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Potatoes: understanding blight

Course: Potato pests and diseases | Last Updates: 12th October 2015

Ruaridh Bain
Biography >>

Potato blight is probably the most devastating crop disease in agriculture and costs UK growers £20m a year in fungicides to keep it in check.

But that task continues to be hard due to the new aggressive genotypes of the pathogen that first emerged in 2006 and have continued to dominate the blight population in UK crops.

That has implications for its control, which requires a thorough understanding of how this pathogen behaves.

What has happened to blight?

Late blight, Phytophthora infestans, spreads fast, with a lifecycle that is generally three to six days long. It reproduces asexually – the greatest numbers of wind-borne spores are produced under warm and humid conditions and the pathogen needs green plant material to develop.

The blight population is made up of a number of different genotypes that may be of two mating types, A1 and A2. Until 2006, the A1 mating type was dominant in the UK.

However, Potato Council-funded research has shown that an A2 mating-type genotype (13_A2) came to dominate the population in 2008. An A1 mating-type genotype (6_A1) partly displaced the A2 type in 2011 and by 2013 both mating types were present in equal amounts.

Since 2003, about 350 blight scouts have monitored late-blight outbreaks in Great Britain. These volunteers identify potential sources of infection and send in samples to Fera for analysis. They are then passed to

The James Hutton Institute to identify and map the genotypes.

What is the significance of the shift in populations?

Where both mating types are present, as has proved the case in 15-20% of blight outbreaks in recent years, sexual reproduction may take place. This creates the potential for the pathogen to evolve more rapidly, which may make the disease harder to control.

Genetic analyses to date have revealed that the number of genotypes within the pathogen population has not risen significantly, but is dominated by a limited number of clonal lineages. If they were recombining, researchers would expect a much more diverse pathogen population.

What are blue 13_A2 and pink 6_A1?

Two genotypes have come to dominate the population.

In just four years the prevalence of genotype 13_A2, dubbed blue 13 because of the colour chosen to represent it on the chart (below), rose to 79% of outbreaks, settling back to 53% in 2010.

Another aggressive genotype called pink 6_A1 increased steadily to under 20% of the population in 2010 and then increased to 80% in 2011.

This was partly due to the early inoculum being checked by a dry period early in the 2011 growing season and a strong "founder" effect where many infections were generated by the pink 6genotype and then dominated the later part of the season.  

Since 2011 the frequency of these two genotypes has balanced out.

Potato Council-funded trials, conducted by The James Hutton Institute, showed these two genotypes to be very aggressive and fit. This means they survive well in a competitive situation and contribute to the gene pool of the next generation. Both  6_A1 and 13_A2 caused more disease than other genotypes of the pathogen under a range of conditions, which makes them hard to control.

Significantly, blue 13_A2 appears to have an advantage at cooler temperatures and the period between infection and sporulation is markedly shorter than previously encountered – typically four-and-a-half days.

In 2006, it was dominant in the south-east England, suggesting it originated on the Continent. The pink 6_A1 was also first found in continental Europe in 2002 and has progressively spread across British crops in recent years.

What are oospores?

If two different mating types come into close contact in an infected plant they reproduce sexually, generating oospores.

Unlike asexual spores, these can survive for many years in soil. Evidence suggests oospores have been responsible for many blight outbreaks in parts of the EU over recent years. So far none have been found in UK crops, although it is highly likely some recombination is taking place.

So it is important to look out for the common signs that will indicate that a blight outbreak possibly originated from this soil-borne source. This can be hard to identify, so the Potato Council has produced a guide to help growers on their website.

An early indication of infection from oospores will be many small lesions affecting lower leaves that are touching, or close to, the soil surface.

Infection may rapidly kill localised patches of young plants with stem-based symptoms and look particularly aggressive.

How do we keep blight at bay?

The consequences of the shift in blight populations are that blight will tend to come into the crop earlier and hit harder, so vigilance early on and planning is crucial.

Growers must stay alert for signs of blight and control sources of infection, such as out-grade piles and volunteers.

As well as monitoring your own crops, make use of a good blight monitoring service to keep yourself abreast of disease developments and weather-based risk.

This can best be achieved through Potato Council’s free Fight Against

Blight and Blightwatch services.

What are the implications for spray programmes?

Although growers are now dealing with a different type of blight, the current fungicide armoury, used well, will still give good control.

The difficulty is that these aggressive and fit genotypes will mercilessly take advantage of any lapse in a fungicide strategy.

Well before the season begins growers should discuss plans with their agronomist, but there are some key pointers that all growers and agronomists should now be including to keep crops protected:

  • Start early. Genotypes that are more aggressive develop more quickly and spread more rapidly. Consequently, if all other factors remain constant the blight threat to crops is earlier and more potent.
  • Avoid the active ingredient metalaxyl-M – blue 13_A2 has been found to be resistant to phenylamide fungicides. Given that 13_A2 combines this resistance with elevated aggressiveness, SRUC is advising that growers avoid the fungicide group altogether. Alternative strategies are to include some of the newer fungicides at short intervals for good protection of new growth or to use propamocarb-based formulations, which offer true systemic activity.
  • Tight timings. Spray programmes should build well-timed applications around a risk-based blight control strategy. Given these genotypes’ aggressiveness, there is now much less scope to extend spray intervals.
  • The use of curative fungicides is essential in high risk years, such as 2012, or periods of elevated risk in other years. Use well-timed applications of fungicides with curative activity, preferably within one day of high risk weather. Other factors remaining the same, the period of opportunity for curative activity is reduced with more aggressive genotypes of P. infestans.
  • Robust rates. Follow manufacturers’ instructions and discuss these with your agronomist. Programme weaknesses are more likely to be found out by more aggressive genotypes, so the advice is to stick to full label rates and maintain them throughout the season. However, growers should be mindful that an integrated approach to disease control is being actively encouraged in the EU.
  • Use fungicides with tuber protection activity no later than the end of rapid canopy development. Recent research has shown that isolates of genotype 13_A2 were more aggressive on tubers than those of older genotypes.
  • To minimise the threat of generating within the UK strains that are more aggressive, rotations should be as long as practicable and at least one in five. Groundkeepers between potato crops in the rotation should be well controlled.

Four golden blight control rules

  1. Vigilance – Stay alert for signs of blight and control sources of infection, such as out-grade piles and volunteers
  2. Planning – Discuss your control strategy with your agronomist well before the season begins and aim for well-timed applications around a realistic risk-based blight control strategy
  3. Timeliness – An early start to the programme may be necessary and there will probably be little scope to extend spray intervals
  4. Perseverance – Maintain good fungicide protection until the haulm is dead
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