Outbreaks of dieback and stem cankers formed by Diplodia pinea occur sporadically throughout New Zealand. Unthrifty trees and trees suffering mechanical injury are attacked. The disease sometimes causes substantial losses in forests of exotic conifers. Curtis (1926) recorded the fungus causing severe dieback on Pinus radiata and P. muricata following drought injury. Birch (1936) associated the disease with a red top symptom in P. ponderosa, P. contorta, P. nigra, and P. radiata among unthinned and overstocked stands on pumiceous soils. Grayburn (1957) recorded a severe attack of Diplodia in north Canterbury on Pinus radiata, P. nigra, and P. ponderosa following hail injury. Gilmour (1964) (1965b) recorded the occurrence of this fungus as a wound pathogen following thinning and low pruning in three- to 10-year-old P. radiata trees growing in pumice soils in the central North Island.