Browsing by Author "Spies, Christoffel, F. J. (Christoffel Frederik Jakobus)"

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    The inoculum ecology of Botrytis cinerea in Rooibos nurseries
    (Stellenbosch : Stellenbosch University, 2005-04) Spies, Christoffel, F. J. (Christoffel Frederik Jakobus); Holz, G.; Lamprecht, Sandra, C. ; Stellenbosch University. Faculty of Agrisciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Grey mould, caused by Botrytis cinerea, is the most important foliar disease of rooibos seedlings. Although the disease is primarily controlled with applications of fungicides, the improvement of cultural methods of disease management should lessen this dependence on chemical control. Such improvements would, however, not be possible without knowledge of the inoculum sources and dispersal of the pathogen. The aim of this study was to investigate the inoculum ecology of B. cinerea in rooibos nurseries in order to identify primary sources of inoculum and to improve the environmentally friendly management of the disease. The study was conducted in four nurseries over two production seasons (March to July 2003 and 2004). Levels of airborne inoculum of B. cinerea were monitored on a monthly basis inside and around the nurseries with spore traps. Samples of plant material and organic debris were taken in the corresponding areas to determine the incidence of plant material infected by the pathogen and the incidences of grey mould in the nurseries were recorded. Low numbers of B. cinerea colonies were observed on the spore traps. Similar levels of airborne inoculum were observed inside and around the nurseries. The incidence of plant material yielding B. cinerea was higher outside the nurseries than inside, indicating the importance of such materials as potential sources of inoculum. Since patterns of airborne inoculum observed in this study confirmed reports of the local dispersal of B. cinerea, the removal of possible hosts outside the nurseries could aid in the management of grey mould in rooibos nurseries. Resistance to dicarboximide fungicides is a genetically stable trait in B. cinerea, and therefore has the potential to be used as a phenotypic marker. This marker can be used to gain knowledge on the dispersal of B. cinerea inoculum inside and outside rooibos nurseries. Isolates of B. cinerea collected from the air and from plant material in and around four rooibos nurseries were assessed for resistance to iprodione at 1 and 3 μg/ml a.i. Some of the isolates showed resistance to iprodione at 1 μg/ml a.i. However, none of the isolates showed resistance at 3 μg/ml a.i. iprodione. The initial incidence of dicarboximide-resistance at the nurseries was slightly higher than expected. As the season progressed, the incidence of iprodione-resistant isolates decreased towards May, after which an increase was observed towards July. A relatively high percentage of isolates collected outside the nurseries was found to be dicarboximide-resistant. Two of the nurseries had a significant higher incidence of resistant isolates on plant material collected inside, than on plant material collected outside the nursery. However, when looking at resistance levels of airborne isolates, no significant differences were found in the incidence of resistant isolates sampled inside and outside the four nurseries. The data indicated the importance of organic debris and seed-borne infections in the survival and dispersal of dicarboximide-resistant isolates of the pathogen. With the current emphasis on organic agriculture the knowledge gained in this study presents valuable possibilities of improving the cultural management of grey mould in rooibos nurseries.