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Update on latest blight research

2 August 2011

New strains of P. infestans may be more aggressive when it comes to causing late blight on leaves, but less is known about their ability to attack tubers.

Indeed, given that tuber blight levels have actually fallen at a time when new strains like 13_A2 and 6_A1 have been increasing, it is tempting to conclude they are actually less aggressive in this department.

As part of a wider project (GB Late Blight Populations: monitoring and implications of population changes) funded by the AHDB Potatoes and led by Dr Alison Lees at SCRI in collaboration with SAC and AFBI, scientists are monitoring the pathogen population and attempting to discover the actual threat these new strains pose compared with their longer-established counterparts.

There are several other potential factors that could be influencing the reduction in tuber blight, including the widespread use of more effective blight fungicides, says the SAC’s Dr Ruairidh Bain.

“Nevertheless, it does seem odd that over the past three years we have not seen so much tuber blight, so we need to find out if there is a strain effect involved,” he explained.

The first area he has been investigating is the effect of temperature on zoospore production among different P. infestans strains. This has not been examined for new genotypes such as 13_A2 and 6_A1.

Zoospores are asexual spores that move about using flagella, tail-like projections, and are one of the ways the blight pathogen spreads. They are a key cause of tuber blight when washed down from leaf lesions in periods of heavy rain or irrigation.

“We know that lower temperatures favour zoospore production, but we needed to find out more specifically how the new strains compared with more established ones,” says Ruaridh.

In theory, if one particular strain produces far fewer of these zoospores or even stops producing them at temperatures prevailing later in the season, it may pose less of a threat when the risk of tuber infection is at its highest.

However, laboratory tests indicate that there is little difference between strains “There didn’t seem to any particular pattern emerging either in terms of optimum temperature or range,” adds Ruaridh.

“For 13_A2, optimum zoospore production occurred at 8˚C, while 6_A1 was good at producing zoospores from 4-10˚C. There was nothing unusual about that - we know some genotypes are favoured by a broad range, and others a single temperature.”

As an aside, what was unusual was that all eight blight strains showed optimum zoospore production within that 4-10˚C range, much lower than the generally accepted optimum of 9-13˚C based on work carried out in the 1930s and 1960s.

“The optimum range we identified is very similar to that obtained from more recent work carried out in 2004, the year before 13_A2 was detected in Britain, assessing spore production from leaf material. This could mean that crops are at greater risk of tuber infection on colder nights than previously thought. It will be interesting to see these findings are repeated in further work.”

The next stage is to assess the extent of tuber infection by the different strains in the field.  “By mimicking the infection process in the field we hope to see if different strains produce substantially different incidences of tuber blight,” says Ruaridh.

“SCRI work has shown that with foliar blight, the new strains have a much shorter latent period between initial infection and new spores being produced. That basically means they are much more effective at producing spores.

“That by itself could pose a bigger threat when it comes to tuber blight. It’s these sorts of questions we now hope to answer.”

In parallel, work at SCRI will investigate the ability of new strains to infect leaves at a range of temperatures from 6-20°C to determine whether they are able to cause disease outside the temperature range previously thought to be optimal.

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