Factors Influencing Suppression of Root Rot of Eucalyptus sieberi and E. obliqua Caused by Phytophthora cinnamomi

Abstract

A krasnozem from the foothills of Mt Dandenong, Victoria, was found to strongly suppress root disease of Eucalyptus sieberi and E. obliqua seedlings caused by Phytophthora cinnamomi. Soil factors associated with disease suppression were examined in a phytotron by altering soil structure, using a hammer mill, sieves of various sizes and changing the soil microflora with steam at 60 and 100°C. E. sieberi, E. obliqua and E. globoidea were used as test seedlings. The results showed that the suppressive activity depended on the soil microflora and their association with the aggregates in the krasnozem. P. cinnamomi was found to kill seedlings by girdling the base of the stem and/or major roots. Some evidence showed that in unsteamed krasnozem aggregates many small roots were infected but few large roots and no stems were girdled, suggesting that post-infection antagonism occurred within the root. Field isolations from dead and dying E. sieberi, E. obliqua and E. regnans showed that P. cinnamomi infected the stems and major roots.