Biocontrol of Botrytis allii Munn the Causal Agent of Neck Rot, the Post Harvest Disease in Onion, by use of a New Iranian Isolate of Streptomyces

Abstract

Problem statement: Soil actinomycetes particularly Streptomyces spp. have showed antagonistic activity against wide range of plant pathogens. In the recent decades they have attracted high interests as biocontrol agents. Onion neck rot or gray mold caused by Botrytis allii have imposed economic post harvest damages to onion bulbs and decreased its storage durability and market value. Approach: To investigate for biocontrol means against the pathogen, antagonistic activity of 50 isolates of soil Actinomycetes were assayed through agar disk method and dual culture bioassays. Active isolates were exposed to chloroform for detection of antibiotic. Minimum Inhibitory Concentration (MIC) value and solubility of active crude extract in organic solvents were determined for Streptomyces isolate No. 347 which showed a unique and stable property of inhibiting Botrytis allii. To investigate the antagonistic effect of Streptomyces isolate No 347 on control of onion gray mold, four different treatments were tested by means of Tukey HSD test. Results: From the tested isolates, 13 showed anti gray mold activities. Exposure of active isolates to chloroform revealed that Streptomyces isolates No 347, 263 and 350 retained their antifungal activities. The active metabolite(s) of Streptomyces isolate No 347 was polar, soluble in H₂O but insoluble in chloroform and methanol. MIC of the crude was determined as 0.05 mg mL^-1 against B. allii. Stability of the active crude in distilled water at room temperature (12-30°C) was about 6 months. Statistical studies indicated that Streptomyces isolates No 347 can decrease losses of neck rot with significant level (p<0.05). Conclusion: The future goals include investigation of the antifungal genes in active isolates as candidates for genetic engineering of onion for increased tolerance against B. allii.