Comparison of the interactions ofErwinia carotovora ssp.atroseptica withPhytophthora infestans, Phoma foveata andFusarium coeruleum in rotting potato tubers

Summary An interaction betweenP. infestans andE. carotovora subsp.atroseptica (Eca) in rotting tubers was confirmed and the biochemical basis for its occurrence investigated. The pH of tissue infected withP. foveata became alkaline whereas tissue infected withP. infestans orF. coeruleum did not rise above neutrality, which could promote pathogenicity of Eca by maintaining pH closer to the optimum for polygalacturonase activity. Polygalacturonase, pectate lyase and galactanase were detected in cultures of all three fungi grown on media containing cell wall material or pectin from tubers. AsP. infestans produced more polygalacturonase than the other two fungal tuber rot pathogens the possibility was investigated that oligogalacturonide products of enzymatic degradation of pectin byP. infestans stimulates pathogenicity of Eca. However, while tubers soft-rotted after infiltration with supernatant from fungal cultures grown on tuber cell wall material, controls showed that rotting resulted from infiltration rather than the products contained in the infiltrated water.