Well-Travelled Fungus

Groundsel cluster cup fungus, looking like minute tarts, on an infected groundsel leaf surface. The spores are produced in vast numbers in the centre of each cup. Each cluster cup is about as large as a full stop (period) on your monitor screen. A minute larva of a fly is crawling across the leaf surface, just to the right of the open cups, and may have been feeding on the fungal spores. In the bottom right corner (below) cluster cups are just forcing their way to the leaf surface, ready to open.

Cluster cups erupting from an infected, swollen stem. Infected stems often develop purple pigmentation and become distorted. Heavy infestations can be fatal.

 

Above: A vertical section through a cluster cup on a leaf surface, x100, showing the chains of spores that are formed in the centre of each cup.  

Below: Although apparently heathy, this plant is infected with the fungus, just visible on the surface of the bottom leaf on the left in the middle of the picture 

Take a look at the weed groundsel Senecio vulgaris stems and leaves in autumn and you’ll often find that they’re swollen and distorted, with a patches of a yellow fungus erupting from their surface. The infection is a fungus called groundsel clustercup Puccinia lagonophorae, which has an interesting history, having travelled more than half-way round the world since the mid-20th. century. Migratory people tend to take weeds, as well as their crops, with them and groundsel was accidentally taken to Australia by early settlers from Britain, where it became infected with this fungus which is native to Australia. Some clustercup-infected groundsel made the return journey and the fungus arrived in Europe in 1961, where it has been spreading ever since. More recently, within the last decade, the fungus crossed the Atlantic and has begun infecting groundsel that had been taken there by early European settlers. The Americans are not altogether sorry that the fungus has arrived, because it weakens groundsel and might offer a means of biological control of this invasive weed. In the photographs you can see the flower-like spore cups, called aecia, that produce the infective golden yellow spores. In the vertical section through one of these cups (second photo from bottom) you can see the chains of spores budding off from the fungal hyphae.

Rust

In Victorian times it was socially acceptable to possess books devoted to smut – provided that the smut was of the fungal variety. It’s hard to imagine that a book entitled Rust, Smut, Mildew and Mould: An Introduction to the study of Microscopic Fungi would sell well with the general public today but M.C. Cooke’s volume of the same name, published in 1870, apparently did. “In these latter days, when everyone who possesses a love for the marvellous, or desires a knowledge of some of the minute mysteries of nature, has, or ought to have, a microscope,” Cooke began confidently, “a want is occasionally felt which we have essayed to supply”. His book seems to have satisfied that want, since the preface to the second edition states that it was produced as a response to “demands from the public encouraging the publisher to proceed with a new edition”. So more smut – together with rust, mildew and mould, is what the public got. One of the plates illustrates the bright coloured patches that appear on bramble leaves, caused by the fungus Phragmidium violaceum, showing its distinctive club-shaped resting spores (teleutospores) that are released from purple pustules on the leaf undersurface (middle left illustrations). I scraped some of these onto a microscope slide and photographed them – a comparison with the plate from Cooke’s book shows that his illustration is pretty accurate. Wight Rambler has recently written about brilliantly coloured bramble leaves at http://wightrambler.blogspot.com/