Blast disease caused by Pyricularia grisea Sacc. is the most serious rice disease in North of Iran because of its epidemic development each year and the subsequent yield loss it incurs in local cultivars. Several fungicides have been tested in the past for its control. Benomyl was used for several years, but Edifenphos application has become prevalent since 1976. In 1997-98, infected leaves and panicles were sampled from rice fields of various Localions in Guilan province. In Lab. causal fungus was isolated from each sample on PDA medium using the single- sopre technique. For evaluation of resistance of isotates to fungicides, PDA medium was amended separately with 0.1, 1,2 and 4ppin of active ingredient (a.i.) of Benomyl and 1, 10, 20, 30, 40, ... and l4Oppm (a.i.) of Edifenphos respectively. Inhibition of the mycelial growth of each isolate on treated PDA plate was determined 5,7 and 14 days after inoculation of a mycelial disc of 7 mm diameter. The EC5O’s of these fungicides were found for all isolates. The results showed that among 50 isolates, one isolate was resistant to Benomyl at 4ppm, while 2 isolates were resistant to Edifenphos at 140 ppm and 8 isolates to 80 ppm. EC5O’s varied from 0.1 to 0.6 and 0.5 to 30 ppm for Benomyl and Edifenphos, respectively. Isolates were classified into 4 categories based on growth in different concentrations of Edifenphos as: 1. sensitive, 66% of isolates (l0ppm) 2. moderately sensitive, 18% of isolates (4oppm)3. moderately resistant, 12% of isolates (80ppm) and 4. resistant, 4% of isolates (1 4oppm).
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