Page 5 – Parasites: disease-causing fungi
Parasitic fungi feed on living plants and animals, and are the main cause of plant disease worldwide.
Most diseases of pasture grasses and crops for horticulture, agriculture, and forestry are caused by introduced fungal parasites. Practices such as spraying fungicides are needed to minimise the damage.
Most parasitic fungi are microscopic and may be better known by their symptoms (such as brown rot in peaches). Domesticated animals are also harmed by toxic introduced fungi – for example Pithomyces chartarum causes facial eczema in cattle and sheep. Another non-native fungus, the chytrid Batrachochytrium dendrobatidis, has been implicated in the global decline of frogs, including the rare Archey’s frog in New Zealand.
A variety of fungi have been collected from human nail infections, and several can infect people who are weakened, for example by damaged skin or an impaired immune system.
Although there are several native parasitic fungi, their impact on native plants and animals is largely in balance as they have co-existed for millions of years.
The fungus known as beech strawberry (Cyttaria species) causes cancerous galls on beech trees. In spring, these develop into yellow, golfball-like fruiting bodies. They are eaten by pigeons and possums, and in South America related species are harvested for human consumption.
The native pūtawa (Laetiporus portentosus) causes wood decay in living silver beech. It forms large, bracket-shaped fruiting bodies that were harvested by Māori for use in fire-starting and fire-carrying, because they smouldered for hours. There are several native fungi parasitic on leaves. For example, Corynelia tropica causes spots on the leaves of tōtara trees.
Vegetable caterpillar (Cordyceps robertsii) was the first new fungus described in New Zealand, in 1836. It is a parasite of the ground-dwelling caterpillar stage of certain native moths. The fungus mummifies the caterpillar, then forms a stick-like fruiting body from the caterpillar’s head, extending up to 20 centimetres above the ground. Māori burned the fungus (known as āwheto) to make powdered charcoal as a black pigment for tattoos. They also considered it a delicacy.
Damage to crops
A few native fungi have become parasitic on introduced crops. Harore, the bootlace mushroom (Armillaria novaezelandiae), usually decomposes dead logs and roots, but it can become parasitic on crop plants such as radiata pine and kiwifruit if it is already present in the soil where they are planted. This readily occurs where native forested land has been converted into forest plantations, and in kiwifruit orchards where willow shelter belts are felled – their stumps and root systems become infected by the mushroom. It then spreads to nearby living roots of kiwifruit and becomes parasitic on the plant.