Bacterial leaf blight (BLB) of rice is a very destructive disease worldwide and is caused by Xanthomonas oryzae pv. oryzae (Xoo). The aim of the present study was to examine if the Xoo virulence pathotypes obtained using phenotypic pathotyping could be confirmed using molecular approach. After screening of 60 Operon primers with genomic DNA of two Xoo isolates (virulent pathotype, Vr, and mildly virulent pathotype, MVr), 12 Operon primers that gave reproducible and useful genetic information were selected and used to analyze 50 Xoo isolates from 7 West African countries. Genetic analysis revealed two major Xoo virulence genotypes (Mta and Mtb) with Mta having two subgroups (Mta1 and Mta2). Mta1 (Vr1) subgroup genotype has occurrence in six countries and Mta2 (Vr2) in three countries while Mtb genotype characterized mildly virulence (MVr) Xoo isolates present in five countries. The study revealed possible linkage and correlation between phenotypic pathotyping and molecular typing of Xoo virulence. Xoo virulence genotypes were known to exist within country and there was evidence of Xoo pathogen migration between countries. Durable resistance rice cultivars would need to overcome both Mta and Mtb Xoo virulence genotypes in order to survive after their deployment into different rice ecologies in West Africa. 1. Introduction Rice is perhaps the most widely cultivated food crop world over, but its production is constrained by diseases of fungal, bacterial, and viral origins. Bacterial leaf blight (BLB) of rice, caused by Xanthomonas oryzae pv. oryzae (Xoo) is a very destructive disease and its incidence has been reported from different parts of Asia, northern Australia, Africa, and USA [1–3]. The disease is known to occur in epidemic proportions in many parts of the world, incurring severe crop loss of up to 50% [1, 2, 4]. In West Africa, disease incidence ranged from 70 to 85% and yield loss ranged from 50 to 90%, indicating a wide spread of BLB disease in farmers’ fields [2, 4]. Some selected Xoo isolates have shown high level of pathogenicity and virulence on the cultivated rice varieties [4]. Research studies have also revealed that BLB is an important rice disease in irrigated rice ecosystems in West Africa, mainly in Sahelian and Sudano-Sahelian countries [2, 5]. Crop loss assessment studies have revealed that this disease reduces grain yield to varying levels, depending on the stage of the crop, degree of cultivar susceptibility, and, to a great extent, the conduciveness of the environment in which it occurs [6]. The severity and significance of damage
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