Population biology and impact of Austropuccinia psidii in South Africa

Abstract

The myrtle rust fungus, Austropuccinia psidii (Sphaerophragmiaceae, Pucciniales), has a wide global distribution, having been recorded from five continents. It has a known host range of over 500 species within the family Myrtaceae and causes disease of young leaves, shoots and flowers. In its most severe manifestation, it leads to tree stunting and mortality. Austropuccinia psidii is one of the most economically important pathogens of species of Eucalyptus commercially planted in Brazil, and has been described as the biggest threat to native Myrtaceae in Australia. Austropuccinia psidii was first discovered on the African continent in 2013 in KwaZulu-Natal, South Africa, on a non-native host, Myrtus communis. It has since been reported from two additional provinces and five host species in the country. A recent study showed that the genotype of A. psidii in South Africa is different from the pandemic genotype which has spread internationally. The aims of this dissertation were to: (i) expand studies on the host range and distribution of A. psidii in South Africa, (ii) investigate the population genetic diversity of A. psidii within the country and the potential presence of other genotypes of the pathogen in South Africa, (iii) determine the threat of A. psidii to the commercial eucalypt industry in the country. Samples of A. psidii from New Zealand and Singapore were received by the Forestry and Agricultural Biotechnology Institute (FABI) between 2016 and 2017, which were included in the research studies presented in this dissertation. Chapter one is a literature review that provides a general background to the research presented in this dissertation. It elaborates on the distribution, host range and population diversity of A. psidii globally. Furthermore, a complete list of known strains of A. psidii and their hosts is included to assist in elucidating the complex global population diversity of this pathogen. Chapter two of this dissertation investigated the current distribution and host range of A. psidii in South Africa as well as the population genetic diversity of the pathogen in New Zealand, Singapore and South Africa. Extensive surveys were done in South Africa in 2016 across natural ecosystems, nurseries and gardens. A large number of samples were collected from seedlings, shrubs and trees exhibiting myrtle rust symptoms in South Africa. Samples from New Zealand and Singapore were included in the study and the genotypic identity of the samples from the three countries described. In Chapter three we determined the potential threat of A. psidii to the commercial eucalypt industry. We screened hybrid clones of Eucalyptus currently grown by South African forestry companies for susceptibility to A. psidii. A total of 13 genotypes of E. grandis × E. urophylla (GU), E. urophylla × E. grandis (UG), and E. grandis × E. nitens (GN) were screened for susceptibility to A. psidii in a controlled environment. In addition to the above research questions, I authored and co-authored three publications that overlapped between the research presented in this dissertation and work at the TPCP disease clinic. These ancillary publications include a new disease report from Singapore, a paper that provides new knowledge on sexual reproduction by A. psidii, and a manuscript currently under review that identifies the strain of A. psidii in New Zealand. These publications and submitted manuscript are included at the end of the dissertation. The information presented in this dissertation provides new insights into the host and geographic distribution of an invasive pathogen in South Africa. It highlights the possible future threats of the pathogen to both native and non-native species of Myrtaceae in the country, raising concerns regarding the limited attention given to the pathogen. We emphasise the need for quarantine, which can prevent the introduction potentially more aggressive strains of A. psidii into South Africa. Through this work, we hope that more attention will be given to plant quarantine and that preventative actions will be improved to reduce the impact of such pathogens in South Africa.