Ganoderma Stem Rot Of Oil Palm: Epidemiology, Diversity And Pathogenicity

Abstract

Basal stem rot (BSR) of oil palm, caused by Ganoderma boninense, a white-rot basidiomycete is the main threat to oil palm cultivation in South East Asia. It can kill more than 80 percent of stands by the time they are halfway through their normal economic life. Attempts to control the disease have been hampered by the lack of understanding of the pathogen, how it spreads and infects and interacts with its host. Therefore this study was designed to better understand its epidemiology, diversity and pathogenicity. Reproducible root infection was obtained using small mycelium-infested rubber wood block inoculum and alternative infested wheat grain inoculum. This revealed that close contact between palm roots and the inoculum are important and that a smaller and alternative grain inoculum has potential for resistance screening. G. boninense was shown to remain viable for up to 4 years through the formation of melanised mycelium, which has implications for plantation management. Basidiospores clearly play a critical role in BSR infections. They were detected in abundance (ca. 3-5000/m3) in aerial samples from plantations in North, South, East and West of peninsular Malaysia. G. boninense was revealed by RAMs, SSRs and somatic incompatibility to be highly genetically diverse within and between those plantations. High genetic diversity would arise from sexual production of basidiospores as this study confirms G. boninense to be heterothallic and tetrapolar with multiple alleles at both mating type loci. Basidiospores applied to cut frond (petiole) surfaces were taken up into xylem vessels where they germinated and must have formed invasive heterokaryons, as on occasion colonisation progressed beyond the anatomical barrier of vessel end walls. Efficacy of detection by Ganoderma-selective medium, DNA and RNA are compared and discussed. Scanning electron microscopy revealed that spores also can adhere, germinate and apparently penetrate oil palm root surfaces, revealing that superficial roots might provide another route of entry. Putative pathogenicity factors are represented by production of wide array of extracellular cell wall degrading enzymes (CWDE) including the three main groups of ligninases and diverse polysaccharidases. All were produced during saprotrophic (on palm wood) and parasitic (root invasion) phases of the life cycle. Regulation in vitro was by nitrogen or carbon metabolite repression respectively and by aromatic or oligosaccharide inducers respectively. This study also showed production in vitro of peroxide and related reactive oxygen species-degrading enzymes, which could be linked both to lignin breakdown and protection from host defences. Preliminary evidence is presented for toxin production. The approaches and knowledge gained from the research described above culminated in the design of model systems, representing the life cycle of G. boninense, in order to facilitate a collaborative transcriptomic analysis. Long overdue, in depth understanding of G. boninense potentially will improve disease control through finding novel fungal targets and via effectoromics locating and assessing resistance genes in the oil palm.

Date of Award	22 Apr 2016
Original language	English
Awarding Institution	University of Bath
Sponsors	Ministry of Education Malaysia
Supervisor	Richard Cooper (Supervisor)