Postharvest applications of fludioxonil and pyrimethanil to control Phlyctema vagabunda on apple in South Africa

Bull's eye rot is a postharvest disease of pome fruit caused by Neofabraea and Phlyctema species infecting the lenticels of fruit in the orchard with symptoms only becoming apparent after months in storage. Chemical control is a valuable method to manage postharvest diseases on pome fruit. In this study, the efficacy of the fungicides fludioxonil (299 mg/L) and pyrimethanil (500 mg/L) was tested on Phlyctema vagabunda Desmazières, the prevalent bull's eye rot species in South Africa. Although both fungicides are registered on pome fruit against other fungal pathogens, neither are specifically registered against bull's eye rot in South Africa. Fruit trials involved curative dip, drench, and thermofogging applications of the fungicides on P. vagabunda inoculated ‘Cripps Pink’ and ‘Fuji’ apple fruit. Furthermore, the variation of pyrimethanil (500 and 1000 mg/L) sensitivity in planta in a dip application was evaluated on six P. vagabunda isolates from the Western Cape of South Africa. The effect of incubation time before treatment was tested in relation to the efficacy of the fungicides.Fludioxonil was highly effective in controlling P. vagabunda incidence as a dip application and moderately effective as a drench and thermofog application. Pyrimethanil had moderate efficacy on both cultivars as a thermofog application. The pyrimethanil dip application was ineffective in controlling disease incidence while the drench application had little efficacy on ‘Fuji’ apples and no efficacy on ‘Cripps Pink’ inoculated fruit. There was no variation in the efficacy of pyrimethanil to P. vagabunda isolates. Incubation time had a significant effect on pyrimethanil efficacy. Delaying pyrimethanil application after inoculation significantly decreased the efficacy of the fungicide in controlling P. vagabunda incidence. The longer inoculated fruit was incubated, the less susceptible the fungicide became. As a recommendation, further investigation into the ability of the fungicides to reduce P. vagabunda as a postharvest application should be conducted on pre-harvest, naturally infected, or conidial suspension inoculated fruit.