IAA production by strains of P. syringae pv. savastanoi (pv. savastanoi) from oleander is necessary for formation of galls on oleander. Pathogenicity on olive and oleander plants of wild-type and IAA-deficient (Iaa-) mutant strains of pv. savastanoi from olive, oleander and privet [Ligustrum japonicum] was compared. All wild-type strains from olive, oleander, and privet induced galls on olive. Only wild-type isolates from oleander were tumorigenic on oleander. An exception was observed with isolate EW1017 from olive which also caused galls on oleander; however, this strain resembled strains from oleander in that IAA genes were plasmid-borne. Iaa- mutants of the strains from olive and privet did not induce galls on olive and oleander. Iaa- mutants of strains from oleander produced no symptoms on oleander plants, but induced atypical galls on olive plants. The role of bacterial production of IAA in gall formation on the olive and oleander knot disease was confirmed.