The bacterial disease blackleg is a perennial problem for potato growers.
Some seasons are much worse than others but there is always infection about and the sight and smell of the slimy stems that result are the stuff of nightmares for seed growers, especially those producing the higher grades.
Essentially Pre Basic 1 (PB1) and Pre Basic 2 (PB2) seed at the very top of the production pyramid should be completely free of the disease. But that is not always the case, with the four years of wet summers leading up to 2012 being particularly challenging.
SAC Consulting potato specialist Stuart Wale told a growers’ meeting at Finavon Hotel that a new project designed to look at the disease had challenged everything so far known about it.
The project, funded by the Potato Council, included James Hutton Institute (JHI), Scottish Advice and Science for Agriculture (SASA) and SAC Consulting as partners.
“We started with the premise that having PB1 stocks infected by the time they had reached the PB2 generation was unacceptable,” said Dr Wale.
It is well enough understood that blackleg bacteria can be spread by machinery, by water-borne aerosol or by infection from mother tubers.
The mother tuber route can be ruled out at the PB1 stage because such stocks are propagated from greenhouse-produced mini-tubers, but all other routes remained a possibility. Over the 2013 season two high-grade seed fields were monitored, one in north Aberdeenshire and the other in the south of the county.
The fields contained several varieties but the study concentrated on Desiree, which over the years has proved to be blackleg susceptible.
Little suspecting the surprise that was around the corner, Dr Wale and his colleague Dr Daan Kiezebrink removed plants for testing periodically throughout the season.
“We found nothing at all until August 16, a week after dessication with Diquat. We found bacteria on the stem base and on the tubers on the north site but how the hell did it get there?
“We did, however, notice that these first signs of infection coincided with the first heavy rainfall of the season. Until then the 2013 growing season had been exceptionally dry.
“The south site showed infection at about the same time,” said Dr Wale.
This link to heavy rainfall led Dr Wale to believe that this was an aerial infection, spread by aerosol.
Raindrops bouncing off infected leaves could pick up bacteria which would then be carried in tiny droplets for some distance.
This seemed a less likely explanation in the north trial, where the whole field was planted with PB1 stock.
The south trial field also had PB2 plots.
The biggest puzzle was still to come, however. Petrobacterium atrosepcticum is by far the most common cause of blackleg-type disease, especially in Scotland, but of the 108 samples sent to JHI for testing none were positive.
Analysis is still under way at JHI to identify the bacteria.
Dr Kiezebrink said: “This could be petrobacterium wasabiae, which is known to be present in the Netherlands, but we don’t know yet.
“We don’t even know if this would show typical blackleg symptoms.
“It may be that it is the more refined analytical techniques available now that are showing up this difference.”
A number of growers have maintained that the resurgence of blackleg has been caused by the banning of sulphuric acid as a desiccant. A separate field-scale trial had been set up to see if this was the case, with two sequential sprays of diquat compared with two of sulphuric acid and one of diquat followed by one of carfentrazem.
The trial sites were first infected with blackleg bacteria to set up a real challenge.
There turned out to be very little difference in infection levels after the first sprays but after the second there were significant variations in daughter tuber infections, with by far the lowest levels on the acid-treated plots.
“This may be right, but the problem is that sulphuric acid is not going to come back,” said Dr Wale.
“I think the main factor with blackleg is that the run of four or five wet years just overwhelmed control efforts.
“Separating PB1 crops from other lower-grade material including PB2 helps, but it is not easy to achieve. Perhaps we need to consider growing PB1 in polytunnels?”