Fine Structure of the Host-parasite Relationship of Diplodia pinea on Pinus radiata

A description is given of the fine structure of Pinus radiala D.Don tissues inoculated with the fungus Diplodia pinea (Desm.) Kickx. which is consistently associated with the Dead Top disease in this species.

The fungus was introduced into the pith of clonal material, and the tissues were examined in optical and electron microscopes. The fungal hyphae were concentrated in the ray cells where they caused disturbances in the distribution of starch. Abnormal resin vesicles had formed in the ray cells below the infection points. It is suggested that carbohydrates may have been converted into resinous materials in response to the fungal infection.

The appearance of traumatic wood rings and narrow bands of cells having the morphology of reaction wood supports the hypothesis that toxins are produced as a result of the infection.

The fungal hyphae penetrated the tissues by lysing the middle lamellae and occasionally causing some erosion of the cell walls where intracellular penetration occurs.

The evidence presented suggests that Diplodia pinea is a soft rot type of pathogen that can remain dormant in the pith cells, has marked preference for the ray cells and can cause considerable disturbance to the metabolism of adjacent host cells.