How to build a balanced late blight programme for 2023 – Farmers Guide
The emergence of new, more aggressive blight strains, and those with resistance to key chemistry, poses serious challenges for control programmes ...
Farmers Guide examines the current situation and what it means for control.
Blight is a constantly evolving pathogen that has changed considerably in recent years, becoming more aggressive and potentially harder to control.
The 36_A2 genotype is a case in point. Since first detected in 2017, it has quickly become dominant in Great Britain, accounting for 53% of the population last year, according to Fight Against Blight (FAB) monitoring.
It is particularly prevalent in eastern counties, while the 6_A1 genotype continues to dominate in Southwest England and west Wales.
“The fact that 36_A2 spread so quickly, replacing older genotypes, indicates it is more aggressive and challenging to manage,’ says David Cooke, plant pathologist at the James Hutton Institute.
Encouragingly though, monitoring also shows a marked decline in37_A2, which has some insensitivity to fluazinam.
“A change in use of fluazinam has reduced the selection pressure and the amount of 37_A2 seen since 2018.
“That is preserving fluazinam in the armoury for the future and is a lesson for how we can cope with CAA resistance should it arrive, by using actives in a smarter way.”
Indeed, one new genotype topping the Euro blight ‘watch list’ is 43_A1, which has insensitivity to CAA-inhibitor chemistry (for example, mandipropamid).
While it has not yet been found in Britain, 43_A1 is spreading from Denmark into Belgium and the Netherlands. It may, therefore, be a case of when, not if, it does appear, as most new blight strains are seen on the continent first, Dr Cooke says.
Another, 41_A2, has spread within Denmark, and was found in Fife in 2021. It was not seen in 2022, which was a low blight year, but is another to watch for.
Potato consultant John Keer says new blight strains are influencing control programmes more than ever, and as genotypes change, so does the most effective chemistry. Historically, the sector has been quick to drop certain actives when new blight strains showing reduced sensitivity have appeared, he explains.
In 2006-2007, for example, growers stopped using phenylamides when Blue 13 became prevalent, and latterly, fluazinam use dropped after the discovery of 37_A2.
Such measures were effective in reducing those populations, but the chemistry still has a place in programmes, he says, as both phenylamides like metalaxyl-M and fluazinam remain effective against 36_A2 and 6_A1.
“The more modes of action we can put into programmes, especially those that are not single site actives, the less chance genotypes like CAA-resistant 43_A1 will become established in the GB population.”
Dr Keer has two golden rules when planning blight programmes, for efficacy and resistance management:
- Never use the same product alone or apply blocks of the same product.
- Mix different modes of action into one spray.
Early control with appropriate products is vital, especially given the speed 36_A2 moves through its lifecycle, Dr Keer adds.
“Lesions grow much faster and have a huge sporulation capacity, so epidemics can develop far quicker. You can’t afford to take any gambles with control.”
This virulence has been clearly seen in trials, where plants can take less than 14 days to go from the first sign of infection to 100% necrotic, compared with three weeks or more for older genotypes.
“Apart from onion downy mildew, I don’t know another disease that moves as fast.”
Fortunately, there is a good range of blight products available and most chemistry appears effective against 36_A2, but it must be used responsibly, Dr Keer says.
During rapid growth, stronger systemic products, such as Zorvec Endavia (benthiavalicarb + oxathiapiprolin), or Infinito (fluopicolide + propamocarb), have an important role in protecting fresh growth appearing between treatments, and not just what’s there at the time of spraying, he advises.
Mixing active ingredients with different modes of action and including multisites where appropriate is central to maximising efficacy and managing resistance risk, Dr Keer says.
“Some products are pre-formulated, but most are single mode of action.”
For key products like Ranman Top (cyazofamid), which is often used later in programmes for tuber blight and zoospore control, cymoxanil, orcymoxanil + propamocarb in higher pressure situations, are effective partners, combining the curative kick-back of cymoxanil with the protectant nature of cyazofamid.
Such mixes may be important earlier in programmes too, as tuber blight activity starts from tuberinitiation, therefore risk should be managed with appropriate chemistry from then onwards, James Cheesman of Certis Belchim adds.
“It’s essential to understand the risks in your area and select the most suitable products.”
Shirlan (fluazinam) was central to blight programmes before 37_A2, due to its cost-effectiveness and zoospore activity.
“Rightly growers moved away from it, but Shirlan still has a place, as it performs well against 36_A2 when used in mixtures,” Dr Keer says.
Again, cymoxanil, mancozeb, or stronger chemistry, are suitable partners.
Mr Cheesman says Shirlan can be a good option early on at the rosette stage, if blight pressure is very low.
In green-top crops, Shirlan in mix with cymoxanil, mancozeb, or stronger chemistry also works well later in the season due to the strong zoospore activity.
However, its use is not supported in the last three sprays of stored crops, so Ranman Top is the main option in these cases.
Shirlan’s other main timing is at flowering, where fluazinam delivers additional benefits against sclerotinia, botrytis and alternaria, he adds.
“We’re only talking percentages rather than total control, but it’s a definite benefit. If we lose mancozeb or other actives at some point, fluazinam, in mix with other chemistry, may have an important part to play in programmes.”
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