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As a plant pathology PhD student at Manchester in the 1970’s I joined the other students and staff on annual research visits to major institutes. I was always amazed that some researchers had devoted their entire working careers to a single disease. Two examples that spring to mind were Michel Perombelon and SCRI who worked only on blackleg of potatoes (now Pectobacterium atrosepticum) and Stephan Buzacki at Wellesbourne who worked on clubroot (Plasmodiophora brassicae), although he later went into TV broadcasting. In light of these thoughts nearly 40 years ago, I found myself at a European Powdery Scab Workshop for the third time. And reflecting on it, I had been studying powdery scab of potatoes (caused by Spongospora subterranea f . sp. subterranea) since I arrived at NOSCA (now SAC) in 1978, so perhaps I was not so different.
Perhaps the fascination of S. subterranea is its intractability. Here is an obligate pathogen that cannot be cultured, it survives for (probably decades) as a tough multi-celled sporeball (or cystosori) and is amazingly well adapted to initiating disease when the correct conditions exist. The plasmodiophorales are difficult organisms to study either in the lab or field, and advances in knowledge have been a series of small steps.
Funders of research projects must be frustrated by the seemingly snail pace of discovery but it is hoped they recognise the difficulty researchers face. So the meeting together of almost all those involved in powdery scab at this 3rd workshop was a fantastic opportunity to both update each other on progress in the last 3 years and to tease out how the new knowledge can advance control of the disease.
Over the years, I have had the misfortune, as a potato adviser, of meeting potato growers who have been forced to stop growing potatoes because of this disease. In Scotland, seed growers fear the disease more than late blight (for example) just because there are no simple solutions. Effective host resistance to powdery scab is the long term solution and there were several breeders or those who evaluate varieties at the workshop. However, considering the array of objectives in potato breeding, powdery scab falls outside them as it is considered a relatively minor problem. In any case, the mechanism of resistance is poorly understood and even the development of markers to assist selection in breeding programmes has not been progressed. A lack of selection for resistance has led to some threequarters of the varieties grown in the UK being susceptible or moderately susceptible. Resistance of tubers to S. subterranea is regularly tested in many countries but the relatively recent realisation that root hair and epidermal cell infection and development of root galls play a major part in the pathogen’s epidemiology has asked awkward questions. For example, the most tuber resistant variety in New Zealand, Swift, produces very large numbers of root galls. The same can be said for Russet Burbank (the world’s most widely grown variety). By growing a tuber resistant variety a grower may be fooled into thinking that powdery scab is not a problem in his fields when all the time, inoculum is building up with sporeball release from root galls. Change to a susceptible variety and the truth is revealed.
One relatively new finding has been that severe root infection can impair root function and lead to yield loss. This is particularly a risk where growing conditions are sub-optimal, perhaps during periods of drought or where a variety has a more limited root structure. It would be good if, knowing the inoculum level, the risk of disease could be predicted. Whilst a reasonable relationship has been found on some occasions, the ability of low inoculum levels to cycle in roots and build up to high concentrations means that if the soil conditions are appropriate for infection, even low initial levels of inoculum can result in severe disease. A PCR soil test is available in the UK (the result of GB Potato Council funding) but detecting even a low level means that there is risk of disease development.
Sessions over the three days of the workshop covered the topics of epidemiology, biology, occurrence and control, with contributions from 5 European countries as well as Australia, Israel, Latvia, New Zealand, South Africa and Sri Lanka. The diversity of climates in which powdery scab occurs demonstrates its adaptability but makes explanation of disease occurrence difficult when the narrow range of conditions under which infection occurs. The final morning was taken up with a round table discussion of the infection process. In some ways, this discussion was depressing in that it threw up more questions than answers. However it did identify the gaps in knowledge and focussed attention on those areas where useful progress could be made. Modern technologies such as PCR detection, and analysis of the pathogen genome are offering insights into pathogen variability (apparently very little in most of the world except South America) and quantification of inoculum during infection and disease developments.
Despite greater understanding, control of the pathogen relies on an integrated approach. This includes a large number of factors and attention to each of the factors is crucial to success. There is no doubt that the few who are dedicated to this disease and pathogen should be meeting again in 3 or 4 year’s time, for this timescale is enough for progress to be made. However, it can be confidently predicted that the pathogen will continue to frustrate for decades yet.
That the three workshops on powdery scab have taken place at all is due to the enthusiasm and dedication of Ueli Merz from UTH in Zurich, Switzerland. He is yet another researcher who has dedicated his life to a disease.
Potato Dynamics Ltd