Factors Affecting Thaxtomin A Production by Streptomyces scabies in Egypt

Document Type : Original Article

Authors

1 Plant Pathol. Dept., Fac. of Agric., Damanhour Univ.

2 Plant Pathol. Dept., Fac. of Agric., Alexandria Univ.

Abstract

Twenty isolates of Streptomyces scabies were isolated from scab lesions on potato tubers collected from three different regions in Behera Governorate during the 2009-2010 growing season. The isolates of S. scabies recovered were totally monomorphic for the morphological and the biochemical characteristics investigated. All isolates were non motile, gram positive, exhibited grey colonies, spiral spore chain and were melanin producers. Meantime the isolates showed positive reaction for catalase activity, starch hydrolysis, casein hydrolysis and gelatine liquefaction. Also, all isolates were tolerant and exhibited growth at NaCl 5, 6 & 7% and at 20, 25 & 30°C at pH 7 & 9. None of the tested isolates were able to grow at pH 5. More variations, however, were revealed for the pathogenicity of twenty isolates. Five out of twenty investigated isolates were found to be highly virulent. The rest of tested isolates were moderately virulent. Spunta was the most tolerant potato cultivar, while cvs. Mondial and Hermis were more susceptible. The highly virulent S. scabies (isolate ST5) was a highly producer of Thaxtomin A, while the moderately virulent isolate (ST10) was poorly toxin producer (1.26 mg/ml medium). The optimal conditions for Thaxtomin A production were 28ºC, pH 8 and 1% cellobiose in medium. The amino acids, i.e. tryptophan, phenylalanine and tyrosine exhibited an in vitro inhibitory effect on the toxin production in the amended oatmeal bran broth medium with highest inhibitory effect was recorded for tryptophan.

Keywords

Egyptian Journal of Phytopathology
Volume 40, Issue 2
December 2012
Pages 131-147

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