Pathogens associated with plantation tree diseases in Zambia

Abstract

Species of Eucalyptus and Pinus are of great importance to Zambia, providing material for the mines, saw milling companies, as well as reducing the pressure on native tree species. The plantation forestry industry plays an important role in the economy of the country. Despite this, very little is known regarding the diseases of these trees in Zambia. In this dissertation I provided information on the first comprehensive survey of plantation forestry diseases in Zambia in the last 30 years. Information presented in the dissertation is even more crucial since it includes detailed species level identification of the causal agents of the identified diseases, as well as information on the population genetics of the most important stem canker pathogen currently. By looking at related native tree species in the Myrtales, I also provided some insight into possible future pathogens of Eucalyptus spp. in Zambia and the rest of the region.

The dissertation started with review of the available literature on diseases of plantation forestry trees in Zambia. Very limited information was available for the review as many disease reports were not published in widely available sources. From the published literature, it was clear that the majority of research conducted on plantation trees had focused on the identification of diseases based only on symptoms and morphology of pathogens. Many of these reports are therefore, inaccurate and deserved fresh study. The review also provided information on factors contributing to disease development in Zambia and on possible solutions to plantation diseases in the country.

The first part of the dissertation focused on the identification of pathogens of Eucalyptus and Pinus spp. in Zambia. The identification was based on the analyses of morphological characteristics and DNA sequence data. More than 11 fungal pathogens, most of them from Eucalyptus spp. were identified. Studies presented in this thesis have increased the geographic distribution of Kirramyces zuluensis, K. epicoccoides, Chrysoporthe austroafricana, Cylindrocladium pauciramosum, Diplodia pinea, Lasiodiplodia theobromae, Neofusicoccum parvum, N. luteum and Botryosphaeria dothidea on the African continent. These studies have further laid a foundation for future programmes that aim to formulate disease control measures.

The fungus K. zuluensis includes the most serious fungal pathogen causing canker disease on E. grandis and E. cloeziana in Zambia. Nine microsatellite primer pairs developed specifically for this fungus in a previous study were used to determine the genetic diversity of K. zuluensis in Zambia. The study presented in this thesis revealed that K. zuluensis was most likely introduced into Zambia from South Africa. This is consistent with the fact that Eucalyptus plantations where K. zuluensis was observed were established using seed from South Africa.

Two novel Chrysoporthe species occurring on Syzygium and Eucalyptus trees in Zambia were described based on morphological characteristics and DNA sequence data. These fungi were described as Chr. zambiensis sp. nov. and Chr. syzygiicola sp. nov. In this study, pathogenicity tests showed that both Chr. syzygiicola and Chr. zambiensis are pathogenic to Eucalyptus, with Chr. syzygiicola being considerably more virulent than strain CMW2113 from South Africa, which in studies of the previous 15 years had been the most virulent isolate known. Thus, great effort should be made to ensure that these pathogens are not introduced into new areas where Myrtales, or possibly hosts in other families could be highly susceptible to infection.

The final part of the thesis focused on the Botryosphaeriaceae occurring on native Myrtales in Zambia. Fungi collected were studied using both morphological and molecular techniques. Four species including B. dothidea, N.n luteum, L. theobromae and a new species N. granulosum sp. nov., were collected. Pathogenicity trials revealed that all species, except B. dothidea, was pathogenic to Eucalyptus sp. and significantly less so on Syzygium cordatum. The most virulent species collected was L. theobromae, which in Chapter two was associated with canker of E. grandis, emphasizing the wide host range of this pathogen. Results from the final part of this thesis show that native Myrtales host several species of Botryosphaeriaceae, most of which are pathogenic to the introduced Eucalyptus, and considerably less so on a native African host. This suggests a possible co-evolution between the fungi and the native hosts, and not between them and the introduced host.

The knowledge generated from studies that make up this dissertation provides an important step in the process of developing a forest health program in Zambia. New techniques for the identification of tree pathogens have been acquired and it is hoped that these can further be applied in the Zambian forestry situation in the future. The government and private forestry companies should form partnerships to establish sustained tree breeding and selection programmes that should include disease resistance as a key feature. Furthermore, strict quarantine measures when importing germ plasm, such as seed, should be established to prevent the introduction of new and important diseases into the country. It is essential for government and forestry companies in Zambia to invest significantly in programmes and activities aimed to understand the biology, population genetics and epidemiology of important plantation pathogens. I hope studies in this thesis will stimulate further research on plantation forestry diseases in Zambia and promote the field of forest pathology in the country.