New 2-phenylpyridazinones, analogs o f 4-chloro-5-methylamino-2-phenylpyridazin-3(2H)ones, were assayed for their phytotoxic activity to interfere with plant pigment formation. Six derivatives with — SCF3, — OCF2CH FCF3, — O CH F2, — CN, or — Br in meta position o f the phenyl ring exhibited similar or better bleaching activity than the m -CF3 analog (norflurazon). The activity is predominantly...