Management of late blight of potato and tomato by antibiotics produced by streptomyces species
Waswa, R M
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This was a study undertaken to formulate antibiotics produced by Streptomyces sp. and test their efficacy in control oflate blight disease of tomato and potato. It was also meant to establish mammalian toxicity of these antibiotics. The study was carried out in the laboratory and in the greenhouse. The isolates used were tentatively identified as Streptomyces sahachiroi ( isolate CS35) and Streptomyces hygrostaticus or S. mitakaensis ( isolate 28P). They produced antibiotics on soybean media with 2% glucose incubated for 7 days on a shaker at 120 Lp.m at room temperature. Isolates 28P and CS35 produced mean inhibitory zones of diameters 18.85 mm and 17.60 mm respectively on Pythium sp. after 48 hours of incubation by paper disc method oftesting antagonism. Partial purification of antibiotic culture filtrates increased activity by 48.9% and 34.4% for 28P and CS35 respectively. Concentration of the antibiotics to 50% by removal of water caused an increase in activity by 48% and 41% for the antibiotics from 28P and CS35 isolates respectively. Filtrates concentrated through freeze-drying dissolved in propylene glycol with minimal sedimentation. Solubility of the filtrates from 28P and CS35 in propylene glycol was Ig/6ml and l g/Sml respectively. These formulated products were free flowing, easy to apply on the crop as a spray and still retained the antibiotic activity. Application of the formulated filtrates at 10,000ppm and 5000ppm was found to reduce the incidence and severity of late blight of potato and tomato in the green house. The higher concentration gave better effects though phytotoxicity seen as wilting from the leaf margins was also observed. Reduction of disease incidence in tomato plants treated with formulated antibiotics of isolate 28P and CS35 at 5000ppm was 34% and 33% respectively. In potato, the reduction in blight incidence was 55% and 52% by the two isolates respectively. Reduction in the incidence of late blight was comparable though not as effective as Ridomil (metalaxyl), which gave a reduction of 69% and 58% in potato and tomato respectively, and Milraz (cyanoacetamide) with a reduction of 60% and 52%. Both products are acyalanines used in the market for control of the disease. In mammalian toxicity studies, it was found that the LDso for culture filtrates from antibiotics of 28P and CS35 were 3.94g and 3.27g/kg body weight of mice respectively, classified as slightly toxic according to Loomis (1974) scale of practical classification of toxicities.