A 4-phenylpyridazinone of the formula or salts thereof in which A and B are in each case independently of one another hydrogen or (C 1 -C 6)-alkyl; G is hydrogen, C(=O)R 1, C(=L)MR 2, SO 2 R 3, P(=L)R 4 R 5, C(=L)NR 6 R 7 or E; E is a metal ion equivalent or an ammonium ion; L is oxygen or sulfur; M is oxygen or sulfur; R 1 is (C 1 -C 6)-alkyl, (C 2 -C 6)-alkenyl, (C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl, di-(C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl or (C 1 -C 4)-alkylthio-(C 1 -C 6)-alkyl, each of which is substituted by n halogen atoms, a fully saturated 3- to 6-membered ring consisting of 3 to 5 carbon atoms and 1 to 3 heteroatoms from the group consisting of oxygen, sulfur and nitrogen which is substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy, (C 3 -C 6) -cycloalkyl, phenyl, phenyl- (C 1 -C 4)-alkyl, heteroaryl, phenoxy-(C 1 -C 4)-alkyl or heteroaryloxy-(C 1 -C 4)-alkyl substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy; R 2 is (C 1 -C 6)-alkyl, (C 2 -C 6)-alkenyl, (C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl or di-(C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl, each of which is substituted by n halogen atoms, or is (C 3 -C 6)-cycloalkyl, phenyl or benzyl, each of which is substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy; R 3, R 4 and R 5 are each independently of one another (C 1 -C 6)-alkyl which is substituted by n halogen atoms, (C 1 -C 4)-alkoxy, N- (C 1 -C 6)-alkylamino, N, N-di-(C 1 -C 6)-alkylamino, (C 1 -C 4)-alkylthio, (C 2 -C 4) -alkenyl or (C 3 -C 6)-cycloalkylthio, or phenyl, benzyl, phenoxy or phenylthio which is substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy; R 6 and R 7 are each independently of one another hydrogen, (C 1 -C 6)-alkyl which is substituted by n halogen atoms, (C 3 -C 6)-cycloalkyl, (C 2 -C 6)-alkenyl, (C 1 -C 6)-alkoxy or (C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl, phenyl or benzyl, each of which is substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy; or R 6 and R 7 together with the nitrogen atom to which they are attached form a 3- to 6-membered ring which contains 2 to 5 carbon atoms and 0 or 1 oxygen or sulfur atoms; m is 1, 2 or 3; n is 0, 1, 2 or 3; X is halogen; Y and Z are each independently of one another hydrogen, halogen, cyano, nitro, (C 3 -C 6)-cycloalkyl or are (C 1 -C 6)-alkyl or (C 1 -C 6)-alkoxy, each of which is substituted by n halogen atoms, with the proviso that neither Y nor Z is a (C 1 -C 6)-alkyl or (C 1 -C 6)-alkoxy radical located in position 6 if n is 0, and that the compound 4-(2,4-dichlorophenyl)-5-hydroxy-2-methylpyridazin-3 -one is to be excluded. A and B are in each case independently of one another hydrogen or (C 1 -C 6)-alkyl; G is hydrogen, C(=O)R 1, C(=L)MR 2, SO 2 R 3, P(=L)R 4 R 5, C(=L)NR 6 R 7 or E; E is a metal ion equivalent or an ammonium ion; L is oxygen or sulfur; M is oxygen or sulfur; R 1 is (C 1 -C 6)-alkyl, (C 2 -C 6)-alkenyl, (C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl, di-(C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl or (C 1 -C 4)-alkylthio-(C 1 -C 6)-alkyl, each of which is substituted by n halogen atoms, a fully saturated 3- to 6-membered ring consisting of 3 to 5 carbon atoms and 1 to 3 heteroatoms from the group consisting of oxygen, sulfur and nitrogen which is substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy, (C 3 -C 6) -cycloalkyl, phenyl, phenyl- (C 1 -C 4)-alkyl, heteroaryl, phenoxy-(C 1 -C 4)-alkyl or heteroaryloxy-(C 1 -C 4)-alkyl substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy; R 2 is (C 1 -C 6)-alkyl, (C 2 -C 6)-alkenyl, (C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl or di-(C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl, each of which is substituted by n halogen atoms, or is (C 3 -C 6)-cycloalkyl, phenyl or benzyl, each of which is substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy; R 3, R 4 and R 5 are each independently of one another (C 1 -C 6)-alkyl which is substituted by n halogen atoms, (C 1 -C 4)-alkoxy, N- (C 1 -C 6)-alkylamino, N, N-di-(C 1 -C 6)-alkylamino, (C 1 -C 4)-alkylthio, (C 2 -C 4) -alkenyl or (C 3 -C 6)-cycloalkylthio, or phenyl, benzyl, phenoxy or phenylthio which is substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy; R 6 and R 7 are each independently of one another hydrogen, (C 1 -C 6)-alkyl which is substituted by n halogen atoms, (C 3 -C 6)-cycloalkyl, (C 2 -C 6)-alkenyl, (C 1 -C 6)-alkoxy or (C 1 -C 4)-alkoxy-(C 1 -C 6)-alkyl, phenyl or benzyl, each of which is substituted by n radicals from the group consisting of halogen, (C 1 -C 4)-alkyl and (C 1 -C 4)-alkoxy; or R 6 and R 7 together with the nitrogen atom to which they are attached form a 3- to 6-membered ring which contains 2 to 5 carbon atoms and 0 or 1 oxygen or sulfur atoms; m is 1, 2 or 3; n is 0, 1, 2 or 3; X is halogen; Y and Z are each independently of one another hydrogen, halogen, cyano, nitro, (C 3 -C 6)-cycloalkyl or are (C 1 -C 6)-alkyl or (C 1 -C 6)-alkoxy, each of which is substituted by n halogen atoms,