Plant-pathogenic bacteria are combated with 3-halogeno-benzotriazine 1-oxides of the formula ##STR1## in which X each independently is halogen, a nitro group, an alkyl, halogenoalkyl, alkoxy, alkylmercapto or alkylsulphonyl group with up to 4 carbon atoms in each case, a phenoxy radical or a phenylmercapto radical,n is an integer from 0 to 4, andHal is chlorine or bromine.Such compounds are produced by reacting the corresponding 3-hydroxy compounds with thionyl chloride in the presence of dimethylformamide.