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Potato (Solanum tuberosum L.) is the worlds fourth most important food crop after wheat (Triticum aestimum, L.), maize (Zea maize L.) and rice (Oryza sativa L.), and provides a balanced source of starch, vitamins and minerals to many communities in the global village (Rowe, 1). Of all primary food crops, potatoes suffer the greatest losses due to disease (Agrios, 17). ). Of the various diseases, common scab is one of the oldest and was first recorded in South Africa by Pole Evans (106). Recent statistics indicate that 0.1 % of all seed potato tubers produced in South Africa are infected with common scab (http//www.potatoes.co.za). Between 16 and 001, the percentage of bags containing infected seed tubers averaged%, with a corresponding rejection or desertification of the seed. The disease also reduces the cosmetic value of ware potatoes and, with the growing consumer-demand for blemish-free produce, increasingly results in the downgrading of consignments on the ware market.
The etiology, epidemiology and control of common scab have been investigated extensively in many countries. However, as the disease has previously been regarded as of little economic significance in South Africa, it received only sporadic interest. Dippenaar (1) conducted a comprehensive environmental and control study of common scab, part of it in Wisconsin with a South African isolate of the pathogen. This was one of the first studies to show that scab can be reduced by increasing soil moisture during the early stages of tuber initiation, and also indicated an enhanced activity of bacteria in the moist soil. The study furthermore verified earlier results (Sanford, 16) that the scab organism and the disease are suppressed at pH lower than 5.0-5.5. Other relevant reports include those by Anonymous (175) confirming the existence of superficial, raised and deep-pitted lesions on common scab-infected tubers in South Africa, and Frean (175) indicating that common scab occurs locally in soils with a pH as low as 4.5. Slabbert et al. (14) reported that 45 % of 10 isolates of Streptomyces scabies from four of the 14 potato production areas in South Africa produced thaxtomin A, a phytotoxin commonly associated with pathogenicity of the common scab pathogen (King et al., 11).
Current research on common scab in South Africa focuses primarily on the screening of genotypes for resistance (Marais & Visser, 18; Nortje et al., 000) and recommendations accordingly. A seed certification scheme, which allows between 0 and 8 % infected tubers depending on the generation and class, is also in place (Republic of South Africa, 18). Chemicals that are presently registered in South Africa for the control of soilborne inoculum of the pathogen include chloropicrin, chloropicrin/ethylene dibromide and quintozene, whereas flusulfamide and mancozeb are registered for tuber treatment (Nel et al., 1).
Despite the above, various aspects related to common scab in South Africa remain unclear. For instance, although S. scabies is the main causal organism of the disease worldwide (Loria, 14), and has been shown to be prevalent in four of the 14 production areas in South Africa (Slabbert et al., 14), at least 1 other Streptomyces species are also associated with the disease (Lambert & Loria, 18; Faucher et al., 15), but no attempt has been made to determine if, and to what extent, any of them occur in the country. Also, the rotation crops recommended by Frean (175) for the control of common scab are not necessarily economically and agronomically feasible in rotation systems with potato, or suitable for the management of other pathogens and pests. Lastly, although the chemicals registered against common scab are effective, the cost of particularly soil applications is extremely high and control not always consistent (De Klerk & Engelbrecht, 16). Growing consumer-resistance to, and the negative environmental impact of, synthetic pesticides furthermore necessitate the consideration of alternative control strategies. The purpose of this study therefore was to (i) elucidate the identity of the common scab pathogen in selected potato production areas, (ii) identify rotation crops compatible with potato growing, and (iii) investigate the potential of biofumigation, i.e. rotation with glucosinolate-containing brassicaceous crops or incorporation of their residues into soil (Brown & Morra, 17), as an affordable and environmentally-acceptable alternative to agrochemicals for the control of common scab.
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REFERENCES
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