The pathogenicity of Diplodia pinea to Pinus radiata D. Don

Abstract

In the Pinus radiata stands of north-eastern Victoria the symptom "dead top" is common and consistently associated with the fungus Diplodia pinea. In order to resolve the pathogenic status of D. pinea, its association with disease symptoms has been studied by histological examination and tissue isolations following greenhouse and field inoculations.

Malformations were associated with infection by D. pinea and although the amount of tissue killed was small, the defect induced was considerable. Damage to the leader caused the greatest loss in wood quality.

The fungus attacked the pith very readily after gaining access to this tissue through injuries and via needle or cone traces. In the pith, the fungus spread slowly downwards and was protected from changes in the external environment. Injection of D. pinea spores into the pith resulted in a very high percentage of infection, and caused the malformations and other symptoms associated with attack by this fungus in the field. Histological studies showed that the fungus spread rapidly in the pith tissues, causing alterations in cambial function which produced traumatic resin canals, parenchymatous cells, and compression wood.

Field trials showed that fast-growing trees were more severely affected than slower-growing ones, and the latter recovered more quickly. The fungus persisted in the infected tissues for up to 400 days.