Studies of camellia flower blight (Ciborinia camelliae Kohn) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science (Plant Pathology) at Massey University, Palmerston North, New Zealand

Abstract

Camellias are popular ornamental plants and the most serious pathogen of this plant is camellia flower blight, caused by the fungal pathogen Ciborinia camelliae Kohn. Ascospores of this fungus attack the flowers, turning them brown, rendering infected flowers unattractive. Little is known about the pathogen and control measures are not particularly effective. In this thesis, various aspects of the pathogen's basic and molecular biology and interaction with host species were studied. Surveys of the distribution and spread of C. camelliae within New Zealand determined that the pathogen was present in most regions of the North Island, and north and east coasts of the South Island. Over the distances and time involved, it appeared that the disease was spreading mainly by windborne ascospores rather than human transfer. Sclerotia were germinated out of season to increase the period during which ascospores were available for infection work. Greatest germination was achieved at low temperatures (5°C-10°C) in 24 h darkness. Isolate-specific primers were designed to the ribosomal DNA Internal Transcribed Spacer region to detect the pathogen in planta and distinguish between New Zealand isolates of C. camelliae and other fungal pathogens. Phylogenetic analysis of the ITS region with other Ciborinia, Sclerotinia and Botrytis species showed that C. camelliae was more closely related to S. sclerotiorum than other Ciborinia species. Two inoculation techniques for infecting Camellia petals with ascospores of C. camelliae were developed and tested. Inoculation using airborne ascospores in a settling chamber was a simple and quick method for testing large numbers of species for resistance. Inoculation of ascospores in suspension produced qualitative data, but was more time consuming. Of the four mechanisms of resistance tested, levels of aluminium hyperaccumulation and the presence of phenolic compounds did not correlate with resistance in Camellia species. The large uptake of aluminium, however, did indicate that Camellia species would be good plants for phytoremediation of acid soils. Some resistant species were found to have cell wall modifications and/or lignification of cell walls in response to C. camelliae infection and chitinase activity was found in most resistant Camellia species tested. Further research into these latter two mechanisms is recommended and indicates that the development of resistant Camellia cultivars is possible.