Pathogenic and genetic diversity of Xanthomonas axonopodis pv.Manihotis in Uganda
Opio, Samuel Morris
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Cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis, is the most destructive bacterial disease of cassava owing to its negative effects on yield and quality of planting materials. Thus, in order to develop a sustainable management strategy, knowledge of its distribution and genetic diversity in Uganda is important. CBB symptomatic leaf samples were collected from 21 districts in Uganda from which bacteria was isolated and cultured. From each sample, the avirulence gene (avrBS2) was amplified (500 bp) using PCR and the resulting amplicons sequenced. These sequences were aligned using MegAlign under DNA Star Navigator software and a pairwise comparison done. A phylogenetic tree was thereafter generated. Pathogenicity tests were later performed for 14 isolates through direct stem inoculation in the screen house. Data were taken weekly up to 42days post inoculation. The most virulent isolate was used to screen 19 cassava cultivars for resistance to CBB. Field surveys revealed presence of CBB throughout Uganda. Incidence was highest (100%) on local cultivar Sanje with mean severity 2 (mild) using a scale of 1-5. A total of 98 isolates grew on media (Yeast extract, glucose, Bacto Peptone, Agar) and had either hyaline or yellow colonies with smooth margins. The phylogeny tree indicated presence of different strains that were grouped into two major clusters implying genetic diversity. The most virulent isolate was obtained from a 4-month-old cassava cultivar (TME14) with average incidence 63.3% and severity 2.0. After inoculation, there were brown necrotic lesions with cream bacterial exudates on the stems of different cultivars. Three groups of cultivars (susceptible, moderate and resistant) were identified. Local cultivar Alado alado was the most susceptible while NASE 14 was the most tolerant cultivar. The remaining cultivars were moderately affected. Cultivars like NASE 14 that are tolerant to bacterial blight and viral diseases (Cassava Mosaic and Cassava Brown streak disease) are recommended to farmers for production. They could also be used in CBB resistance breeding programs.