Faculty of Natural and Agricultural Sciences
School of Biological Sciences
Department of Microbiology and Plant Pathology
Selected Highlights from Research Findings
South Africa has, until recently, escaped major outbreaks of cholera. This situation has, however, changed dramatically since August 2000 when the first cases of the present epidemic were reported in Kwa-Zulu Natal. By February 2001 this epidemic had not shown any signs of abating and more than 30 000 cases had been reported with a number of cases also detected in other parts of the country. Because cholera is spread primarily through drinking water the effectiveness of solar pasteurisation as a water disinfection method suitable for the production of potable water in rural areas was researched. Water samples were collected from Tshwane river and the water was then heated by solar energy to different temperatures. Indicators of microbiological pollution including heterotrophic bacteria, total coliforms, faecal coliforms and coliphages were used to monitor the disinfection process. The results indicated that solar pasteurisation can effectively decontaminate (disinfect) water cost-effectively.
Contact person: Prof TE Cloete.
Research has identified microbial resistance to nonoxidising biocides. Very little information is furthermore available on the biodegradability of these compounds in natural water systems. This makes them hazardous from an environmental point of view. Chlorine is the most widely used oxidizing biocide, with its own limitations. An environmentally sensible alternative to both these classes of biocides is needed. Electrochemically activated water (ECA) solutions may provide such an alternative. Water of varying mineralisation is passed through an electrochemical cell, the specific design of which, permits the harnessing of two distinct and electrically opposite streams of activated water. Aside from its distinctive attributes, the negatively charged anti-oxidant solution (Catholyte) can also be channeled back into the anode chamber, thereby modulating the quality of the positively charged oxidant solution (Anolyte) that is produced. The anti-microbial activity of the current ECA technology has been confirmed in this study. Electrochemically activated water (ECA) is less toxic, less volatile, easier to handle, compatible with other water treatment chemicals, effective against biofilms and generates no by-products compared to currently used biocides.
Contact person: Prof TE Cloete.
Citrus Black Spot (CBS). Further development and improvement of a rapid diagnostic technique to distinguish between the citrus black spot pathogen, Guignardia citricarpa and the omnipresent endophyte, G. mangiferae was highly successful. Previously species-specific primers, of the primer set Citric 1 and Camel 2 in conjunction with the ITS 4 primer yielded PCR amplicons of approximately 580 bp and 430 bp for G. citricarpa and G. mangiferae respectively. What is unique in the research done during 2002 is that the technique has been reduced in time to a 6h assay and that leaf litter and fruit can be used directly. This new technique will enable researchers to make predictions in terms of the inoculum load of G. citricarpa in orchards and can be used in disease forecasting systems. This technique is also highly specific and a BLAST search revealed no matches with other simliar pathogens i.e. Colletotrichum gloeosporioides, the most common contaminant in black spot lesions. We are, therefore, able to distinguish G. citricarpa and G. mangiferae unequivocally using a PCR based technique, thereby shortening the time needed to test export consignments. By improving the technique citrus growers can prevent incorrect rejection of export consignments, retain their competitive edge in the European markets and have access into the United States of America, which requires citrus exports to be CBS free, due to sanitary, and phytosanitary issues.
In response to the EU feedback on the CBS Pest Risk Assessment report specific critical issues had to be addressed for the industry. Although ascospores can be regarded as the major source of infection, we confirmed that pycnidiospores also play an important role in the spread of the disease. The pathogen could be re-isolated from the leaves consistently up to four months from pycnidiospore infection trials and could be re-isolated from 6-month-old leaves. The most significant breakthrough during 2002 was the development of a new Quest inoculum monitor (QIM), which was used in the laboratory to determine the presence of G. citricarpa inoculum on plant material. Information thus obtained can now be applied to estimate the theoretical inoculum load available to cause new infections by plant pathogens.
Contact person: Prof L Korsten.
In South Africa it is estimated that root diseases of citrus are causing an annual loss of about R60 million in foreign exchange. Mechanisms of resistance against root pathogens were investigated. The role of total phenolic content in rootstock resistance was studied. Using the Folin-Ciocalteu reagent, a colorimetric method was established to determine extracted phenolic concentration. It was found to be a rapid, yet sensitive technique for screening citrus rootstocks for phenolic content. Total phenolic concentration increased upon infection by Phytophthora nicotianae and it was higher in tolerant Swingle, Macrophylla and Troyer rootstocks in comparison to susceptible Rough lemon and Volckameriana rootstocks. Fosetyl-Al treatment further elevated phenolic content that provides evidence that elevation of phenolic levels is involved in the mechanism of action of Fosetyl-Al in control of Phytophthora root rot.
The hydroponic industry in South Africa is suffering great losses due to root diseases, particularly Pythium wilt and root rot in the major hydroponic crops. The etiology of Pythium wilt and root rot was studied in selected commercial hydroponic systems. The following new Pythium-host associations in South Africa emerged from the study: P. acanthicum on strawberry; P. aphanidermatum on parsley; P. coloratum and P. irregulare on lettuce; P. perplexum on tomato; P. group F on basil, celery, chive, cucumber, endive, fennel, lettuce, strawberry and a number of oriental herbs; P. group G on lettuce; P. group HS on cucumber and lettuce; P. group P on strawberry and P. group T on lettuce and strawberry. New Pythium-host associations not previously reported internationally include P. group F on celery, fennel, sorrel and endive and P. irregulare as well as P groups F, HS and T on lettuce. Positive results have been obtained in controlling Pythium by means of biological control agents and water sanitizers.
Contact person: Dr N Labuschagne.
Cowpea — commonly known as black-eyed beans and possibly one of the most ancient food sources — is indigenous to southern Africa, probably originating in Botswana. It is a heat-loving, drought-tolerant crop with a high protein content and lower soil fertility requirements than many other crops. Cowpea has a wide variety of uses including hay, grazing, green manure and as a vegetable; it is an ideal alternative crop, especially in southern Africa and other developing areas where extensive food production should be cost effective, with the minimal input from growers. Cowpeas are grown intercropped with other crops such as maize and sorghum by subsistence farmers.
Cowpea seeds have a high protein content, often replacing meat protein eaten by people living in poor rural areas. Storage conditions are often inadequate in these areas and the seeds become infected by fungi and mycotoxins, such as aflatoxins and fumonisins. The production of fumonisins in cowpea seeds by Fusarium proliferatum was reported for the first time. This is also the first report of this fungus producing fumonisins in legumes. Since these mycotoxins cause adverse health effects in animals and humans, this is an important discovery. Research is underway to try and reduce the levels of mycotoxins in cowpea seed by improving storiage conditions and facilities.
Contact person: Prof TAS Aveling.
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