Remember those August days last year where highs of 34degC were reached and the night-time temperature did not drop below 20degC? Those ‘tropical nights’ were part of the perfect conditions that triggered the worst outbreak of cercospora in sugar beet in the UK.
Cercospora needs warmth and humidity to develop, with temperatures above 25degC, especially combined with humidity over 90%, triggering exponential growth, says Dr Simon Bowen, head of knowledge exchange at British Beet Research Organisation (BBRO).
“In those August days it all came together and we saw symptoms within three or four days.”
Infections tended to peak in inland areas, where tempera tures were highest and on dark soils around the Wissington factory, he adds.
Unfortunately, it also seemed that cercospora was more aggressive in crops hit hard by virus yellows.
“It was clear from the crops we monitored there was an interaction. While there isn’t much proven science, it’s logical that virus-affected plants lose resilience and cercospora infects and develops more easily.”
The other factor that could have exacerbated infection levels was the prime infection period was between the first and second fungicide sprays, when protection levels were at their lowest, he says.
“In Spain and North America where cercospora is an established disease, the two factors leading to poor control are late applications of the first spray and extending the intervals between the first and second spray.”
To help growers understand when infections might occur, BBRO is using a prediction model this season.
Developed in the USA and road tested in Spain, it uses a combination of temperature and humidity to estimate a daily infection value, which is related to the likelihood of symptoms developing in the crop, Dr Bowen says.
Retrospectively testing last season’s weather with disease development on BBRO trials sites suggested it worked equally well in UK conditions.
“We’re going to run that model this year,” says Dr Bowen.
“One slight difference is we’re going to use Weatherquest two-day forecasts to keep ahead of the disease to hopefully give growers more warning.”
The Weatherquest data will be translated to postcodes and communicated via a BBRO bulletin every Tuesday to growers, with an update on a Thursday, possibly via text, he adds.
“When BBRO believes conditions are conducive to disease development, the message will be that it is vital growers check crops now for symptoms, to see whether they need treatment.”
Information will be supple mented by a network of 40 Sencrop weather stations – 10 for each factory area – equipped with leaf sensors which can measure humidity within the canopy and leaf wetness.
Identifying symptoms at the earliest possible stage is crucial for control, Dr Bowen says.
“You’re looking for small dots with a brown/purple halo and a grey centre – you want to capture it at that stage before they start coalescing into larger areas.”
In BBRO trials at Bracebridge, Lincolnshire, last season, Escolta used at precise timings, at early onset of disease and with a tight interval gave good control, albeit at a site where pressure was not as high as in some other areas, he says.
That is despite the finding of resistance to strobilurins and insensitivity to azole fungicides in previous years.
Results “We’re drawing some comfort from those results, but we need to be really precise with our timings,” says Dr Bowen.
With epoxiconazole-containing products no longer available to buy, restrictions on the number of applications of strobilurin-containing fungicides and intervals between sprays, potentially there could be tricky decisions ahead for when to apply fungicides if onset of rust or mildew doesn’t coincide with conducive conditions for cercospora, Dr Bowen admits.
Weather dependent “It’s going to depend on the weather because cercospora is driven by it. If cercospora is not forecast to be prevalent at onset of rust or mildew, do you use one of your Escoltas up or do you keep it back? Personally, I think the latter is a sensible strategy because we’re going to need two strong sprays to control a cercospora epidemic.”
But choosing an alternative product to use in that situation is not straightforward, says Agrovista technical manager Mark Hemmant.
Most of the alternative products also contain a strobilurin and include a label warning to make applications with due regard to current FRAG-UK guidelines for strobilurin (QoI) compounds.
Mr Hemmant’s reading of that phrase is that limits growers to two applications of strobilurincontaining fungicides, although FRAG-UK’s website does not currently have specific guidance for sugar beet.
The global Fungicide Resistance Action Committee also does not specify a maximum number of applications of strobilurins in relation to cercospora resistance but does state they should not exceed 50% of the total number of sprays.
“The wild card is Impact, which contains flutriafol. My understanding is, while there is hardly any data, for an alternative spray for an interim spray for cercospora, which may or may not happen, that’s about all you’ve got, unless you have any straight epoxiconazole on-farm.”
“The other one that’s claimed to have some activity on cercospora is sulphur. It’s labelled for mildew, but could have a small effect,” says Mr Hemmant.
Monitor His advice is to monitor carefully for the onset of disease, whether that is mildew, rust or least likely cercospora, and decide based on the season.
“If onset of rust or mildew comes at the normal timing, you probably stick with your Escolta or Angle spray. If a big cercospora event then happens 10-14 days later you probably use your Impact or straight epoxiconazole if you already have it then and then time your second big gun later.”
One new product that could offer some help, if approved is Caligula (fluopyram + prothioconazole), which is currently approved in potatoes but not yet in sugar beet.
In BBRO trials it gave 20% improved control against cercospora compared with Escolta three weeks after the second spray, says Bayer’s Lizzie Carr-Archer.
“We’re hoping for registration in time for this season, although it is not certain.”