Catalysis of the reaction: 5-amino-4-chloro-2-(2,3-dihydroxyphenyl)pyridazin-3(2H)-one + O2 = 5-amino-4-chloro-2-(2-hydroxymuconoyl)pyridazin-3(2H)-one + 2 H+.
5-amino-4-chloro-2-(2,3-dihydroxyphenyl)-3(2H)-pyridazinone 1,2-oxidoreductase (decyclizing)