Transmission of Xanthomonas campestris pv vignicola, and screening cowpea lines for resistance to bacterial blight in Uganda

Experiments were carried out at Kabanyolo and Namulonge to (a) determine the level of seed infection and ability of cowpea seeds to transmit Xanthomonas campestris pv vignicola, (b) determine the most suitable techniques for inoculating cowpea seedlings, and (c) to screen cowpea germplasm for resistance to bacterial blight. A total of 140 seed samples from different parts of Eastern and Northern Uganda were assayed for infection by Xanthomonas campestris pv vignicola using yeast dextrose calcium carbonate agar (YDCA) as media. The level of seed infection was low and variable (0.03-0.69%). The highest seed infection was recorded from seeds collected from Kumi district while the lowest was from Nebbi district. A separate study was conducted in the screenhouse at Kabanyolo between April 1995 and January 1996 to determine the ability of cowpea seed to transmit Xanthomonas campestris pv vignicola. the experimental design was a split-plot with cowpea varieties as the main plots. The three bacterial concentrations (10^1 , 10^4 , 10^7 colony forming units/ml)formed sub-plots.Percentage seed germination and seedling mortality, and percentage pod infection significantly (p≤ 0.05) varied with cow pea varieties. Bacterial concentrations significantly influenced p≤ 0.05 percentage leaf and pod area blighted, and pod infection during the first trials, and bushy and stunted growth in the second trial. In all cases, bacterial concentration of 10^7 colony forming units/ml resulted in the highest leaf blight, leaf and pod area blighted, pod infection and bushy stunted growth. Significant variety X bacterial concentration were observed on percentage pod infection and bushy stunted growth during the first and second trials, respectively, implying that bacterial blight infection depended on host genotype and inoculum load. Secondly infection expressed as percentage leaf and pod area blighted did not differ significantly at p≤ 0.05 among the cow pea varieties during the second trial. In a separate experiment, three techniques for inoculating cowpea with Xanthomonas campestris pv vignicola were evaluated in the screenhouse with cowpea varieties constituting the main plots and inoculation techniques the sub-plots. Inoculation techniques significantly p≤ 0.05 influenced the occurrence of the disease, but the levels of disease development did not vary with the cowpea variety used.