Studies on seedling mortality associated with Eucalyptus regnans forest regeneration in southern Tasmania
thesisposted on 2023-05-26, 20:01 authored by Michael LaceyMichael Lacey
Burning of logging slash resulted in apparent sterilisation of the soil surface to a depth of about 2cm and also increased soil surface pH, which in localised areas reached sufficiently high values to be toxic to seedlings. In vitro studies showed that high pH conditions caused seedling mortality in E. regnans although the lethal effect of a transient elevated pH was reduced at temperatures sufficiently low to delay germination. The pH above which detrimental effects were expected was in the range 8.4 to 9.0. Suppression of seedling growth was observed in field soil with glasshouse studies showing that this suppression was partly overcome by prior slash-burning or by addition of N and P fertilisers. Availability of soil moisture appeared to be the main factor limiting seedling establishment in the field. Slash-burning was associated with increased seedling emergence but also with increased mortality rates compared to an unbumt control. Although fungi were found to be associated with the majority of dying seedlings from the field, their involvement in seedling pathology was unclear. Most common among these isolates was a species of Phoma, representative isolates of which caused cliscolouration and stunting of E. regnans seedlings in vitro, but were not otherwise markedly pathogenic. Species of Alternaria and Cladosporium were also commonly found on seedlings and probably present as saprophytes. Pythiaceous fungi were isolated from seedlings but pathogenicity of these was not examined. Rhizoctonia solani (pectic zymogram groups ZG5, ZG7 and ZG10) were isolated from seedlings that had died in field soil under glasshouse conditions. Representatives of R. solani groups ZG5 and ZG7 were pathogenic to E. regnans in vitro‚Äö however Rhizoctonia was not commonly isolated from the field. Three bacterial rhizosphere isolates (two Pseudomonas fluorescens and Bacillus circulans) and a fungus (Epicoccum purpurascens) from E. regnans were evaluated as fungal antagonists. Three of these were shown to reduce mortality (due to R. solani ZG7) of seedlings in vitro following inoculation in the vicinity of seeds, with E. purpurascens providing 100% protection after 30 days (compared with 22% survival of controls). Surprisingly, B. circulans and E. purpurascens also increased seed germination rates. Of the antagonists, only E. purpurascens improved seedling establishment in a glasshouse trial, both in the presence and absence of R. solani ZG7. However neither treatment with E. purpurascens nor the fungicide Raxil (applied to seed in combination with Mancozeb) resulted in improvement in seedling establishment in a small field trial.
Rights statementCopyright 1995 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Thesis (M.Agr.Sc.)--University of Tasmania, 1995. Includes bibliographical references (leaves 113-125)