Two aggressive genotypes continue to dominate the GB blight population, with experts warning growers that they need to consider how to achieve effective control if conditions favour the disease this season.
The weather in 2012 was by far the biggest driver for blight with the split season in terms of planting, further exacerbating the problem, explains Potato Council blight expert Gary Collins. “It was very wet and in terms of cumulative Smith periods, it was the second worst year since we started monitoring in 2003.” Smith periods are a measure of the ideal conditions for blight speed.
Last year’s high disease pressure meant more sprays, tighter intervals and a reliance on more expensive chemistry to protect the crop.
Figures produced by the Potato Council put blight spend at around £591/ha in high-pressure situations such as last year, compared with £316/ha in a low-pressure year.
The aggressive blight strains A1-Pink 6 and A2-Blue 13, which are able to infect earlier, sporulate more and spread more efficiently in the crop, are now recognised as the norm.
According to David Cooke of the James Hutton Institute, the frequency of A1-Pink 6 was down from 80% in 2011 to 62% in 2012 and A2-Blue 13 correspondingly increased from 7% to 24% of the 683 isolates genotyped in 2012.
The frequency of the genotypes by country indicated that there was also a geographical difference with more A2-Blue 13 in England and Wales than in Scotland. “This reflected the local frequencies of the genotypes in the previous season and indicates that much of the primary inoculum is locally generated.”
Oospore levels in 2012 remained unchanged at 7%, suggesting that other sources of inoculum are more important, and that rotations in the UK are sufficiently long and populations are declining over time, he says.
Validity of Smith
With studies showing that all genotypes can infect at temperatures lower than the Smith criteria cut-off of 10C, and infection also occurring at shorter periods of humidity than 11 hours at a relative humidity greater than 90%, questions have been raised as to whether Smith periods are still valid?
They certainly are, insists Mr Collins.
“Smith periods are still a good indication and the message is to continue using them, but don’t wait for a Smith period. Go online to the Blightwatch website and look at the data. If the relative humidity is not quite 90% and temperature not quite 10C, but nearly there, that is high risk as well.”
For these reasons the Potato Council, in conjunction with the James Hutton Institute, is looking at the development of a new more complex prediction model that might include other parameters such as wind speed, direction, humidity, leaf wetness and source of infection.
“There is still a lot to do and it could take a while. If it takes up to five years to end up with a robust model we might need a Smith Plus in the interim,” adds Mr Collins.
Researcher John Keer would welcome an updated model that takes into account leaf wetness, which he believes was one of the biggest factors in last year’s epidemic. The other factor was the split planting situation, whereby later planted crops emerging into clouds of spores originating from infected earlier plantings.
“The problem is that while we have a lot of good new chemistry, it does not offer eradicant activity. Therefore, we can’t protect new growth, so late-planted crops were immediately susceptible to blight and even if growers were able to keep to a two-day programme, they still wouldn’t have been able to keep blight out of the crop. It was an oddity of the season that we had such high pressure early.
“Although we have good diversity of blight fungicides, we don’t have systemic eradicant curative type products since metalaxyl use died out due to the resistance of A2-Blue 13.”
Dr Keer believes the biggest lesson from last year was the use of curative plus protectant chemistry in a tank mix. “Without exception across trials in the UK and the Continent, this mix of good curative and protectant scored highest.
“That is a very different set of results than anyone has seen.”
And it didn’t matter which protectant you used in tank mix with the curative. Ranman Top (cyazofamid), Revus (mandipropamid) and Infinito (fluopicolide + propamocarb), all provided good levels of control. But alone they were left wanting, he adds.
That meant reliance on the older chemistry – cymoxanil, propamocarb and dimethomorph – for additional curative activity.
But product choice is only one element of a successful blight programme, the other is spray interval. “And last year there were situations where we couldn’t get on often enough, especially in late-planted crops that were emerging into extremely high blight pressure.”
His advice for this year will be to take good note of what happened in 2012, but only apply those lessons in similar situations in the future.
“Odds are we will be into a more protectant-type season with pressure building towards the end.
“If we do have another season like 2012, lessons need to be remembered and growers need to be in with a curative tank mix earlier in the season, should the need arise,” he adds.
With such high levels of blight last year, there is likely to be a large inoculum source from discard piles, volunteers, tubers left in the ground and blighted tubers, explains Mr Collins.
However, levels of tuber blight in 2012 were lower than expected and the reasons are not yet fully understood. Effective chemistry or infected tubers rapidly rotting in field are possible, he says.
So what advice would Mr Collins give growers this season? “Be vigilant, it’s not a new message but an important one. Also look harder and if you haven’t got blight be careful you haven’t missed it, and be aware of early infection.”
He also urges growers to address outgrade piles and adopt a zero tolerance approach to green leaf, and finally use the new blight guide, produced by the Potato Council.