Joseph Poppe

Pathogenicity, electrophoretic characterisation and in planta detection of the cocoyam root rot disease pathogen, Pythium myriotylum

Cocoyam (Xanthosoma sagittifolium), an important staple food crop for many people in the tropics and subtropics, suffers great losses from a root rot disease which is most probably caused by Pythium myriotylum, although it has been claimed that a complex of three root pathogens is needed to cause the disease. In this study, we compared two Pythium isolates from diseased cocoyam roots, CRPm and Bokwai, with other putative P. myriotylum isolates from culture collections and from Cameroonian soil, with respect to host range and isozyme patterns. Pathogenicity was tested on tomato, bean, cowpea, tobacco and cocoyam. CRPm and Bokwai were only pathogenic to tobacco and cocoyam. On cocoyam, these isolates caused typical symptoms within 48 h on 100% of the inoculated plantlets. Only two other isolates of P. myriotylum from culture collections were moderately to weakly pathogenic to cocoyam. Isolates of P. myriotylum were very variable in their pathogenicity to bean, cowpea, tomato and tobacco. Isozyme patterns of α- and β-esterases were used to differentiate CRPm and Bokwai from all other isolates. Unlike the other P. myriotylum strains, cocoyam isolates were unable to grow at 37°C. Malate dehydrogenase isozyme bands originating from CRPm were consistently detected in CRPm-infected cocoyam roots grown in vitro and in vivo. These findings indicate that CRPm can penetrate cocoyam roots and cause disease in the absence of other root pathogens. This study also indicates that P. myriotylum from cocoyam developed a certain degree of host specialisation.