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• W57952554 abstract "Macadamia is the only native Australian food plant in large-scale cultivation in Australia and overseas. Annual Australian macadamia production is worth AUD$150 million, but the husk spot disease caused by the fungus Pseudocercospora macadamiae can result in production losses worth AUD$10 million annually without adequate disease control. Yield losses in orchards result from premature abscission of diseased fruit. Paucity of knowledge of the biology of P. macadamiae has led to difficulty in husk spot management in orchards. An understanding of the factors associated with the growth and spread of the pathogen and infection of the fruit pericarp (husk) will provide the basis for effective disease control. Specifically, this study provides information on: (i) identification and culture of P. macadamiae in vitro; (ii) the process of infection, colonisation and sporulation of P. macadamiae on fruit; (iii) timing of infection of fruit by P. macadamiae and climatic effects on the pathogen and disease; (iv) the source of P. macadamiae inoculum in orchards and methods to characterise husk spot susceptibility in the field; and (v) the level of genetic diversity among P. macadamiae populations in relation to the presence and the possible influence a teleomorphic stage may have on the husk spot disease cycle in orchards. In culture P. macadamiae can be identified and cultured with optimal growth and sporulation on potato dextrose agar (PDA). Colonies are slow growing (typically <1mm/day), light to medium grey in colour, margins entire, with light brown and septate hyphae. Identification by culture morphology were supplemented by comparison to DNA sequences of the internal transcribed spacer (ITS1 & ITS2) and 5.8S gene region that have been generated and submitted to the GenBank sequence database (Chapter 2). The process of infection, colonisation and sporulation of P. macadamiae on macadamia fruit was determined using scanning electron microscopy and fluorescent light microscopy. P. macadamiae infects the pericarp through open stomata within 20 h without specialised infection structures such as appressoria. Colonisation of the entire thickness of the pericarp occurs intercellularly without specialised feeding structures such as haustoria. Conidia and conidiophores are produced on substomatal and protuberant subepidermal stromata that forcefully erupt through the epidermis and cuticle of the pericarp about 4 months after infection. These findings indicate that infection in the field can occur rapidly, but visible symptoms on the pericarp may not appear until several months after infection (Chapter 3). Results from field trials showed that fruit are most susceptible to infection after fruit set, when they are 3 mm to 10mm in diameter. Fruit exposed to infection at these stages of development had up to a 5-fold increase in husk spot incidence and a 9-fold increase in premature abscission, compared to unexposed or protected fruit. More severe husk spot epidemics coincided with wet climatic conditions with mean temperatures close to 26°C; conditions found to be optimal for the germination of conidia and growth of germ tubes of P. macadamiae in vitro (Chapter 4). Field observations revealed a significant relationship between husk spot incidence and the presence of fruit that failed to abscise from the canopy (sticktights), indicating sticktights are a significant source of P. macadamiae inoculum in orchards. In addition, newly formed diseased sticktights continually produced viable conidia of P. macadamiae for 30 months. In order to assess different macadamia cultivars for resistance or susceptibility, sticktights were used as a source of inoculum in field trials. The results indicate that macadamia cultivars vary in terms of resistance to infection by P. macadamiae, resistance to premature abscission, and/or their ability to spatially escape inoculum by being less prone to sticktight formation. Hence, diseased sticktights can serve as a reliable assay for evaluating macadamia germplasm for husk spot resistance (Chapter 5). Polymerase chain reaction – restriction fragment length polymorphism (PCR-RFLP) revealed low levels of genetic diversity in P. macadamiae populations from each of the three major commercial production regions in Australia. Six different PCR-RFLP genotypes were identified, with the same single genotype dominating in all the regions. The overall low levels of genetic diversity of P. macadamiae populations in commercial macadamia orchards suggests a greater contribution of asexual conidia, and a low, if any, contribution of sexual spores, to the husk spot disease cycle (Chapter 6). The findings from this study have greatly enhanced existing knowledge of the husk spot disease. Having deduced the husk spot disease cycle provides essential information for designing appropriate management options such as fungicide application and cultivar selection. However, further research is needed to clarify: (i) the nature of the pathogen’s latency between infection and symptom expression; (ii) the biochemistry of symptom expression, including possible roles of hormones and toxins; (iii) the mechanisms of induction of premature abscission; and (iv) methods for the rapid screening of juvenile germplasm for resistance to husk spot. Further gains in disease management could be achieved by addressing these future research aims." @default.
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• W57952554 date "2011-05-01" @default.
• W57952554 modified "2023-09-27" @default.
• W57952554 title "Husk Spot Disease of Macadamia" @default.
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