Invasive fungi


Invasive fungi

As with all life forms, fungi is not always benign and, given their super-efficient system of spore dispersal, it is not surprising to find that those that have malignant intentions can be extremely invasive, creating, in fact, rampant and microscopic death squads.

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Honey fungus Armillaria mellea

Honey fungus Armillaria mellea © Ita McCobb

Probably the most well-known tree destroyer is the selfish Honey fungus Armillaria mellea, which is known to have formed some of the largest living organisms on the planet that are thousands of years old.

Unlike most fungi that control their rate of growth to run in symbiosis with their living host tree or plant, Honey fungus just lets caution to the wind and is happy to form a large clump at the base of healthy trees, grow on relentlessly while at the same time sapping the nutrients from the tree and creating White-rot root disease. It then lives off the decaying material of the tree it is killing.

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Penicillin Penicillium growing on an orange © Ita McCobb

Then there’s the tiniest of powdery fungi – Penicillium – which has proved to be both mankind’s saviour and a destroyer of fruit crops. Penicillium is one of the oldest known fungi. It leads a strange double life – on the one hand many of its 300 species are used by man to create antibiotics and in the creation of foodstuffs, then, on the other hand, Penicillium is a plague to fruit farmers.

Powdery white-edged and blue-green in colour, Penicillin Penicillium is what you sometimes find growing on your decaying oranges at home, but for a fruit farmer this can mean crop failure. While it’s a particular problem for apple-growers its mould can also destroy pear, strawberry, tomato, corn and rice crops. Added to this, penicillin mould produces the carcinogenic metabolite Patulin.

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Fungal tree diseases

Artist’s bracket Ganoderma applanatum © Ita McCobb

The cause of the devastation of elm trees throughout Europe and America, Dutch elm disease is the result of infection from a particular type of flask and cup fungi (called Ascomycota), which is then spread throughout the tree by the elm-bark beetle.

Ascomycota species are particularly important to mankind because of their use in medical compounds and food production and are the largest group of fungi species known, with an estimated 64,000 species worldwide. The fruiting body (the bit we see above ground) of these fungi is either cup shaped or more or less spherical.

Elm trees Ullmus are not the only ones to suffer the attacks of the minute species of fungi, the Walnut Juglans, Ash Fraxinus and Oak Fagus are all threatened with similar fates.

Geosmithia morbida is a fungus that causes Thousand-cankers disease in species of walnut tree. The fungus is transmitted by the walnut twig beetle Pityophthorus juglandis, and the resulting cankers restrict the flow of a tree’s nutrition, killing it in two or three years.

Hymenoscyphus fraxineus is a fungus that causes Ash dieback, which is characterized by leaf loss and crown dieback in infected trees.

Ceratocystis fagacearum is a fungus that rapidly kills oak trees and is spread by insects or tree-root connections. Initially it causes general leaf discoloration, wilt and defoliation but ultimately will kill the tree.

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Fungal crop diseases

Ergot fungus Claviceps purpurea on ear of Rye © Rasbak

Finally, there’s the notorious Ergot fungus Claviceps purpurea, which attacks cereal crops. It contains alkaloids similar to LSD and not only reduces the harvest but also is dangerous if eaten by cattle or mankind.

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References: New Scientist; Kew Botanical Gardens, London; Fungi, Paul Sterry, WH Smith; Wikipedia.