SENSITIVITY OF MYCOSPHAERELLA-FIJIENSIS, CAUSAL AGENT OF BLACK SIGATOKA OF BANANA, TO PROPICONAZOLE

One hundred monoascosporic isolates of Mycosphaerella fijiensis were c ollected in February and November 1994 from each of two banana (Musa s pp.) plantations in Costa Rica. Locations at San Pablo and Coopecariar i had been sprayed with propiconazole for the past 7 years to control black Sigatoka. One hundred monoascosporic isolates from a third locat ion, San Carlos, with no history of fungicide use, also were tested fo r sensitivity to propiconazole. Fifty percent effective concentration (EC50) values were calculated for individual isolates by regressing th e relative inhibition of colony growth against the natural logarithm o f the fungicide concentration. In the February sample, the mean EC50 v alues for San Pablo and Coopecariari populations were 0.06 and 0.05 mu g a.i. ml(-1), respectively, which were not statistically different ( P = 0.05). The mean EC50 value of the population at San Carlos was 0.0 08 mu g a.i. ml(-1), which was significantly lower (P = 0.001) than th e mean EC50 values obtained at San Pablo and Coopecariari. Frequency d istributions of EC50 values of isolates from the three populations col lected in February showed that 80% of isolates from San Pablo and Coop ecariari had EC50 values greater than the highest EC50 value from San Carlos, indicating a significant shift in reduced sensitivity to propi conazole. Isolates collected in November 1994, after eight treatments of propiconazole at San Pablo and Coopecariari, showed a significant i ncrease in mean EC50 values compared with the means observed in Februa ry. The high proportion of isolates with reduced sensitivity to propic onazole may account for the unsatisfactory control of black Sigatoka b etween 1992 and 1993 in the two banana plantations at San Pablo and Co opecariari.