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AHDB Agronomists’ Conference 2016: Blight

16 December 2016

 

AHDB Agronomists’ Conference 2016 – AHDB Potatoes

Blight: The view from the field 2016

Severe blight pressure in the north west from June onwards put control programmes to the test, but a good balance of product choice and spray intervals kept disease under control throughout the rest of the 2016 season.

In the north and borders the disease hardly got going at all, and the north east remained clean until June.

In other areas the situation was much as normal. Initial outbreaks were reported from late May but ensuing good weather meant growers were able to prevent the disease from gaining a foothold.

Speaking at the agronomists conference in Peterborough, John Sarup of SPUD Agronomy and Consultancy said he first sighted the disease under a fleeced crop in Cheshire in early June.

This posed the question whether the outbreak originated from oospores, although it had since been confirmed this was not the case.

“However, you know where potatoes are grown in very tight rotations of one in four years or less you’ll find blight. These are the sorts of conditions that are conducive to oospore production.”

Small patches cut back to ground level helped control the outbreak.

Blight pressure built across the north west following two weeks of incessant rain from 12 June, after the first Smith Period had occurred, that prevented crops being sprayed at this critical time.

“The season underlined the difficulty of targeting and controlling blight once it became established on stems,” said Mr Sarup. “It also demonstrated the use of robust products through rapid growth is vitally important.

 

Established blight

“We were left fighting the disease with expensive products, and we were spraying every three to four days to get on top of established blight and seven days thereafter. We managed to keep it under control, but it was always there.”

In other areas where the disease did not get going, growers were able to adopt recommended timings. “Seven-day intervals may allow the use of cheaper products and provide the greatest flexibility in fitting product choice to weather conditions,” said Mr Sarup.

“With more robust products the interval can be stretched to 10 days, with the ability to tighten it if required.”

 

Right product at right time

He emphasised that it was crucial to use the right product at the right time. The crop was particularly vulnerable during the rapid growth stage.

“That’s the time to spend money on robust sprays,” sais Mr Sarup. “I favour products with good protectant activity to protect fresh growth and kickback properties to remove established blight.

“Where you are on short intervals you may be able to use cheaper products containing mancozeb at higher rates. And any product containing fluazinam is a good protectant and good on tuber blight.”

Agronomists and growers needed to be prepared to switch at short notice to adjust to disease conditions. “Flexibility is key,” Mr Sarup concluded.

 

Sarup’s product choices
Protectant: Shirlan, Revus, Ranman Top
Curative: Sipcam C50, Proxanil, Curzate, Infinto, Invader
Rapid growth: Infinito, Revus +/- Sipcam C50, Invader
Established blight: Infinto + Sipcam C50, Ranman Top + Proxanil, Shirlan + Curzate
Tuber blight: Shirlan, Ranman Top
Alternaria: Invader, Profilux/Zetanil WG

 

Blight 2016 population dynamics

Blight populations remained largely unchanged during 2016, with most outbreaks in British crops caused by the dominant clones 6­­_A1 and 13_A2.

However, one clone, 37_A2, appeared for the first time this season and 33_A2, not seen since 2012, was discovered in one sample.

“2016 was something of a contrast to the previous season when blight levels remained very low,” said Dr David Cooke of the James Hutton Institute.

“We were worried last winter that the mild weather had encouraged growth of groundkeepers and tubers in discard piles. In fact, the season started with a drier spring and we didn’t see many samples until mid June in the west, although it quickly moved across the country.”

Overall, 178 outbreaks were recorded in 2016, compared with 58 the year before. From a total of 664 samples, 454 isolates were purified and 481 genotyped.

Most data were based on samples sent in by scouts taking part in the AHDB’s Fight Against Blight service, said Dr Cooke.

“I am continually impressed by the enthusiasm, interest and support for this programme from agronomist across the country. We could not have carried out this detail of analysis without it.”

 

Dominant clones

Dominant clones 6_A1 and 13_A2 were again the most common causes of outbreaks. “Although first seen in the UK in 2004 and 2005 respectively, they remain very fit in the UK,” said Dr Cooke.

The 6_A1 clone accounted for almost 60% of the total isolates tested, while 13_A2 accounted for a further 20%. “This suggests most primary inoculum is locally generated – discard piles and volunteers need to be managed.”

Miscellaneous clones accounted for 15%. “This group is primarily found in Scotland, and is genetically much more diverse – in some cases four lesions from the same outbreak all come back with different genetic fingerprints.”

Although that level of diversity suggests sexual reproduction, Dr Cooke said no direct evidence of oospore production had been found.

“There is no evidence of a spread of novel genotypes from season to season, or out of the region.”

 

New clone found

A new clone to the UK, 37_A2, was found in 3% of isolates.

“It was first found in the Low Countries in 2015 and has since spread further afield. This year it was responsible for a few outbreaks in the Midlands, where blight pressure was particularly high.

“We may not have paid it much attention had it not been for recent reports of tuber blight problems in other areas caused by the same clone.

“It is one to keep an eye on – we don’t know how fit it is, but we have passed these isolates on to the European IPM Blight 2.0 project to assess its virulence, aggressiveness and resistance to fungicides.”

The 33_A2 strain, which is known to have reduced sensitivity to fluazinam, was also found in a sample of tubers after a three-year absence.

 “Although only an isolated incident, this highlights the importance of sticking to fungicide resistance guidelines and rotating products,” said Dr Cooke.

In mainland Europe, observations continue of another new clone, 36_A2, which has appeared in the past two seasons in crops in the Netherlands and northern Germany.

 

Future blight control

New risk criteria that will greatly improve the reliability of late blight alerts across the UK were outlined at the conference.

Arising from research carried out at the James Hutton Institute and funded by AHDB Potatoes, the Hutton Criteria offers significant advances on the current forecasting model, the Smith Period, which was developed 60 years ago.

Although the Smith Period has been immensely valuable over that time, the need for an enhanced alerting system had been driven by changes in blight pathogen populations and climate, said Siobhan Dancey, PhD researcher at The Hutton.

“In recent seasons, blight has been found on crops before any Smith Periods were recorded. In addition, there has been a lack of uniformity in Smith Period performance across different climatic regions of the UK.

“This suggests it was no longer reliable enough to support precision decision making.”

 

New thresholds

This was confirmed by historical data analysis to examine relationships between reported outbreaks and recorded Smith Periods to try to determine new thresholds.

This was based on over 2000 reported late blight outbreaks supplied by the Fight Against Blight programme between 2003 and 2014.

“These outbreaks were compared with Met Office data to assess how well Smith Periods had performed, if they needed to be improved, and how,” said Ms Dancey.

Detailed experimental work on the effect of relative humidity and temperature on late blight progression, coupled with testing of five different models, showed that existing minimum temperature and relative humidity thresholds did not need to change.

However, reducing the duration of relative humidity at 90% or above to six hours, rather than the 11 used in the Smith Period, proved the most accurate match for conditions leading to blight outbreaks.

 

Better UK performance

“As well as improving forecasts of outbreaks at farm level, the models show we will get a much better spread of performance across the UK,” said Ms Dancey.

“While we had a fair system before, we believe the Hutton Criteria is an excellent one.

“While it will not result in a big increase in the number of days in which alerts occur, these alerts will provide growers with a more accurate indication of risk, providing a more robust early warning system for the future.”

Claire Hodge, knowledge exchange manager for AHDB Potatoes, said Smith Periods had been immensely valuable in assessing blight risk to date.

“However, late blight, caused by the pathogen Phytophthora infestans, remains the single most important disease for the British potato trade.

“Spreading quickly in the foliage, a typical blight pressure season can be costly to the industry.

“We need to continue to optimise our response to changes in climate and developments in technology and the Hutton Criteria provides that timely enhancement.”

 

Blight alert systems

Smith Period

Triggered when the following criteria are met on two consecutive days:

- Minimum air temperatures are at least 10°C
- Relative humidity is 90% or above for at least 11 hours

Hutton Criteria

Triggered when the following criteria are met on two consecutive days:

- Minimum air temperatures are at least 10°C
- Relative humidity is 90% or above for at least six hours

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