The role of oxalic acid and polygalacturonase in the pathogenicity ofPythium aphanidermatum on different cowpea varieties

Abstract

The amounts of oxalic acid and polygalacturonase (PG) produced in the tissues of cowpea (Vigna unguiculata Walp.) plants infected byPythium aphanidermatum were higher in vars. IT81D-1020 and VITA 5 than in vars. IT82E-32 and TVX3236. Oxalic acid accumulated early at the infective stage of the disease but its production decreased as the plants became older or as the disease developed. At the peak of oxalic acid and PG production (8–10 days after infection) the pH of the tissue fell from 7.2 to 3.1 and thereafter rose to 4.3 (within 18 days). It seems, therefore, that the combination of oxalic acid and PG and the accompanying reduced pH of infected tissue play an important role in the pathogenesis of susceptible varieties of cowpea byP. aphanidermatum. In the two varieties found to be resistant in this study, oxalic acid and PG production were lower than in the susceptible varieties and the pH did not fall as low. Early accumulation of oxalic acid in cowpea tissue during pathogenesis may be a useful tool for monitoring disease severity, and hence susceptibility or resistance toP. aphanidermatum, when there is a compatible interaction between host and pathogen.