Etiology and integrated control of common scab on seed potatoes in South Africa

Abstract

Common scab, as the name implies, is one of the most common and widespread diseases affecting production of potato in virtually all parts of the world where the crop is grown. In South Africa, the percentage of bags containing scab-infected seed tubers averages 32 %, with a corresponding rejection or decertification of the seed. The disease also reduces the cosmetic value of ware potatoes and, with the growing demand for blemish-free produce, increasingly results in the downgrading of consignments on the ware market. Control is mostly attempted by means of agrochemicals. Streptomyces scabiei is generally considered as the main cause of common scab, also in South Africa. However, various otherStreptomyces species are known to be associated with the disease, either parasitically or saprophytically, but no attempt has yet been made to determine if any of them occur in South Africa. The purpose of this study was to elucidate the etiology of common scab in the country and to evaluate some strategies that could be included in an integrated control programme for managing the disease. The results indicated the following: Isolation and characterisation of streptomycetes from symptomatic seed potato tubers from the five potato-production areas in South Africa worst affected by common scab, showed that the disease is caused by three phenotypically distinct Streptomyces groups, designated 1, 2 and 3, whereas a further three groups were saprophytically associated with scabby tubers. Streptomyces group 1, which corresponded morphologically to the description of S. scabiei, was by far the most common, representing 52 % of all and 82 % of the pathogenic isolates, and occurred in all the regions surveyed. The two other Streptomyces groups containing pathogenic isolates may represent atypical S. scabiei isolates, or could be separate species. Analysis for the production of thaxtomin A, an important pathogenicity factor, confirmed a positive correlation between the pathogenicity of isolates and their ability to produce thaxtomin A. However, 14 % of the pathogenic isolates did not produce thaxtomin A, whereas 6 % of the non-pathogenic isolates tested positively for production of the toxin. Another interesting observation was the ability of 15 % of both all and of the pathogenic isolates to grow at a pH of 4.0, which is atypical of S. scabiei. In the greenhouse, densities of S. scabiei were significantly lower in sand artificially infested with the pathogen and planted to triticale or cabbage than in fallow sand or sand planted to soybean, spinach, maize, pumpkin or sunflower, three months after planting of the crops. Survival in sand planted to rye and pea was also relatively low, albeit not significantly lower than the other crops. Roots of soybean, spinach, rye, sunflower, pea and cabbage were colonised significantly less by S. scabiei than those of pumpkin and maize. Cluster analysis separated the crops into two groups according to their effect on S. scabiei. The first group, comprising pumpkin, maize, soybean, sunflower and spinach, had little impact on survival of the pathogen. The second group, consisting of rye, triticale, pea, and cabbage, showed a distinct suppressive effect and on average supported only 0.23 % of the S. scabiei population sustained by the first group. Incorporation of fresh and dry cabbage residues at rates of 0.25 and 0.1 % (m/v) into soil naturally infested with S. scabiei reduced common scab in the greenhouse by a significant 32 and 41 %, respectively. Amendment of scab-infested soil in the field with dry residues of cabbage, cauliflower, broccoli and Brussels sprouts at 0.33 % (m/v) resulted in reduction in disease of approximately 90 %.