To control late blight effectively, you now need a thorough understanding of the pathogen you’re dealing with.
David Cooke of
SCRI (left) and
Ruaridh Bain of
SAC (right) summarise recent research and changes to best practice
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 is now harder because the genotypes that make up the blight population in UK crops have shifted. Therefore potato growers are dealing with a different type of blight from the one they encountered more than five years ago.
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 life cycle that is generally three to six days long. It reproduces asexually – the greatest number 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, grouped into two mating types, A1 and A2. Until 2006, the A1 mating type was dominant in the UK. But the newer A2 mating type, first identified in the mid-1990s, now dominates the population, which is monitored through a Potato Council-funded research programme.
Since 2003, outbreaks in Great Britain have been monitored by around 350 blight scouts – volunteers who identify potential sources of infection and send in samples to the Food and Environment Research Agency for analysis. They are then passed to SCRI to identify and map the genotypes.
Latest results reveal that the A2 mating type is now present in 77% of outbreaks.
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 so far have revealed that the number of genotypes within the pathogen population has not risen significantly and is dominated by a limited number of clonal lineages. If they were recombining, researchers would expect this figure to be much higher.
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 61% during 2009. Over the same time pink 6_A1 has increased steadily to 23% of the population.
Potato Council-funded trials conducted by SCRI show these two genotypes are very aggressive and fit. This means they survive well in a competitive situation and contribute to the gene pool of the next generation. Each strain is responsible for causing a considerable amount of disease to a plant in a particular situation, 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 suggesting it originated on the Continent.
The origin of pink 6_A1 is unknown, but it appears to have spread from the north of England. It also appears to be displacing blue 13_A2 as its prevalence has grown. Both genotypes are now fairly evenly distributed across the country.
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 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 recombination is taking place.
So it is important to look out for the common signs which 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, available at www.potato.org.uk.
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 will be localised, look particularly aggressive and in need of urgent treatment.
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 are crucial.
Growers must stay alert for signs of blight and control sources of infection, such as outgrade 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 (FAB) and Blightwatch.
What are the implications for spray programmes?
Although growers are 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 robust 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 - Blue 13_A2 is one of the first strains to appear in the crop. It is active at low temperatures and is prevalent in blight populations throughout the growing season. Assume this is the genotype you are tackling.
• Avoid metalaxyl - Blue 13_A2 has been found to be resistant to phenylamide fungicides. Given its dominance early on and prevalence throughout the season SAC is advising that growers avoid the fungicide group altogether. Alternative strategies are to use some of the newer, highly active contact 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 realistic threat-based blight control strategy. Given these genotypes’ aggressiveness, there is now less scope to extend spray intervals.
• 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 it throughout the season. However, growers should be mindful that an integrated approach to disease control is being actively encouraged in EU member states.
Three Golden Rules |
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1 Vigilance – Stay alert for signs of blight and control sources of infection, such as outgrade 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 threat-based blight control strategy
3 Timeliness – An early start to the programme may be necessary, and there may be less scope to extend spray intervals |
