Variation in sensitivity of Magnaporthe oryzae isolates from Korea to edifenphos and iprobenfos

In this study, the sensitivity of Magnaporthe oryzae isolates from different geographic areas in Korea to two phosphorothiolate (PTL) fungicides, edifenphos and iprobenfos, was determined. A total of 1080 M. oryzae isolates were collected from rice-cultivating fields in 11 locations throughout Korea in 1997 and 1998. The minimum inhibitory concentrations (MICs) of edifenphos (20 μg a.i. ml −1 ) and iprobenfos (55 μg a.i. ml −1 ) against seven representative sensitive isolates of M. oryzae were determined from the dose-response curves, and the MICs were then used as single discriminatory concentrations for the detection of fungicide resistance. Of the 1080 tested isolates, the isolates less sensitive to edifenphos and iprobenfos were 57 and 84%, respectively, and 53% of the isolates were less sensitive to both fungicides. Approximately 11% of all tested isolates showed no growth at the MICs of the tested fungicides. Isolates with a relative mycelial growth of 0.05 to edifenphos occurred at the greatest frequency (33%); isolates with a relative growth of 0.3 to iprobenfos occurred at the greatest frequency (23%). The frequency distribution of isolates sensitive to either edifenphos or iprobenfos varied with geographic location. The two-sample median test yielded high medians in the Kwangju and Chonju isolates for edifenphos, and in the Kwangju, Donghae, and Yangyang isolates for iprobenfos, but low medians in the Kangnung isolates for edifenphos and in the Kangnung, Gwangju, and Chunchon isolates for iprobenfos, respectively, as compared to the isolates from the other locations. Analysis of Spearman's rank correlation test showed a significantly positive relationship ( r = 0.490, P M. oryzae populations collected from different geographic areas in Korea, and isolates less sensitive to these fungicides exhibit cross-resistance with each other.