Abstract
Abstract : Since 2012, brassica production in South Africa has been threatened by the polerovirus associated disease, termed Brassica stunting disease (BSD), with high incidences reported on cabbage crops. Poleroviruses (genus: Polerovirus, family: Luteoviridae) are aphid (Hemiptera: Aphididae) transmitted viral plant pathogens that infect economically important crops throughout the world. Cabbage crops affected by BSD are characterized by the flattening and or purpling of the leaves, small or no head formation, vascular discoloration, severe stunting and poor root development. Currently, farmers attempt to control BSD using cultural practices and spraying a broad spectrum of insecticides with the aim of controlling insect vectors. With cabbages being an economically important crop that is grown by both small and large-scale farmers and considered as a staple food by resource poor populations, the development of effective control strategies for BSD is essential for sustainable production of cabbage crops in SA. This study was undertaken to investigate the BSD-vector-pathosystem on three levels, i.e. (i) to determine the identity and abundance of aphid species found in brassica crop production areas in South Africa, (ii) to identify potential polerovirus-vectoring aphid species and (iii) to evaluate insecticides to control BSD by targeting aphids infesting cabbage crops. The information provided by these investigations can be used for the development of knowledge-based BSD management strategies by the agricultural industry. Considering the absence of information regarding the aphid species diversity found on brassica crops in South Africa, a three-year survey was conducted in an open field-brassica producing region located in Brits (North West Province, 2016 - 2018) and Groblersdal (Limpopo province, 2016 – 2017). The diversity and abundance of aphids alighting in various brassica crops, i.e. cabbage, cauliflower, broccoli, purple cabbage and Swiss chard as a non-brassica crop, were monitored using yellow bucket traps and the captured aphids were characterized using morphological characteristics and by DNA barcoding. In total, 36 different aphid taxa were identified in both regions, with Aphis spp. and Myzus persicae predominantly occurring in all crops. Although the aphid species diversity monitored over the three years remained relatively consistent, the aphid activity varied considerably with either half or double the number of winged aphids counted in consecutive years. This fluctuation in winged aphid numbers could not be explained by weather patterns (temperature or rainfall) for that area over the corresponding three years. As such, the best current explanation for the bi-annual pattern in aphid numbers is the predator/parasitoid-cycle effect that has been reported in other areas around the world, where the presence of high wingless aphid populations is associated with the presence of predators, ii resulting in a decline in aphid numbers in the subsequent year. This theory was supported by the presence of high numbers of parasitized aphids in 2017, followed by drastically reduced numbers of colonizing aphids in 2018. The close association between aphid transmitted poleroviruses and the presence of BSD symptoms in brassica crops prompted further investigation of this polerovirus- aphid vector relationship (Chapter 2). As such, the 36-aphid taxa collected over the three-year period were screened for the presence of poleroviruses. Seven aphid species were excluded from this analysis due to low numbers of samples. From the remaining 29 taxa, 19 tested positive for the presence of a polerovirus, including Acyrthosiphon pisum, Aphis gossypii (Aphis spp.), M. persicae, Rhopalosiphum maidis, Rhopalosiphum padi and Sitobion avenae, which have previously been reported to vector poleroviruses and provided evidence for 13 potentially new polerovirus vectoring species. To further clarify the role of each of these polerovirus positive aphid vector in the BSD epidemiology, their transmission ability and efficiency should be investigated in future...
M.Sc. (Biochemistry)