Biology and control of Sclerotium rolfsii on groundnut in South Africa

Information gleaned from this study has stimulated new questions regarding the genetic structure, population dynamics and control of Sclerotium rolfsii. This work can hopefully serve as a foundation for the further study of this fungus in South Africa and elsewhere in the world. The population of the fungus in South Africa was studied and the 121 isolates collected were found to belong to 13 different mycelial compatibility groups (MCG), occurring on various crop and other plants in different geographic localities. Vague associations between MCG, host plant and geographical area could be observed. The genetic fingerprinting technique of amplified fragment length polymorph isms was found to be an effective tool to distinguish between MCGs of S. rolfsii. This technique has never before been used for this purpose, even though other genetic techniques have been used in other studies. No previous study, however, was able to distinguish isolates within MCGs, as this study was able to do. The hypothesis was also made that isolates within a MCG are not clonally derived, based on the differences observed between isolates in the same MCG, the different host plants and the different geographic areas from which the isolates originated. The pathogenicity of MCGs of S. rolfsii to groundnut appeared to be significantly different from group to group, but the results of field trials suggested that isolates were not fixed into specific MCGs, even though a controlled glasshouse experiment did not support this. The phenomenon of MCG switching has never been reported for this fungus, even though it has been recorded for other fungi. Certain isolates also appeared to be more pathogenic to groundnut than others. Mycelial compatibility group E consistently emerged as a virulent pathogen of groundnut. No groups were found to e specific to any of three other crop plants tested. The control of the fungus in groundnut fields was found to be possible with the use of an inversion plough in trials conducted over two consecutive seasons. A fungicide was identified that suppressed S. folfsii but not Trichoderma harzianum, a naturally occurring biological antagonist of the pathogen in the soil. The application of the fungicide difenoconazole at 12.5% of the registered concentration for groundnuts did not reduce disease in the field. The application of T. harzianum and difenoconazole in the early stages of groundnut plant development had no effect on yield or disease incidence. levels of disease in the field were found to increase with a decreased plant density. This observation was attributed to increased soil erosion in the field as a result of the sparser plant density. Sclerotium roltsii is a virulent pathogen of numerous crop plants in South Africa and this study has provided some answers to questions regarding the biology and control of this fungus. However, much still has to be discovered and may future scientists find some inspiration from what was done here. Key words: Sclerotium faItsii, southern stem rot, population, mycelial compatibility groups, AFlP analysis, pathogenicity, control, inversion plough, integrated pest management.