Aspects of interaction between environmental factors, Eucalyptus spp., and Pythiaceous fungi, especially Phytophthora cinnamomi

Abstract

The root rot fungus Phytophthora cinnamomi is associated with decline and death of native plant communities in various parts of Australia, and in some of these areas species of Eucalyptus are affected. It is generally recognised that species of the eucalypt sub-genus Monocalyptus are severely affected by the fungus and that many species of the sub-genus Symphyomyrtus are tolerant of it. The sub-genus Monocalyptus is also reported to be intolerant of waterlogging and it has been suggested that sensitivity of the Monocalyptus species to both P.cinnamomi and waterlogging may be related in some way. Periodical waterlogging of soil would encourage activity of Pythiaceous fungi, including P.cinnamomi and other species of Phytophthora and Pythium known to be pathogenic to eucalypts. Thus a contributory factor to intolerance of waterlogging could be susceptibility to Pythiaceae. This aspect formed part of the present study. The response of a range of Eucalyptus spp. to P.cinnamomi was studied by exposing roots of young seedlings to zoospores of the fungus. Zoospores accumulated, often in large numbers, on the zone of elongation and also on cut ends and wounds. Provided inoculated roots remained in water, sporangia of P.cinnamomi formed rapidly, sometimes in about 24 hours, indicating successful infection of all the species studied. Rating scales were used to assess both zoospore accumulation and sporangial production and it was found that there were significant differences between eucalypt sub-genera and species. In general, species of eucalypt known to be susceptible to P-cinnamomi showed greater zoospore accumulation and faster and greater sporangial production than did the tolerant species. It is suggested that susceptible species contribute to increase in disease firstly by having root exudates which are highly attractive to P.cinnamomi zoospores and secondly by favouring rapid and substantial inoculum build-up. There was almost one hundred percent mortality with all eucalypt species tested when seedlings were less than 1 week old, whereas with seedlings older than 2 weeks some of the species had developed tolerance to P.cinnamomi. When seedlings between 2 and 3 weeks old were inoculated with zoospores of P.cinnanomi, the subsequent mortality closely reflected the known relative susceptibility of the species, in that most individuals of the susceptible species died compared with few individuals of the tolerant species. Inoculation of seedlings of this age could thus provide a realistic, simple, and rapid method of assessing susceptibility to P.cinnamomi. Potted seedlings of five species of eucalypt, two of them Monocalyptus and the others Symphyomyrtus, were inoculated with a total of eight Phytophthora isolates from five species. The response of the eucalypts to these Phytophthora isolates, and to Pythium afertile, which occurred as a not-uncommon contaminant throughout the study, suggests that the species of Eucalyptus which are highly susceptible to P.cinnamomi are also more susceptible to other Pythiaceae than are species tolerant of P.cinnamomi. The effect of waterlogging on Eucalyptus spp. was studied in the glasshouse under rigid conditions designed to exclude Pythiaceous fungi. All of the eucalypt species initially responded adversely to waterlogging, with death of most of the root system. Tolerant species, however, overcame this setback by the formation of adventitious aerenchymatous roots. Eucalyptus robusta (Symphyomyrtus) is one of the few eucalypts known to occur in swampy habitats and in the case of this species, aerenchymatous roots effectively replaced the original root system killed by waterlogging. The Monocalyptus species are normally regarded to be intolerant of waterlogging but in the studies reported in this Thesis some seedlings of Monocalyptus showed slight signs of aerenchymatous root growth. The present studies have confirmed that Monocalyptus are both susceptible to Pythiaceae and sensitive to waterlogging and that, in general, Symphyomyrtus are tolerant of both factors. A swamp inhabiting species such as E.robusta could be expected to be highly tolerant of both Pythiaceae and waterlogging, as is indeed the case, as a result of natural selection over a long period of time. By contrast, species without the potential to tolerate both Pythiaceae and waterlogging would not have the ability to become established on wet sites and would not necessarily develop their potential for resistance to one or other factor. For example, E.delegatensis and E.obliqua are both highly susceptible to Pythiaceae and hence would not be expected to successfully colonise waterlogged sites even though the present studies suggest that these species may have the ability, albeit undeveloped, to tolerate waterlogging. On the other hand, E.microcorys, although highly tolerant of Pythiaceae, would be unlikely to colonise wet sites because it is extremely sensitive to waterlogging. These studies indicate that there is not an obligate association between susceptibility to Pythiaceae and sensitivity to waterlogging even though in general it appears that there is such an association. It has been suggested that P.cinnamomi may have been partly responsible for the natural distribution of eucalypts because species occurring on moister sites tend to be tolerant of the fungus. As the eucalypts susceptible to P.cinnamomi are also susceptible to other Pythiaceae, it is possible that the group of pathogens as a whole, rather than one species has played a role in eucalypt distribution. Little is known of the reasons for the different reactions of eucalypts to P.cinnamomi. The young seedling inoculation studies suggest avenues of study which may lead to an understanding of these species differences.