Characterization of pathotypes among isolates of Xanthomonas axonopodis pv. manihotis in Colombia

Cassava bacterial blight, caused by Xanthomonas axonopodis pv. manihotis (X am) is a destructive disease occurring in most cassava growing-areas. Altho ugh Colombian isolates of Xam differ in DNA polymorphism and pathogenicity, no suitable host differentials have been identified to demonstrate physiol ogical specialization. A set of 26 Xam isolates from three edaphoclimatic z ones (ECZs) in Colombia was selected for inoculation on a set of 17 potenti al cassava differentials. Leaf inoculation and stem puncture were used in o rder to detect possible specific interactions between cultivars and isolate s. Cultivar x isolate interaction was highly significant (P < 0.001) after stem inoculation, but not after leaf inoculation. The stem inoculation tech nique was selected as a method for resistance screening of cassava cultivar s for bacterial blight resistance. A highly significant interaction was als o detected when cultivar behaviour was rated as area under the disease prog ress curve (AUDPC) after stem inoculation. Different pathotypes were define d among the 26 isolates and differential cultivars were proposed to define the pathotypic composition of Xam populations in three ECZs in Colombia. Th e results should help to improve selection of sources of resistance to cass ava bacterial blight.