Alternaria blight (AB) of sweet potato has been reported from tropical and sub-tropical regions of Africa, Asia and South America with the causal agent as A. bataticola. A taxonomic revision of Alternaria sect. Porri showed that another three Alternaria species, A. argyroxiphii, A. ipomoeae, and A. neoipomoeae, previously thought to be A. bataticola, also occur on sweet potato in South Africa (Woudenberg et al. 2014). The reference isolate used in the first report of A. bataticola from South Africa (Narayanin et al. 2010) was shown to be A. ipomoeae (Woudenberg et al. 2014).
During a study to investigate all the fungi associated with AB symptoms from naturally infected fields, fungi were characterised based on morphology and multilocus sequence analysis and representatives from each species evaluated for pathogenicity against two known AB susceptible sweet potato cultivars, Resisto and W-119. The same fungal species were consistently retrieved from typical and atypical AB symptoms from natural infected fields in Gauteng and KwaZulu-Natal Provinces of South Africa. Alternaria argyroxiphii, A. bataticola, A. ipomoeae, and A. neoipomoeae were strongly pathogenic to both cultivars, while Peyronellaea americana and Phoma tropica were only pathogenic towards Resisto, but not W-119.
Alternaria alternata, although consistently isolated from sweet potato, was non-pathogenic on both cultivars tested. This is the first report of A. argyroxiphii, Pe. americana and Ph. tropica causing disease on sweet potato. A. neoipomoeae is currently only reported from South Africa. Additional work will be required to establish the true worldwide distribution of A. argyroxiphii, A. bataticola, A. ipomoeae, and A. neoipomoeae on sweet potato, as the correct identification of Alternaria isolates from sweet potato requires multilocus phylogenetic analyses.
Among fungicides tested under field conditions, the mixture azoxystrobin-difenoconazole was the most effective in reducing AB intensity. Fungicides pyraclostrobin-boscalid, unizeb, azoxystrobin-chlorothalonil and cymoxanil-mancozeb were also effective against the disease (Kandalo et al., 2016). In field screening trials, cultivars 199062-1, Mvuvhelo, Impilo, Ndou and Monate had the lowest General Disease Index (GDI) value, and was the most tolerant to AB.
Contact: Dr Mariette Truter
References:
Kandolo, S.D., Thompson, A.H., Calitz, F.J., Laurie, S.M., Truter, M., Van der Waals, J.E. & Aveling, T.A.S. 2016. Field tolerance of selected varieties to and fungicide efficacy against Alternaria blight of sweet potato. African Crop Science Journal 24(3): 235-243. http://dx.doi.org/10.4314/acsj.v24i3.2
Kandolo, S.D., Aveling, T., Van der Waals, J.E., Truter, M. & Laurie, S.M. 2018. Effects of wetness duration, inoculum concentration and temperature on the development of Alternaria blight on transplants of sweet potato. Acta Horticulturae 1204: 211-217. https://doi.org/10.17660/ActaHortic.2018.1204.27.
Narayanin, C.D., Thompson, A.H. and Slabbert, M.M. 2010. First report of Alternaria blight of sweet potato caused by Alternaria bataticola in South Africa. African Plant Protection 16: 7-9.
Woudenberg, J.H.C., Truter, M., Groenewald, J.Z. & Crous P.W. 2014. Large-spored Alternaria pathogens in section Porri disentangled. Studies in Mycology 79: 1-47. http://dx.doi.org/10.1016/j.simyco.2014.07.003.
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