Factors associated with coniothyrium canker of Eucalyptus in South Africa

[摘要] English: English: In chapter one of this thesis, the literature pertaining to the genusConiothyrium and its importance in plant pathology, is reviewed. Specialattention is given to Coniothyrium species associated with Eucalyptus but thefocus is on Eucalyptus stem canker pathogen, C. zuluense. Coniothyriumzuluense is an important pathogen in South Africa and has, since itsdiscovery, become widespread throughout plantation areas of KwaZulu-Natal.The current means for reducing the impact of this disease is to plant diseaseresistant species and clones of Eucalyptus. It is evident from this review thatvery little information is available pertaining to the biology, reproductivesystem, or the population structure of C. zuluense. Such information isessential for managing the disease successfully in the future.The strategy currently used to reduce the impact of Coniothyrium canker inplantations is to deploy Eucalyptus species or clones that are resistant to thedisease. Considerable success has already been achieved in this regard,but the long-term durability of resistance is of concern. Results of the studyrepresented in chapter two showed that there is considerable variation incolony colour and pathogenicity of a large collection (344) of C. zuluenseisolates. Conidial morphology and growth requirements are, however, similarfor all isolates tested. The considerable variation in pathogenicity indicatesthat C. zuluense has been present in South Africa for an extended period oftime, or that virulence is changing rapidly due to strong directional selectionpressure.Results of the taxonomic and pathogenicity studies in chapter two, suggestthat the C. zuluense population is well established. In chapter three, thepopulation diversity of 108 C. zuluense isolates, differing in their pathogenicityto a susceptible Eucalyptus clone, was investigated using Amplified FragmentLength Polymorphism (AFLP) technology. Results indicated that the level ofgenetic diversity is relatively low, but higher than expected for an asexuallyreproducing pathogen. Genetic similarity values also indicated a significantpopulation differentiation between different plantation regions (subpopulations), suggesting that gene flow, together with selection, might beresponsible for most of the gene diversity. New epidemics would, therefore,not be as a result of the emergence of new aggressive strains, but wouldrather be due to the introduction of susceptible Eucalyptus species, togetherwith environmental conditions favouring disease development.A Coniothyrium species associated with similar symptoms to those associatedwith C. zuluense in South Africa was observed on E. camaldulensis inThailand in 1996. It was previously thought that C. zuluense was restricted toSouth Africa. In chapter four, I show using morphological and molecularcomparisons, as well as pathogenicity studies, that C. zuluense and theConiothyrium sp. from Thailand are the same organism. This is, thus, the firstrecord of this important Eucalyptus stem canker pathogen, C. zuluense,outside South Africa.Bacteria commonly exude from necrotic cankers on severely infectedEucalyptus clones in plantations. In chapter five, it was shown that bacteriaassociated with Coniothyrium canker in the field are species of the genusPantoea. These species were identified based on 16S rDNA sequence dataas P. ananatis pv. ananatis and a species closely related to P. stewartii subsp.stewartii. It was also shown that a synergistic interaction between C.zuluense and both Pantoea species exists. Inoculation studies, using bothPantoea species together with C. zuluense isolates, resulted in a significantincrease in pathogenicity as opposed to inoculations where the bacterial andfungal isolates were used alone. Future studies should consider the presenceor absence of both bacteria species in disease development in Thailand.During plant-pathogen interactions, pathogens are known to produce cell wall degradingenzymes, in particular pectin degrading enzymes.Polygalacturonase (PG) is the first enzyme produced during such interactionsand is known to be a determining factor in pathogenicity. Chapter six showedthat C. zuluense isolates and both Pantoea species, P. ananatis pv. ananatisand an unknown Pantoea sp., produce PG. Experimental assays show thatlevels of PG activity for both Pantoea spp. are significantly higher than thoseobtained for C. zuluense isolates. As PG is the first enzyme produced duringdisease development it is hypothesised that the two Pantoea species mightplay a significant role in the development of Coniothyrium canker. Productionof PG could also be used as an assay to evaluate pathogenicity in differentisolates of C. zuluense.Pathogen-produced cell wall-degrading enzymes play a key role in activatingplant defence responses. Most inducible defence responses are the result oftranscriptional activation of genes. Various plant resistance (R) genes, as wellas pathogenesis-related proteins, such as polygalacturonase inhibitingproteins (PGIPs), have been linked with resistance to various fungal andbacterial pathogens. In chapter seven, a partially sequenced resistance genefrom disease resistant E. grandis clone, TAG 5, was shown to be similar to agene associated with a disease resistance gene in Arabidopsis thaliana. Themost exciting aspect of this study was, however, the discovery of a shift inreading frame of this gene for the susceptible Eucalyptus clone, ZG 14. Thecomplete sequence of this gene should provide a more complete view of itsimportance in disease resistance. Screening for similar interruptions in theopen reading frame of various commercially available Eucalyptus clones couldsignificantly speed up breeding programmes aimed at producing improveddisease resistant clones.

[发布机构] University of the Free State