Fungal diversity of [Tylosema esculentum (Burch.) A.Schreib.] and T. fassoglense (Kotschy ex Schweinf). Torre & Hillc. seeds, leaves and tubers from South Africa

Abstract

The marama bean, encompassing both Tylosema esculentum ((Burch.) A.Schreib.) and Tylosema fassoglense ((Kotschy ex Schweinf.) Torre & Hillc.), has been identified as an orphan crop of southern Africa with a great potential for commercialisation. The potential of the marama bean to be developed into a valuable crop poses a question regarding possible biotic constraints, including fungal pathogens and mycotoxigenic fungi. This study investigates the mycoflora and mycotoxins associated with marama bean in South Africa, as well as the potential pathogenicity and mycotoxigenicity of various fungal species on the marama bean. Various fungal isolates were obtained from marama bean leaves, seeds and tubers collected from eight locations in South Africa, ranging from the Gauteng and North West to Limpopo and Mpumalanga provinces. The various fungal isolates were identified using a dual morphological and molecular approach. A total of 116 species spanning 27 families were molecularly identified, several of which have been previously reported on marama beans in Namibia, including various Alternaria, Epicoccum, Fusarium, Penicillium, Phoma and Rhizopus species. Other notable fungal genera isolated from the South African marama bean include Aspergillus, Lasiodiplodia, Neofusicoccum, Botryosphaeria, Chaetomium, Diaporthe, Bipolaris, Pestalotiopsis, Neopestalotiopsis, and Talaromyces. Multi-mycotoxin analysis was performed on the plant material as well as on isolated fungal cultures in vitro using maize patty cultures, focusing on Aspergillus, Fusarium and Penicillium. Several of the species analysed indicated the potential to produce aflatoxin iii B1 and fumonisins B1, B2 and B3 in vitro, however, no mycotoxins were detected in any of the collected plant material. Twenty of the fungal isolates (spanning thirteen different species) were selected to investigate the potential of the fungal species to be pathogenic to the marama bean, and subjected to detached leaf assays and further pathogenicity analysis. A disease symptom rating system was established to account for the various symptoms produced by the different fungal species. Black leaf spots, leaf lesions, stem lesions and leaf discoloration were the most prevalent symptoms observed. Lasiodiplodia pseudotheobromae, Botryosphaeria dothidea, Neofusicoccum parvum, Bipolaris sp., and Diaporthe rhusicola were the top five fungal species with the highest disease rating scores after the five-week period. However, Pestalotiopsis maculans was predicted to be pathogenic over a longer time period. Koch’s postulates confirmed that eleven of the thirteen fungal species were re-isolated after the pathogenicity trial, with the exceptions being Colletotrichum fructicola and the Fusarium solani species complex isolates. Multi-mycotoxin analysis of the seedlings indicated that zearalenone was detected in the Alternaria alternata, F. fujikuroi and F. oxysporum species complex treatments. In conclusion, an unexpectedly large diversity of fungal species was found to be associated with the marama bean in South Africa, several of which have the potential to be pathogenic to the marama bean and produce mycotoxins in vitro and in vivo. Further studies would be required to determine the extent of the pathogenicity of the various fungal species, as well as investigating under what conditions the pathogens would thrive and produce mycotoxins on the plants, thereby identifying future obstacles for marama bean cultivation and commercialisation.