Soil bacteria research could cut water use for common scab control in potatoes

Common scab in potatoes could be suppressed using less water in the future thanks to new molecular DNA technology helping researchers to understand the disease better.

Around 88m cubic metres of irrigation water is used every year to control common scab. One of the aims of a DEFRA-funded LINK project was to reduce the amount of water used, while still achieving control, Richard Thwaites of the Food and Environment Research Agency told growers at the Potato Council‘s West Midlands Potato day.

The starting point of the research was gaining better knowledge of the bacterial species that caused the disease. Traditionally it had been thought that a single species, Streptomyces scabiei, caused common scab, he explained.

But molecular technology advances had found that at least six commonly found Streptomyces species in the UK could cause scab symptoms. Indeed, usually several species were often present in a field, or on a single tuber or scab lesion, he said. “Each species is ubiquitous across the country.”

Control of common scab relied on suppressing these pathogenic species during the critical four week infection period around tuber initiation, he explained.

Irrigation was typically used by growers to achieve that, and the research showed using irrigation successfully reduced the amount of pathogenic Streptomyces species present.

That wasn’t a surprise, but the researchers also measured the total levels of all Streptomyces species, including the majority that didn’t cause scab, and found that total levels of Streptomyces species increased when irrigation was used.

The vast majority of that increase in beneficial bacteria occurred within four weeks of tuber initiation, Dr Thwaites said. “That potentially means water savings could be possible through less frequent irrigation over a shorter period.”

The finding correlated well with the discovery that in soils where there was a history of scab development there was typically a lower total amount of these beneficial Streptomyces species than in fields where scab levels were lower, he added.

It had got researchers looking more closely at what potential there was for control of common scab beyond simply using water, he said. “For the first time we were not looking at the pathogen, but at the total biology of the soil.”

But identifying whether there were other soil bacteria that could also suppress the pathogenic Streptomyces species even more effectively had been impossible until the past couple of years, he said.

“In a gram of soil there are maybe 10,000 bacterial species, 90% of which you cannot grow in the lab. You can’t get good data on what controls pathogens in the soil, if you can’t identify it.

“However, in the last two years, we have been using a new technique, pyrosequencing, that allows you to describe the soil community in lots of detail.”

Using the results of that analysis, the researchers had been able to identify types of bacteria that react positively to irrigation, he said. “The largest population increase is of Pseudomonas species, which are known to be suppressive to disease, and are used as biocontrol agents in some parts of the world.”

Further research was needed to find out whether this discovery could be used to suppress common scab levels using specific management practices aimed at increasing levels of these bacteria, he concluded.

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