Persistence of the protectant efficacy of potato late blight fungicides

The persistence of the most important protectant fungicides registered in The Netherlands was measured under field conditions in 1997 and 1998. Fungicides were sprayed every week and the protectant efficacy was evaluated at the end of each spraying interval with a bioassay. After a number of sprays in 1997, the full rate of dimethomorph-fentinhydroxide and reduced rates of fluazinam showed a lower persistence of efficacy compared to the other treatments. In 1998, again dimethomorph-fentinhydroxide and fluazinam showed a lower persistence of efficacy on several occasions. The lower persistence of dimethomorph-fentinhydroxide coincided in a number of occasions with situations when rain was recorded after spraying. The lower persistence of reduced dose rates of fluazinam was observed after periods with high maximum temperatures, indicating that volatilisation might have occured.