BSPP News Spring 2000 - Online Edition

The Newsletter of the British Society for Plant Pathology
Number 36, Spring 2000 

Conference and Travel Reports

The First International Powdery Mildew Conference Avignon, France : 29 August - 2 September 1999

Avignon, summer residence of the Popes and home to the First International Powdery Mildew Conference. This  conference was the perfect opportunity for those of us researching this fascinating group of fungi to get together, share results and indulge in some decent red wine.

Chris Ridout with Rebecca Wyand (left) and Lonneke Mulder
doing some applied research at the vineyard of the Châteauneuf du Pape.

We started with an overview of powdery mildew research presented by Dr Bill Bushnell, setting the scene for all the themes covered in the conference. The economic importance of these fungi was immediately apparent with various species attacking a diverse  range of important crops. Their ability to overcome fungicides and genetic resistance is quite remarkable, and the life-cycle is supremely adapted for efficient colonisation and spread. Dr Bushnell illustrated how mildews are suited for microscopic  investigation, making these obligate biotrophs ideal for the study of the haustorial/plant interaction. It is not surprising that this group of fungi have become firm favourites for investigation since the evolution of plant pathology as a recognised discipline.

Starting with  taxonomy, the first thing our group (cereal mildews) learnt  is that Erysiphe should be called Blumeria since the former describes only the anamorph! The session covered the classification of powdery mildew fungi by microscopic investigation of features such as ascocarps and appendages, asci, ascospores, hypae, appressoria and conidia. It was pleasing to see that recent molecular data generally supported the classical approach to taxonomy. Dr Kiss focussed on tomato powdery mildew, including the analysis of  re-hydrated specimens from the turn of the century. The general conclusion in this case was that a Euoidium anamorph is present in Australia and that a single Pseudoidium taxon is present in the rest of the world.

Moving on to genetics, and at the last minute I had to stand in for James Brown who unfortunately couldn't make it to the conference. I described how the ability to make sexual crosses with Blumaria graminis f.sp.hordei has enabled us to make extensive investigations into the genetics of avirulences and fungicide resistance. Following on from this, it was encouraging to see the analysis of sexual crosses in Uncinula necator (grapevine powdery mildew), this work being presented by Dr Belinda Stummer. Other talks in this session covered the analysis of point mutations in the demethylase gene, believed to be important for resistance to DMI fungicides, and the identification of chaperonins in pea mildew haustoria.

A summary of chemical control was presented by Dr Derek Holloman, who described the broad arsenal of weapons that have been developed since the dawn of agriculture. Target sites of action have become more precise, and rates of application have been drastically reduced, yet the threat of resistance developing is always prominent in this group of fungi. He finished with  an investigation of alternative respiration in mitochondria, possibly involved in the resistance of mildew to Strobilurins. Following on, Margaret McGrath detailed the gradual advancement of resistance to all major classes of fungicides during the last 20 years in the United States, the pattern of spread depending on the chemical class. At-risk fungicides should be used in alternations or in mixtures, the management strategy being to delay the onset of resistance. It was interesting to see the development of biological alternatives, including plant extracts, bacteria and several fungi including Trichoderma harzianum.

An understanding of epidemiology is essential to managing the spread and control of powdery mildews. Dr Bill Jarvis introduced this topic, and was followed by several speakers covering different crop/environment locations and including the development of models to predict outbreaks. The ADEMô model is PC based and is driven by environmental variables to predict apple scab, resulting in more precise application of fungicide and significant cost saving. Critical parameters in the initial development of disease were also considered by Dr Lydia Bousset in an analysis of founder effect with a specific mildew genotype within a barley field.

The next session on plant cell responses was introduced by Dr Kunoh, and covered  interactions with pathogenic and non-pathogenic mildews.  Physiological investigations on the nature of extracellular material affecting germination were described,  indicating the existence of suppressors blocking the E. pisi elicitor.  Ultrastructural studies illustrated the gathering of cytoplasmic strands at the site of appressorial attachment. Dr Zeynen developed the theme by illustrating the movement of the nucleus to the site of infection, localisation of paramural bodies in appressoria at the site of attachment and the use of X-ray microanalysis to investigate increases in metal ions during penetration. Other talks in this session described work on systemic acquired resistance, the activation of single cell hypersensitive response, induced accessibility following pre-inoculation and characterisation of the extra-haustorial matrix.

The genetics of the plant was introduced by Dr Paul Schulze-Lefert, covering the recent work in his group on the cloning and characterisation of Mla, Mlo and Rar1, the latter functioning downstream of the initial recognition. Here, there was a curious link to the animal world through the identification of a common CHORD domain  representing a novel zinc finger motif, and work on silencing this gene in C. elegans was described. Structural studies concluded that there are seven membrane-spanning domains of Mlo, the protein having an extracellular N-terminus. Dr Ouchi described the procedure of PCR-based analysis of gene expression in barley/mildew interactions, and in situ cDNA synthesis in individual appressoria. We moved swiftly onto Arabidopsis with physical mapping of RPW8, this gene being localised to 20 kB cosmid clone which rendered transgenic Col-0  plants resistant to E. chichoracearum. The conference concluded with work on the characterisation of Arabidopsis EMS mutants that became susceptible to E. chichoracearum, enabling the definition of 6 pmr  loci to be characterised  in this collection.

All the themes were embellished by about 100 posters covering many other aspects of powdery mildew research, and ample time for viewing stimulated lively discussion and the establishment of new contacts. The Organising Committee and the Local Arrangements Committee are to be  congratulated on pulling together this event, first conceived in a bar after a few glasses of wine. Personally, I would like to thank the BSPP who made a generous contribution enabling me to attend the conference.

Chris Ridout
Sainsbury Laboratory & John Innes Centre, Norwich