PURPOSE: An agricultural and horticultural fungicide effective against various blights, comprising a phenoxyphridazine derivative as an active constituent.

CONSTITUTION: A fungicide comprising a compound of the formula (X is halogen; both R1 and R2 are lower alkyl or alkenyl groups, or one of them is lower alkyl group, and the other is halogen), e.g. 3-chloro-6-(2,6-diallylphenoxy)-pyridazine, as an active constituent. The fungicide is effective against a wide variety of plant blights, particularly blast and sheath blight of rice blants, early blight of tomatoes, and powdery mildew of cucumbers.