Tactri, Wufeng, Taiwan

Background and objectives
Mango anthracnose, caused by Collectotrichum gloeosporioides, is the major limiting factor in mango cultivation in Taiwan. Prochloraz, an imidazole fungicide, was introduced into Taiwan as control agent since 1985 under the trade name 'Sporgon' that premixed with manganese chloride. The numbers of prochloraz application were between one to nine sprays among different mango growers. In recent years, occasional reports indicated that the control efficiacy of the fungicide seemed dwindle. In order to understand the control efficacy of this fungicide in fields, a survey of the sensitivity of anthracnose fungus to the fungicide was conducted throughout Yujing areas that covers 4,000ha mango plantations in southern Taiwan.

Materials and methods
All cultures were single spore isolated from either leaves or young fruits by using a paraquat dip method [1] to induce sporulation or from matured fruits that already showed lesions. Totally forty locations were surveyed that covered major mango cultivation areas and a cluster of mango trees located in Taiwan Agricultural Research Institute in central Taiwan that never received any fungicide spray was served as baseline population. At least 30 isolates were collected randomly from each orchard. Global positioning satellite was used for re-locate sampling orchards. The fungicide, prochloraz was gifted by Schering Chemical Ltd. (Taiwan) with a.i. 95%. Quantitative data of sensitivities were collected based on the inhibition of mycelial growth [2]. In addition to the dose-response relationships on the relative growth of mycelium, the effect of the fungicide on the germination, appressorium formation and the production of the extracellular matrix were also studied.

Results and conclusions
Results from locations around Yujing areas indicated that no significant resistant were detected although the fungicide sensitivity were some fluctuations among different locations. The results also indicated that a gradual shift of fungicide tolerance seemed occur in commercial plantations that sprayed prochloraz. The EC50 of the baseline population towards prochloraz was 0.015+/-0.008 mg/ml and 0.077+/-0.037 mg/ml in one commercial orchards that receieved four sprays of prochloraz annually. Significant test indicated that these two populations were different; however, the IC50 was only about five fold higher in prochloraz sprayed orchard than the baseline population. For over 2000 isolates collected from 40 different locations, IC50s were ranged from 0.018-0.115 mg/l. A further survey using 10 mg/l as threshold dosage in the fields was conducted and the results showed no isolate could survive at this dosage. Based on the results, no sign of prochloraz resistance were existed in mango plantations 13 years after the application of this eradicative fungicide. In study the effect of prochloraz on the germination behavior of the anthracnose fungus, the results showed that the development of the appressorium was impeded and the IC50 was 10.34mg/l which was much higher than the effect on the mycelium growth.

1. Cerkauskas RF, Sinclair JB, 1980. Phytopathology 70, 1036-1038.
2. Smith FD, Parker DM, Koeller W, 1991. Phytopathology 81, 392-396.