Abstract
Abstract Some agricultural fields of Razevi and North khorasan provinces were sampled for identification and resistance to metalaxyl of Pythium ultimum species isolates. Isolation was done between years of 2007 to 2009. Van der plaats- niterink key to species of Pythium and specific ITS region of rDNA were used for morphological and molecular identification respectively. Micrometric characteristics of isolates sexual and asexual organs corresponded to van der Plaats-niterink criterion. Fourteen isolates were randomly selected among fourthy identified isolates of Pythium ultimum for determination of sensitivity to metalaxyl. Concentration of metalaxyl which causes fifty percents growth inhibitions (EC50) obtained using logistic function; I=KC (erC). That varied between 0.0228-0.044 µg ml-1 i.e. the results indicate that all isolates were sensitive to metalaxyl, but there was significantly differences among isolates in response to metalaxyl. Keywords: Pythium ultimum, metalaxyl sensitivity, cucumber, Khorasan
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