Pathogenicity of Pythium spp. And Phytopththora drechsleri to Eucalyptus spp

Abstract

A number of Pythium spp, and Phytophthora drechsleri have been associated with Phytophthora cinnamomi in the die-back disease of eucalypts in forests of southern and eastern Australia. Although P. cinnamomi is generally considered to be the most important pathogen causing root rot and die-back, there is some uncertainty whether it is the only pathogen.

The pathogenicity of Pythium ultimum, P. irregulare, P. debaryanum, P. vexans, P. mamillatum, three unidentified Pythium species from eucalypt forest soil and Phytophthora drechsleri was compared with that of Phytophthora cinnamomi on juvenile and intermediate-aged seedlings of Eucalyptus sieberi and juvenile seedlings of other eucalypt species. Of the Pythiums, only P. ultimum, P. irregulare and P. debaryanum could kill small juvenile seedlings at the two-leaf stage of development. Neither any of the Pythiums nor Phytophthora drechsleri could kill E. sieberi at the intermediate growth stage. In marked contrast, Phytophthora cinnamomi showed considerable pathogenicity, killing seedlings and saplings with rapidity. The unidentified Pythium spp, from native forests were completely innocuous.