Scientists chart the UKs crop disease risk

19-11-2013 | |
Scientists chart the UKs crop disease risk
Scientists chart the UKs crop disease risk

The UK disease map is changing as existing dangers spread, more virulent and aggressive species evolve and new strains from overseas threaten to invade.

There are many drivers behind these trends, yet climate, although difficult to predict, will play a big part both long term and seasonally on disease dynamics.

The biggest challenge will be to delay the onset of disease resistance and to make sure we have a good pipeline of products for the future, says BASF’s Rosie Bryson. “Even if conditions change in the UK, in all likelihood, we would have come across those conditions somewhere in Europe anyway.”

Within climate change, growers will need to deal with more erratic weather patterns which cause extreme disease risks like the septoria epidemic in 2012, says NIAB TAG’s Bill Clark.

Those conditions, combined with the highly adaptive nature of the septoria pathogen and growers continuing selection for high input varieties with a heavy reliance on fungicide use will continue to challenge the industry.

“We’re probably at our lowest point in efficacy [triazoles] and it’s difficult to see how many more mutations septoria can have without it suffering some fitness penalty [compromised activity and spread], so we may have the triazoles for some time to come.”

But target-site mutations are getting more complex and there are fears that other reported mutations could occur in already more resistant strains putting control further under threat.

Potential loss of triazoles and chlorothalonil under proposed changes to EU rules add to the pressure, making anti-resistant fungicide strategies more difficult, he adds.

There are also threats from more aggressive strains of the disease with shorter latent (incubation) periods that have been observed for some time in the UK and Europe. Very little is known about these, and they don’t dominate the population suggesting they carry some fitness penalty, he adds.

New strains

More aggressive and virulent strains of yellow rust are also an immediate problem in the UK as the pathogen continues to successfully overcome host resistance with new races evolving every three to four years.

Key drivers behind disease change

Long-term climate change
Extreme seasonal fluctuations
Fungicide resistance
Varietal susceptibility
Changes in agronomic practice
International travel and trade
Loss of active ingredients

“The driving force behind the emergence and spread of the new races is the new varieties that farmers are growing and the resistance genes they carry,” says NIAB TAG’s Rosemary Bayles.

But the Warrior race, first observed in 2011, which appeared in several countries at the same time, appears to be different and has not evolved from existing races.

“Its DNA is significantly different, which would be consistent with it coming from somewhere else in the world. So the hunt is on to identify international isolates with a similar DNA profile.”

There is a real risk of overseas strains of yellow rust reaching the UK. It is a worldwide disease and airborne spores can travel thousands of miles and be moved around the world via international travel.

While aggressive yellow rust races found in Denmark, Asia and Africa, which grow much faster, don’t have the right set of virulence genes to overcome or infect UK varieties, they could do with a bit of reassortment between races, says Dr Bayles.

And although yellow rust could struggle with warmer summers, there is a threat from high temperature-adapted aggressive strains. These dominate the US population and could be a real problem for the UK in the future, adds Mr Clark.

“We could have yellow rust that infects in the autumn, starts early in the spring then keeps going right through until harvest. Now that’s a real challenge to control.”

But Dr Bayles believes brown rust is a more imminent threat, especially as springs get warmer.

Geographical changes

Although DEFRA survey data shows regional distribution of wheat diseases to be very stable, there is evidence that fusarium and tan spot are becoming more prevalent.

Tan spot, which until 2000 was considered a minor disease, is beginning to establish itself according to Food and Environment Research Agency’s (FERA) Judith Turner.

“It is increasing in its geographical distribution, incidence and severity depending on the season.

“The disease tends to be more prevalent in the South East and as it starts to establish itself and inoculum levels increase, then it will start to spread outwards.”

Although the UK is just on the limit of its geographical climate preference, Mr Clark believes tan spot could progress to be a more severe problem, especially in southern regions.

“It is a disease of min-till systems and is difficult to control as it has a short latent period.”

Fusarium graminearum has also increased both in prevalence and geographically over the past 10 years, adds FERA’s Phil Jennings.

“It is spreading further north, and where it was edging in before it is now establishing itself, such as North Yorkshire. Also year on year, there are more infections in The Wash and Lincolnshire.”

As principally maize trash is the main source of inoculum, the geographical distribution has been reflected in the increased rotational use of maize and direct drilling, while the wind borne ascospores further aid its spread.

Should climate change benefit the pathogen, Dr Jennings believes complying with existing legislative setting maximum levels of the associated DON and ZON mycotoxins in wheat will challenge growers.

Warmer summers could also see the introduction of more grain maize in the UK, further increasing inoculum and the potential to introduce new fusarium pathogens such as F moliniforme and F proliferatum.

Both are found on the Continent and produce fumonisin toxins, which have been detected in cereals from France and Italy and in maize from Spain.

OSR rotations

Short rotations will exacerbate oilseed rape diseases, while large seasonal variations will continue to challenge growers.

Climate change predictions anticipate phoma levels to increase and light leaf spot to decrease, according to ADAS’s Peter Gladders. But over the past six years the pattern has been different with light leaf spot recognised as the number one disease.

“The national average of light leaf spot has been running high since 2008. And between this date and 2011, infection levels have increased from 60-80% with the South East being bad in the past two out of three years.

Short rotations, the lack of resistant varieties and reduced sensitivity to azole fungicides increases the future risk from light leaf spot unless a shift in environmental factors change its prevalence, adds Dr Gladders.

“The debate now is whether phoma or verticillium wilt is number two and that will vary year to year.”

Verticillium wilt levels and severity have increased since it was first picked up in 2007. Levels are now up to 80% premature ripening plants in very bad fields. The DEFRA survey indicates that since 2010, up to 20% of crops are affected each year with the highest incidence in the East and South East.

Climate change could favour the pathogen making variety choice, good agronomy and rotations critical to keep the disease at bay.

“Longer term, we are heading for longer rotations. From German experience this is one in four, or more if infections are bad.”

Dr Gladders also believes growers need to be fearful of wet weather and water movement in terms of clubroot and phytopthora root rot.

Battle to stop the rot setting in

A significant shift in blight populations in recent years has already put potato growers under pressure to adapt to a more aggressive disease.

The proportion of each genotype alters each season, but the now well-established 6-A1 and 13-A2, which are fitter and more aggressive, are able to infect earlier and spread more efficiently in the crop.

The changes to blight populations are slow due to clonal (non-sexual) reproduction, says the Potato Council’s blight expert Gary Collins.

“If oospores, produced by sexual reproduction, became a source of blight infection in Britain we would see a much greater variation in genotypes and the possibility of an even more aggressive, fitter late blight.

“New strains of blight usually appear on the Continent before we see them in Britain, so there is also a benefit of studying what is happening abroad to give us a warning if anything new appears,” adds Dr Collins.

“Keeping blight out of crops remains the biggest challenge as well as dealing with extreme weather. We have been lucky this season in that we haven’t seen the extremes of weather experienced in 2012.

The introduction of new diseases is also a concern with the biggest fear being ring rot from Europe, according to Potato Council’s head of seed and export, Robert Burns.

“Importing seed is the biggest risk and any country that has ever had ring rot has never managed to eradicate it,” he says. “Another threat is zebra chip, caused by a bacterium and transmitted by a psyllid, it would be devastating from a processing point of view.”

But ramularia could be one to watch. Although it is more common in the North, it is underestimated everywhere else.

“It is found in the wetter South West and monitoring has picked up spores in Norfolk. Climate predictions don’t look as though that risk will be any greater going forward. But if fungicides are lost and disease resistances don’t improve, it could become more of a worry.”

Fungicide resistance is a concern, strobilurins are not effective and early work shows similar mutations to those seen in septoria. Additionally ramularia has two mating streams at equal levels in the population making it a higher risk pathogen, she adds.

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