James Iremonger is a third year microbiology student at Heriot Watt University and is currently undertaking a summer mycology project funded by the BSPP. 
I’ve just finished my first week studying rust fungi with Dr. Stephan Helfer, a senior scientist and mycologist at the Royal Botanic Gardens in Edinburgh (RBGE). We have been working in the Cryptogam unit, situated next door to the Herbarium, which hosts nearly three million specimens of plants and fungi!
Our project aims to investigate the distribution, pathology and species diversity of pathogenic rust fungi in an urban environment (Edinburgh). Our hope is to gather enough data to provide a useful picture of the distribution of rusts, as well as some of the variation in susceptibility between trees and woody hosts, which can be revisited and developed in future research. We’ve been lucky enough to secure funding from the British Society for Plant Pathology (BSPP) to run the project over the next 7 weeks.
Above: all rusts are plant parasites. Some inflict great losses in agriculture (wheat, oat and barely) as well as forestry (pine, birch and willow). Salix caprea, or goat willow, above, is particularly susceptible.
As a long time Edinburgh resident I’ve been enjoying the Botanics since childhood and have spent many hours idly wandering through the lush grounds and exploring the exotic glasshouses. I even played the guitar at and event in the palm house last year! However, in spite of all this comes a slightly guilty admission – I know absolutely nothing about gardening! It was actually an interest in parasitism (partly inspired by the fantastic This Week in Parasitism podcasts) that initially drew me to the project. I had also heard of the pathogen Puccinia graminis during an introductory plant science lecture, and found it quite fascinating. Commonly known as wheat stem rust, a particularly vilurent strain of the pathogen called Ug99 has recently received much press attention for decimating crop yields and threatening global food security.
Above: rust specimen from the RBGE Herbarium, collected nearly 200 years ago!
Rust fungi are obligate biotrophs (that is, parasitic) often with a complicated life cycle involving two completely unrelated hosts. Spores settle on the foliage or soft tissues of a plant, and form haustoria, which envelop the infected plant’s cells, sequestering amino acids and sugars for the fungus. Often, as autumn draws to a close and leaves wilt and die back, rust forms hardy, overwintering teliospores (below) before transferring onto another, evergreen host (e.g. barberry, in the case of wheat stem rust).
The big challenge so far hasn’t been in the molecular details, however. We’ve decided on a list of 11 target genera (all trees and woody plants), which I must now become intimately familiar with. Members of the Salicaceae such as Salix and Populus (Willow and Poplar) are badly affected by rust, as well members of the Rosaceae family, such as Sorbus and Prunus species. Furthermore, given the time of year, one leaf from any of these trees might house a vast array of diseases, be they insect, bacterial or fungal. This can make identifying rust (even with a handlens) quite difficult! Thankfully I have an experienced plant pathologist on hand, as well as three million herbarium specimens for reference!
Above: teliospores of the genus Phragmidium, x200. These spores produce basidia, each with four haploid basidiospores, which is why rust is considered a Basidiomycete – like the common field mushroom!
Devising a fieldwork strategy has also been a challenge. It isn’t possible simply to pick a few pretty, floristic regions in which to sample; our approach must be statistically representative, and we’ve had helpful advice from both a statistician and a lichen specialist on the finer details of infection scoring and random sampling. We have generated a list of 600 random points around the city in which to sample. The wonders of GPS technology means that affected trees in our study can be revisited and monitored in the future; in addition to this it makes the coordinates very easy to find, which is great as I’m useless with a map and compass! This week has got off to a shaky start with my GPS unit breaking down, hopefully easily rectified! Other than these small challenges it’s been great fun so far – I’ve definitely got the bug for plant pathology.
James is keeping a blog about his experiences at www.wannabebio.wordpress.com