I am an amateur naturalist trying to find out what lives in my garden.
I ended my most recent posting by saying that that a certain "elderly gentleman" had taken up residence in my garden and that he was not in the best of health. Photo 1 (click to enlarge) shows one of his leaves and the rusty brown patches afflicting it.
I know almost nothing about plant diseases, but a short time spent searching the Internet for diseases on groundsel convinced me that the cause of the brown patches on the leaves is Puccinia lagenophorae, an example of a fungal rust.
A close up (photo 2, 40x magnification) shows the cup-like fungal craters produced in the leaf. I believe these may be what are technically termed the rust's spore producing aecidia.
The technical name for the rusts is the Urediniomycetes. From my copy of Fungi (Spooner and Roberts, Collins NN, 2005) I learn there are about 7000 species worldwide and 260 in Britain of which 14 are new here since 1966, suggesting there are still more to be discovered. They have a complex and fascinating life cycle, though one rather alien to us. They can go through periods of sexual cross-fertilization in which they exude nectar-like liquids to attract "pollinating" insects, other periods as airborne spores and still others as thick-walled teleutospores designed to lie dormant over winter and germinate in the spring. Some parasitise different plants at different points in their life cycle, changing their form and appearance as they do so. This site has some more details. I find it wonderful to think that such intricate and complex patterns of life are carrying on quietly in the little places of my garden.
Puccinia lagenophorae has an interest all of its own: it is a relative new comer to the UK having arrived here from Australia only in the 1960's (it was first recorded on groundsel in Dungerness in Kent in 1961). Until recently it was unknown in the US, but is apparently now making an appearance there also. P. lagenophorae's ability to attack groundsel - an agricultural weed - has led to it being investigated as a 'biological weedkiller'. Marigolds and Oxford Ragwort may apparently also be attacked
Finally, one thing I'm not clear about from my reading is how terminal infection is for my plant. Is my weed almost certainly 'doomed' or is it more akin to him having caught a bad cold? If someone out there can tell me, do please leave a comment.