A standardized bioassay measuring the growth inhibition of Aspergillus niger in vitro allowed the detection of small concentrations (0.1 to 20 μg/ml) of chlorothalonil present in dew water on both the adaxial and abaxial surface of banana leaves in a commercial plantation receiving aerial sprays. Chlorothalonil concentrations detected in dew water on the banana leaf surface were within the range of concentrations required to prevent Mycosphaerella fijiensis (causal agent of black Sigatoka) ascospore germination in laboratory bioassays. When 9-cm-diameter banana leaf disks inoculated with M. fijiensis ascospores were immersed for 4 h in water containing 0.1 to 0.6 μg of chlorothalonil per ml, ascospore germination was inhibited by 96.9%. The EC50 values for inhibition of ascospore germination were between 0.01 and 0.03 μg/ml for chlorothalonil and between 3.2 and 3.7 μg/ml for mancozeb. Following a 4-h exposure to chlorothalonil and mancozeb, and subsequent removal of the fungicides by a washing step, ascospores failed to germinate, indicating that both fungicides are fungicidal to M. fijiensis, not fungistatic. Recovery analysis of chlorothalonil spray droplet deposits and active ingredient on deposition cards in the field during aerial spray applications indicated that detectable fungicide deposition on the abaxial leaf surface occurs only when the banana leaf-target is vertical or nearly so. The significance of this observation in relation to the control of black Sigatoka with protectant fungicides is discussed.