Studies on the expression of resistance of Coffea selections to Hemileia Vastatrix.
Physiological races of Hemileia vastatrix in southern Africa were identified. Prevalent races were I (2.2%), II (88.9%) and III (2.2%). Six samples could not be identified. Twelve biotypes of race II were distinguished. In some cases, the biotypes only occurred in specific regions. It was established, using fluorescence microscopy, that, in some cases, the percentages of germinated urediospores that did not form appressoria, appressorium formation over stomata, and aborted appressoria, were significantly different between susceptible and resistant selections of the host, and non-hosts. The sequence of events leading to successful infection was investigated using scanning electron microscopy (SEM). When a stoma is encountered by a germ tube tip a uniquely shaped appressorium forms over one end of the stomatal slit. A distinct appressorial foot is wedged within the stomatal vestibule. In coffee, a torpedo-shaped substomatal vesicle initial (SSVI) develops bilaterally from the apex of the infection wedge, While in bean, the infection wedge protrudes into the substomatal chamber. The substomatal vesicle (SSV), at 48 hours post inoculation (hpi) is anchor-shaped. Haustorial mother cells are formed on stubby primary infection hyphae which curve back onto subsidiary cells. No differences in appearance of these structures were noted between resistant and susceptible coffee selections. A much-branched mycelium ramifies through the intercellular spaces of the mesophyll cells 96hpi. In bean, the SSV began to collapse 48hpi. Bayfidan ® only slightly suppressed fungal development on the leaf surface. However, within susceptible tissue, this systemic fungicide had an effect on the morphology of the fungus. Extracellular material accumulated on the SSVI and the SSV. The SSV appeared swollen, and disruptions in the vesicle wall was noted. The discovery of teliospores on locally infected trees led to a SEM study on their structure, development and germination. Infection structure formation on the leaf surface, latent period, reaction score and urediosorus concentration differed between susceptible coffee leaves of different ages. Generally, mature leaves are more susceptible than very young or old leaves. A range of fungicides, mainly systemics, were tested in the field on naturally infected coffee trees. Various epidemiological and climatic factors influence rust development in the field. The role of these factors at the fungicide site and in commercial coffee-growing regions of southern Africa was evaluated.