Pathogenicity, electrophoretic characterisation and in planta detection ofthe 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 ha s been claimed that a complex of three root pathogens is needed to cause th e disease. In this study, we compared two Pythium isolates from diseased co coyam roots, CRPm and Bokwai, with other putative P. myriotylum isolates fr om culture collections and from Cameroonian soil, with respect to host rang e and isozyme patterns. Pathogenicity was tested on tomato, bean, cowpea, t obacco and cocoyam. CRPm and Bokwai were only pathogenic to tobacco and coc oyam. On cocoyam, these isolates caused typical symptoms within 48 h on 100 % of the inoculated plantlets. Only two other isolates of P. myriotylum fro m culture collections were moderately to weakly pathogenic to cocoyam. Isol ates of P. myriotylum were very variable in their pathogenicity to bean, co wpea, tomato and tobacco. Isozyme patterns of alpha- and beta-esterases wer e used to differentiate CRPm and Bokwai from all other isolates. Unlike the other P. myriotylum strains, cocoyam isolates were unable to grow at 37 de grees C. Malate dehydrogenase isozyme bands originating from CRPm were cons istently detected in CRPm-infected cocoyam roots grown in vitro and in vivo . These findings indicate that CRPm can penetrate cocoyam roots and cause d isease in the absence of other root pathogens. This study also indicates th at P. myriotylum from cocoyam developed a certain degree of host specialisa tion.