Can any readers identify this object (see photo) growing on a stinging nettle of the species Urtica dioica? The nettle was beside a gravel drive, and there are some small patches on the underside of the leaf ribs.
• Such orange-red swellings on the stinging nettle’s stem, which tend to cause it to curve, are the result of an infection by the fungus Puccinia caricina.
Small orange cusps are found on the surface of these deformations, or galls. These contain one of the several different types of spore that this fungus produces. Puccinia is a representative of the group of fungi known as rusts. But in contrast with fungi such as mushrooms, which feed on dead or decaying organic material, rusts infect living plants and are known as obligate parasites – organisms that cannot complete their life cycle without exploiting a suitable host.
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Members of the rusts cause diseases in many cereal crops, and have extremely complex life cycles that often involve different host species.
“Members of the rusts cause diseases of cereal crops and have extremely complex life cycles”
In the case of P. caricina, another host in addition to stinging nettles is the grass-like sedge plant, and it also forms galls on the leaves and berries of the redcurrant.
Frank Wuytack, Herent, Belgium
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This article appeared in print under the headline “Nettling conundrum”
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