Abstract

Cassava bacterial blight (CBB), caused by Xanthomonas campestris pv. manihotis (Berthet and Boudar) Dye, caused 18 to 92% yield reduction on susceptible cassava (Manihot esculenta Crantz) clones relative to resistant clones depending on location, planting time, and level of simultaneous infection by superelongation disease caused by Elsinoe brasiliensis. Root dry matter content of susceptible clones was much lower than that of resistant clones. Susceptible clones did not produce planting stakes for next planting. No cassava clone under study was immune to CBB. Cultivar order of resistance was stable over years. The CBB on leaves of resistant clones spread slowly while on susceptible clones it spread rapidly causing defoliation and death of stem. Resistance appeared to be a quantitatively inherited trait (h² = 0.63) largely controlled by additive genetic factors and was not negatively correlated with yielding ability per se. Cassava resistance to CBB is highly effective in minimizing damage caused by CBB, and use of resistant parents in hybridizations combined with simple phenotypic field selection under high natural disease pressure should effectively improve resistance of cassava cultivars.

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