Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616
One hundred monoascosporic isolates of Mycosphaerella fijiensis were collected in February and November 1994 from each of two banana (Musa spp.) plantations in Costa Rica. Locations at San Pablo and Coopecariari had been sprayed with propiconazole for the past 7 years to control black Sigatoka. One hundred monoascosporic isolates from a third location, San Carlos, with no history of fungicide use, also were tested for sensitivity to propiconazole. Fifty percent effective concentration (EC50) values were calculated for individual isolates by regressing the relative inhibition of colony growth against the natural logarithm of the fungicide concentration. In the February sample, the mean EC50 values for San Pablo and Coopecariari populations were 0.06 and 0.05 μg a.i. ml-1, respectively, which were not statistically different (P = 0.05). The mean EC50 value of the population at San Carlos was 0.008 μg a.i. ml-1, which was significantly lower (P = 0.001) than the mean EC50 values obtained at San Pablo and Coopecariari. Frequency distributions of EC50 values of isolates from the three populations collected in February showed that 80% of isolates from San Pablo and Coopecariari had EC50 values greater than the highest EC50 value from San Carlos, indicating a significant shift in reduced sensitivity to propiconazole. Isolates collected in November 1994, after eight treatments of propiconazole at San Pablo and Coopecariari, showed a significant increase in mean EC50 values compared with the means observed in February. The high proportion of isolates with reduced sensitivity to propiconazole may account for the unsatisfactory control of black Sigatoka between 1992 and 1993 in the two banana plantations at San Pablo and Coopecariari.