-D-glucosamine, malonic acid and L-proline were used only by strains from Benin and Thailand, The strains from Venezuela differed considerably in the metabolization of cis-aconitic acid and hydroxybutyric acid from the other strains.

Ninety-one per cent of the strains from Benin and Thailand (18% of all the strains) were identified as X. campestris pv. vignicola. None of the other strains was identified as Xcv. The Biolog data base identified 80% of the strains as X. campestris, however belonging to seven different pathovars. Twenty per cent were identified as Pseudomonas cissicola.

According to the variations in substrate utilization (but similarities within a group) the strains could be categorized into four groups: (1) Cameroon, Nigeria, Niger, Sudan and Uganda; (2) Benin and Thailand; (3) Venezuela and Brazil; and (4) Mozambique. The Biolog System revealed a considerable variation in metabolic fingerprints of the strains, which were more or less correlated with their origin and identification, but not to blight or pustule development and pathogenicity. It is concluded that the strains isolated from blight and pustules from West Africa belong to the same pv. vignicola.

1. Patel PN, Jindal JK, 1982. Journal of Plant Diseases and Protection 89, 406-409.
2. Khatri-Chhetri, GB, Wydra K, Rudolph K. 1998. Proceedings 8th International Conference Plant Pathogenic Bacteria, Madras 1996, India, in press.