A total of 231 different soil Streptomyces isolates were recovered from 16 different locations in North Jordan. They were assessed for their phytotoxic activity on seeds of cucumber (Cucumis sativus L.) and ryegrass (Lolium perenne L.) placed adjacent to a 2 cm wide Streptomyces culture strips grown at 28C° for 3 weeks on starch casein nitrate (SCN) agar. Phytotoxicity was ascertained on the basis of suppressed seed germination, discoloration of the root tip, reduced root and the shoot growth and eventual death of the root. Twenty one of the isolates exhibited adverse effect against growth of germinated cucumber seeds, germination and growth of ryegrass seeds. Using filter paper bioassay method, culture filtrate from the SCN broth of the isolate R9; identified as Streptomyces aburaviensis, significantly inhibited seed germination, radicle and shoot growth of ryegrass, reduced radicle and shoot growth of cucumber and suppressed the shoot growth of milk thistle (Silybum marianum L.). Also, culture filtrate from the glucose-peptone-molasses (GPM) broth diluted (1:1) with sterilized distilled water caused complete inhibition of seed germination of redroot pigweed (Amaranthus retroflexus L.). Dichloromethane extracted fraction of S. aburaviensis (strain R9) culture filtrate from GPM broth completely inhibited seed germination of ryegrass when applied at doses of 3 and 5 mg of dry weight, and the seedling growth of cucumber and milk thistle was severely reduced by the same doses.
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