Fusarium wilt (FW) and surface rot on the tubers are caused by the fungal pathogen Fusarium oxysporum. The pathogen is widespread in sweet potato production areas in South Africa (Thompson 2004; Thompson et al. 2011) and has been reported worldwide. Symptoms associated with FW includes wilting of the plant, leaf yellowing and browning of the vascular tissues in the lower stems (Thompson et al. 2011). Fusarium surface rot consists of circular, dark brown to pale, firm, dry and superficial depressed lesions on the storage roots. Infection from surface rot do not penetrate deep into the storage roots, unlike root rot, caused by Fusarium solani, that usually infect the inner root tissue as well. Although FW have been reported to occur throughout South Africa, the disease occurrence and severity varies greatly between seasons.
During a recent study, Nkosi (2020) characterised the isolates of the Fusarium oxysporum species complex (FOSC) from sweet potato in different production regions in Gauteng, Limpopo, North West, Mpumalanga and Western Cape Provinces and revealed two additional FOSC, namely F. oxysporum f. sp. tuberosi and F. oxysporum f. sp. vanillae associated with Fusarium wilt on sweet potato. Symptoms on sweet potato plants include wilting, leaf yellowing and browning of vascular tissues in the lower stems. The study has contributed to a new body of knowledge and understanding of FOSC associated with FW of sweet potato in South Africa. The identification of new formae speciales associated with FW of sweet potato in South Africa can have an impact on South African agriculture as it should be considered in determining risk evaluation approaches, control measures for farmers and assist breeders in making informed choices on which F. oxysporum formae speciales associated with FW of sweet potato to use when screening during resistance breeding to FW.
We further are involved in developing an integrated disease model for FW in sweet potato. Non-pathogenic F. oxysporum and Trichoderma species, known for their ability to reduce the incidence of Fusarium wilt in other crops, are screened for suppression in disease development. In addition, we are investigating the effect of plant growth promoting rhizobacteria (PGPR) on FW disease development. As the cost of chemicals are high as well as not environmental friendly, a disease management strategy involving biological control is a feasible alternative. These results will increase sustainable sweet potato production in areas in South Africa where the disease has devastated fields.
Contact: Ms Zama Nkosi and Dr Rene Sutherland.
References:
Nkosi, B.Z. 2020. Characterisation of Fusarium oxysporum species complex associated with Fusarium wilt of sweet potato in South Africa. Master of Technology Dissertation. University of South Africa. South Africa.
Thompson, A.H. 2004. Fungal diseases. In: J.G. Niederwieser (Ed.). Guide to sweet potato production in South Africa Pretoria. ARC-Roodeplaat, Pretoria pp. 81-84.
Thompson, A.H., Narayanin, C.D., Smith, M.F. & Slabbert, M.M. 2011. A disease survey of Fusarium wilt and Alternaria blight on sweet potato in South Africa. Crop Protection 30: 1409-1413.
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