The inoculum ecology of Botrytis cinerea in Rooibos nurseries

[摘要] ENGLISH ABSTRACT: Grey mould, caused by Botrytis cinerea, is the most important foliar disease of rooibosseedlings. Although the disease is primarily controlled with applications of fungicides, theimprovement of cultural methods of disease management should lessen this dependence onchemical control. Such improvements would, however, not be possible without knowledge ofthe inoculum sources and dispersal of the pathogen. The aim of this study was to investigatethe inoculum ecology of B. cinerea in rooibos nurseries in order to identify primary sources ofinoculum and to improve the environmentally friendly management of the disease.The study was conducted in four nurseries over two production seasons (March to July2003 and 2004). Levels of airborne inoculum of B. cinerea were monitored on a monthly basisinside and around the nurseries with spore traps. Samples of plant material and organic debriswere taken in the corresponding areas to determine the incidence of plant material infected bythe pathogen and the incidences of grey mould in the nurseries were recorded. Low numbersof B. cinerea colonies were observed on the spore traps. Similar levels of airborne inoculumwere 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 suchmaterials as potential sources of inoculum. Since patterns of airborne inoculum observed inthis study confirmed reports of the local dispersal of B. cinerea, the removal of possible hostsoutside 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, andtherefore has the potential to be used as a phenotypic marker. This marker can be used to gainknowledge 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 rooibosnurseries were assessed for resistance to iprodione at 1 and 3 μg/ml a.i. Some of the isolatesshowed resistance to iprodione at 1 μg/ml a.i. However, none of the isolates showed resistanceat 3 μg/ml a.i. iprodione. The initial incidence of dicarboximide-resistance at the nurseries wasslightly higher than expected. As the season progressed, the incidence of iprodione-resistantisolates decreased towards May, after which an increase was observed towards July. Arelatively high percentage of isolates collected outside the nurseries was found to be dicarboximide-resistant. Two of the nurseries had a significant higher incidence of resistantisolates 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 werefound in the incidence of resistant isolates sampled inside and outside the four nurseries. Thedata indicated the importance of organic debris and seed-borne infections in the survival anddispersal of dicarboximide-resistant isolates of the pathogen.With the current emphasis on organic agriculture the knowledge gained in this studypresents valuable possibilities of improving the cultural management of grey mould in rooibosnurseries.

[发布机构] Stellenbosch University