Kauri rust

Aecidium fragiforme. There are two rusts known on Agathis, Aecidium fragiforme and Aecidium balansae. The latter occurs in New caledonia.

Southeast Asia and Oceania. It is recorded from Australia, Fiji, New Caledonia, Solomon Islands, and Vanuatu.

Agathis species (kauri).

The fungus forms raised galls, up to 20 mm diameter, and 5 mm high (Photos 1). The lower surface is often depressed. White to yellow pustules are formed within the galls, usually on the upper surface of the leaf (Photo 2). The pustules contain spores of the rust fungus.

There are two types of spore-forming structures that occur on the raised spots or galls: pycnia and aecia. The pycnia are small, 0.1 diameter. They produce spores which are exchanged - a form of mating - between pycnia, probably by insects. This "mating" produces the aecia, which are much larger, 1 mm diameter (Photos 1&2). From aecia, aeciospores form, and these are spread in wind and rain to infect other parts of the same leaf, other leaves, or other trees.

The galls are more common on younger plants, sometimes causing leaves to fall. However, this is not an important disease, although details of its impact are not well reported.

Look for raised yellow spots on the leaves becoming reddish brown, circular, up to 2 cm diameter. The two rusts differ in that Aecidium fragiforme occurs on the upper leaf surface and A. balansae occurs on the lower leaf surface. Differences also occur in the ornamentation on the surface of the spores (called aeciospores).

There is no treatment for this disease.