Effect of Phytophthora cinnamomi on Microbial Populations Associated With the Roots of Forest Flora

Abstract

The effect of Phytophthora cinnamomi Rands on the microbial populations associated with plant roots of three forest communities was investigated by the dilution plate method. Rhizosphere and rhizoplane populations for susceptible and tolerant hosts and soil microflora were compared for diseased and unaffected areas. Populations of P. cinnamomi were determined concurrently from the same root and soil material. Samples were collected every three months over a 2-year period from sites for which rainfall, soil water potentials and soil temperature were continuously recorded. Independently of the pathogen, microbial numbers from soil, rhizosphere and rhizoplane from the dry shrubby sclerophyll forest of the Brisbane Ranges were significantly lower than those from the savannah woodlands of Wilson's Promontory and one-tenth of those from wetter sclerophyll forests of Narbethong. Microbial populations from soil and roots of the three forest communities exhibited statistically significant seasonal periodicity with maxima in the autumn-winter period and minima in the spring-summer seasons. This contrasts with P. cinnamomi populations, which decreased almost to zero in winter and became maximal in spring-summer. The susceptible roots examined were associated with smaller populations of microorganisms, particularly aerobic bacteria and aerobic sporing bacteria in both rhizosphere and rhizoplane compared with tolerant roots. Disease due to P. cinnamomi did not affect the relative microbial numbers occurring in soil, rhizosphere and rhizoplane, but was always associated with a reduction in numbers of actinomycetes on all sites. Severe disease in the Brisbane Ranges was associated with a significant increase in total microorganisms, particularly aerobic bacteria, in the rhizoplane and rhizosphere of both susceptible and tolerant plants, coincident with the production of decayed and dead root tissue. Anaerobic bacteria increased and actinomycetes decreased in number from all rhizospheres sampled from diseased sites. Finally in areas of severe disease in the Brisbane Ranges there was a subsequent highly significant reduction in numbers of soil microorganisms associated with a decrease of plant cover.