Isolates of Sclerotium rolfsii, the causal organism of stem rot or southern blight of groundnut, can be placed in mycelial compatibility groups (MCGs) based on hyphal interactions between isolates. The aim of this study was to determine whether amplified fragment length polymorphism (AFLP) analysis was a suitable technique to assess genetic variability between isolates and MCGs of S. rolfsii. For preliminary genetic analysis, 10 isolates were selected from each of two MCGs and compared with each other using the restriction enzymes EcoRI and MseI and 4 primer pairs. The number of polymorphisms ranged from 10 to 36 per primer combination, with an average of 22.5. AFLP analysis clearly showed genotypic differences (22%) among MCGs B and C, with a maximum variation of 6.41% between any two isolates per group using four primer pairs. Certain isolates could not be distinguished from each other. A more in-depth study of 10 isolates from MCG B, using 8 additional primer pairs, showed small genetic differences (maximum of 4.2% and minimum of 0.2%) between isolates. These results suggested that DNA could be pooled for comparison of MCGs. Pooled DNA from isolates within groups using 20 primer pairs confirmed differences between 9 MCGs. This technique effectively differentiated MCGs of S. rolfsii from each other and also detected differences between isolates within a single MCG.

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Sclerotium rolfsii Sacc. causes disease of numerous crop plants worldwide, including groundnuts. Control of this pathogen is difficult as it produces sclerotia which overwinter in the soil to emerge as inoculum and cause disease the following season. Various chemical, biological and cultural control strategies have been suggested and implemented, some of which have reduced disease incidence in the field. No studies have yet been undertaken in South Africa to control this disease on groundnut, either chemically, biologically or by cultural practices. In this study, several strategies were investigated for the control of S. rolfsii on groundnuts. Difenoconazole was identified as a fungicide that could possibly be applied in combination with Trichoderma harzianum, a biological antagonist of S. rolfsii, above carbendazim and flusilazole, and chlorothalonil. Difenoconazole significantly reduced the growth rate of S. rolfsii but not of T. harzianum. The cultivation of infected fields with an inversion plough significantly reduced infection of groundnuts by S. rolfsii and also improved the quality of the produce, while yield was not increased. Lower plant density increased the incidence of disease in an infected field, and is therefore not considered to be a viable form of cultural control.

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