3. The Incidence, severity and aetiology of a bacterial canker disease of citrus in Ghana

Abstract

A disease of citrus characterised by slightly raised spots with chlorotic halo was observed in the citrus orchard of the Soil and Irrigation Research Centre of the University of Ghana, located at Kpong in 2009. Samples of the diseased leaves and fruits were taken and a bacterium isolated and identified using cultural, morphological and polymerase chain reactions with three different sets of species specific primers. The pathogenicity of the bacterium was established using citrus seedlings. The bacterium isolated was found to produce yellow mucoid colonies and the cells were short rods and gram negative. It showed pectolytic activity on potato plugs and was pathogenic to artificially inoculated seedlings. The expected PCR product of 222 bp, 179 bp and 197 bp were obtained in the PCR using the primer pairs 2/3, J-RXg/J-RXC2 and J-pth1/J-pth2 respectively. It was concluded that the bacterium was Xanthomonas campestris pv citri A and the disease was a form of citrus canker. The disease incidence decreased from 100% to 13.3% in six years (2009-2014). Correspondingly, the disease severity declined from 88.3% to 1.7% in six years (2009-2014) without the application of any control measure. It was conjectured that the prevailing microclimate and environmental conditions might have influenced the survival of the pathogen. The practical implications of the findings are discussed and future studies suggested.