An In Vivo Bioassay for Estimating Fungicide Residues on Peach Fruit

Recent fungicide efficacy studies indicated that brown rot fruit rot at harvest, caused by Monilinia fructicola, was being controlled by residual activity from protectant fungicides applied during the time between bloom and the preharvest fruit ripening period. To determine the extent of this residue, a simple in vivo bioassay was developed by assaying M. fructicola spore germination directly on sampled fruit. A 1.5-cm section of clear flexible tubing was placed upright on harvested fruit to create a small incubation well. After the tubing-fruit interface was sealed using silicon grease, a suspension of M. fructicola conidia was pipetted into the well. The spores were suspended in a buffer-substrate medium consisting of 0.025 M potassium phosphate, 0.1% sucrose, and 0.1% yeast extract. A rubber stopper with an aeration hole was inserted into the well's top and the fruit was placed in an incubator at 25°C. Results of a time-course study indicated that the optimal conidial incubation time was 6 h. Bioassay sensitivity was evaluated by examining test results from varying concentrations of captan fungicide. Results indicated that captan residue levels as low as one-thousandth the standard field rate could be detected using spore germination as the predictor. Fitting of the logistic decline model to the data created a standard curve to allow quantitative estimation of fungicide residue based on observed level of spore germination. A modified version of the bioassay, which can be used to detect carbohydrate or nutrient sources on the fruit surface, was also demonstrated.