GENETIC DIVERSITY, CROSS PATHOGENICITY AND CONTROL OF BACTERIAL BLIGHT OF COWPEA USING BACILLUS SP. AND SELECTED BOTANICALS

Abstract

Cowpea (Vigna unguiculata (L.) Walp.) is an economically important crop cultivated both for domestic and commercial purposes. It is one of the most resilient crops suited to arid and semi-arid areas. Cowpea production is affected by several diseases caused by phytopathogens. Bacterial blight caused by Xanthomonas axonopodis pv. vignicola has been reported in many cowpea producing areas causing reduction in both quality and quantity of the harvestable leaves and grains. However, in Kenya, the disease occurrence status has not been exhaustively documented, similar to the causal pathogen characterization and management using biological control agents and botanicals. This research conducted field surveys in farms from six counties representing different zones to analyse the disease occurrence levels. The identification of the pathogen was done by morphological and biochemical features as well as by analysis of 16S rDNA and its genetic diversity was determined using inter-simple sequence repeat markers. The pathogen cross pathogenicity was tested by inoculating other legumes (beans, soya bean, green gram, garden peas and lentils) with the most virulent isolate. Dual culture and inverted plate techniques were used to test bio-efficacy of the biological control agents and botanical extracts. Xanthomonas axonopodis pv. vignicola was confirmed as the causal pathogen in 48 farms out of 80 farms. The mean disease incidence was 44.89% across the sampled sites, being highest in Kakamega County (50.49%) and the least in Uasin Gishu County (33.57%). Xanthomonas axonopodis pv. vignicola isolates displayed slight variance in morphological and cultural characteristics on nutrient agar. The biochemical tests and analysis by blasting of sequence from 16S rDNA region confirmed Xanthomonas axonopodis pv. vignicola as the causal agent. The polymorphic information content ranged from 0.2384 to 0.4486, indicating genetic variations which was strongly correlated with the differences within populations. The pathogen, infection was observed in all the cowpea varieties tested and cross infection on soya bean and lentils. The percentage disease severity ranged between 25.83% to 51.67%, which was significantly higher in cowpea varieties. The bio-efficacy of selected bio-agents showed varying levels of percentage inhibition against X. axonopodis pv. vignicola, depending on the method and the duration of exposure. Bacillus subtilis displayed the highest antibacterial activity between 68.33% and 87.79% by dual culture technique and between 38.33% to 71.33% in inverted plate method over seventy-two hours. Bacillus amyloliquefaciens showed a statistically significant antibacterial activity between 45.00% and 76.12% in dual culture technique and between 45.00% and 73.89% by inverted plate method. Cyprofloxacin the antibiotic used for positive check, ethanolic extracts of neem, garlic and ginger inhibited X. axonopodis pv. vignicola by 56.3 mm, 38.5 mm, 30.8 mm and 25.0 mm respectively, but no potency was noted for Salvia nilotica. Genetic variations of Xanthomonas axonopodis pv. vignicola was noted from different regions of Kenya. This study showed that B. subtilis and B. amyloliquefaciens, neem, garlic and ginger extracts are useful biocontrol options in management of X. axonopodis pv. vignicola and therefore can be recommended for integration in the management of this pathogen in cowpea.