AbstractOne‐year‐old nursery‐grown trees of Populus deltoides X P. trichocarpa parentage exhibited stem necrosis and lesioning during the unusually moist 1985–1986 growing season. A yellow‐pigmented, Gram negative, aerobic, monotrichous bacterium which utilized salicin was isolated from infected tissue almost in pure culture. The bacterium was identified as Xanthomonas campestris pv. populi and pathogenicity tests revealed it to be the causal organism. This is the first recorded outbreak in New Zealand of stem necrosis of poplars caused by X. campestris pv. populi. The bacterium appears to be an opportunistic