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Basidiospores : their influence on our thinking regarding management of Ganoderma root-rot disease in tropical hardwood plantation crops

Page, DE 2020 , 'Basidiospores : their influence on our thinking regarding management of Ganoderma root-rot disease in tropical hardwood plantation crops', PhD thesis, University of Tasmania.

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Forest products are a major source of national revenue in Indonesia. Nearly 33% (69.9 million ha) of the country’s land area includes production forests. Several Acacia species are planted mainly for pulp production. By 2014 the total area of hardwood plantations established in this country was reported as approximately 1.2 M hectares with 600,000 ha of Acacia mangium. However due to susceptibility to two diseases (Ganoderma philippii and Ceratocystis manginecans) the A. mangium has been replaced by less susceptible eucalypts.
By the early 2000s it was acknowledged that G. philippii, a basidiomycete root rot pathogen, was causing serious damage to A. mangium. In Central Sumatra, by the third rotation, several sites were found to be no longer capable of providing a commercial yield at harvest. Root diseases caused by basidiomycetes are among the most studied diseases of trees in the world, in particular for the genera Armillaria and Heterobasidion. Much can be inferred from those studies for G. phillipii but very little has been substantiated. The ability to trade-off concepts such as disease risk with the cost, effectiveness and deployment of various management strategies will depend on how well the pathosystem is understood and the context imposed by the particular cropping system and commercial environment. The main objective of the present study was to define fundamental aspects of G. philippii disease biology and aetiology (such as sexuality, gene flow and capacity for genetic variation, population dynamics) in order to ascertain the importance of basidiospores in disease dispersal and incidence.
The study comprised three experimental investigations of; (1) G. philippii basidiospore germination, (2) G. philippii breeding systems, and (3) population structures of G. philippii in acacia plantations. Two other saprophytic Ganoderma species that are frequently found fruiting in Acacia and Eucalyptus plantations were included in the first two studies, for comparison with G. philippii.
Spores were collected opportunistically and non-destructively from fresh basidiocarps emanating from the stems of mature-aged trees. Microscopic observations and empirical mathematical modelling enabled a description of the germination dynamics of the basidiospores of G. philippii, G. mastoporum and G. australe as influenced by factors such as spore density, simple or complex carbohydrate availability in the nutrient media, biotic carbon sources such as sawdust and/or ethanol media additives, and incubation temperature. This study is the first to describe simple, reliable protocols for in-vitro germination of G. philippii basidiospores. A standardised method for optimum axenic spore germination was described for each species. The germination dynamics of the basidiospores as observed in vitro were discussed in relation to their ecology.
The sexuality and mating systems of G. philippii, G. mastoporum, and G. australe were determined. Observation of somatic interactions between monokaryotic and dikaryotic mycelia using genetically defined material confirmed the incompatibility mechanisms which operate to delimit individuals of these species in a population. Ten monokaryotic siblings; harvested as single spore isolates from each of two G. philippii, one G. mastoporum and two G. australe basidiocarps were paired in every possible combination. The macro- and micro-morphology and nuclear behaviour of interacting individuals were recorded at 7-day intervals. Ratios of ~1:4 compatible to incompatible crosses were observed between monokaryotic G. philippii, G. mastoporum, and G. australe siblings are consistent with ratios found for other Ganoderma species, indicating a tetrapolar mating system with alleles for heterothallism at two loci.
The incidence and severity of red root disease increases with rotation. This may be linked to changes in the mode of dispersal of G. philippii, the balance between vegetative and spore dissemination. Preliminary somatic incompatibility tests indicated that genetic diversity increased with successive rotations implying an increasing, not decreasing role of spores. It was decided to further explore this observation with microsatellite analysis.
Primers targeting microsatellite loci were screened against three isolates of G. philippii from geographically separate locations. All but two of the primer pairs successfully amplified G. philippii DNA, but the majority amplified a single product that did not vary among the test isolates. Eight of the primer sets were polymorphic and heterozygous in at least one of the test isolates.
Populations were genetically distinct with high levels of inbreeding and clonal spread to adjacent trees increased after the first rotation. Despite the high levels of genetic diversity seen at all sampling scales, migration rates appear low. Measures to reduce the under-ground spread of the pathogen as well as methods to prevent the initiation of new infections from basidiospores will be needed to reduce the incidence of root rot in Acacia mangium plantations.
The final chapter of the thesis reviews and discusses the significance of the above results in relation the hypothesis tested i.e. red root-rot disease, caused by G. philippii, predominantly occurs and spreads in tropical acacia plantations as a function of vegetative (clonal) infection pathways. The contribution of new information generated in this thesis to possible management strategies are discussed and recommendations are given for further research.

Item Type: Thesis - PhD
Authors/Creators:Page, DE
Keywords: Root rot, Ganoderma philippii, Fungal mating systems
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Copyright 2019 the author

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