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Fine tuning blight control

In less than a decade novel blight strains, dubbed ‘Blue 13’ and ‘Pink 6,’ have instigated a transformation in the UK’s blight population which has seen it change beyond all recollection.

Untreated plot at trial site in France last summer

In less than a decade novel blight strains, dubbed ‘Blue 13’ and ‘Pink 6,’ have instigated a transformation in the UK’s blight population which has seen it change beyond all recollection. 

Despite last year’s low blight-pressure season and in the knowledge that the 2012 situation could be very different with the advent of the newest blight strains, planning for this season’s blight strategies is well under way.

Starting from the position of relative unknowns in 2005, the two genotypes 13_A2 (Blue 13) and 6_A1 (Pink 6) have very rapidly taken the positions of dominance amongst the two mating types, A2 and A1 respectively, which structure the potato blight pathogen Phytophthora infestans. 

According to Malcolm Smith, Agrii’s technical support manager, these dominating and virulent blight strains mean that reducing fungicide rates or stretching spray intervals, are a complete ‘faux pas’ for the season ahead.

“They have very rapidly changed the threat faced by growers and thus our approach to control. We now see, as the norm, much more aggressive blight with a faster reproductive lifecycle.”

Although aware that the UK’s entire blight population will continue to evolve – evident in the recent reports of an emerging ‘Green 33’ strain in continental Europe, Malcolm believes that for this season at least, the prevalence of these two now established, dominating strains (Blue 13, Pink 6) are the primary focus. “This is where we should concentrate our efforts and target with a robust, finely tuned programme.”

In practice this now means we are recommending that growers adopt a seven day spray interval as a standard, he explains. 

“If very bad conditions arise again, such as those in 2007, where we needed to go on as often as we could, we may need to adopt even shorter spray intervals of four or five days,” he believes.

“However there are a lot of products on the market at present which have a minimum seven day spray interval, restricting their use in close succession.” For this reason Malcolm advocates an approach of alternating sprays rather than adopting a system of blocked sprays. “Stopping us falling foul of legislative issues.

“It’s also good practice from a resistance management point of view, helping to get the best out of the products available.”

Discussing the principles for this year’s blight spray programmes Malcolm is all too aware that no blight programme will be the same. “Get 20 agronomists in a room and they will all have slightly different programmes outlined or product favourites, and they will all be right,” he says.

But, he maintains that having blight programme principles in place as we advance through the season is important. “An understanding of blight product attributes and strengths and various modes of action enables us to match the product to the situation and target treatments to match the disease risk, giving us robust control.”

The availability of blight sprays to UK growers is largely considered as being strong, but there is evidence of some fungicide treatments now expressing demonstrably weaker activity, notes Malcolm, commenting upon a trials site visit to France last summer.

“The visit gave the chance to see severe levels of blight infection – a good opportunity given we were seeing lower disease pressure in the UK. The untreated plots were almost gone, with a very limited amount foliage remaining because of the blight pressure,” he notes.

“With new chemistry now on the market there was a clear take home message that some treatments are no longer cutting the mustard – not delivering the control they used to.”

As a supporter of the newer chemistry, Malcolm points to Percos (ametoctradin + dimethomorph), Revus (mandipropamid) and Valbon (benthiavalicarb + mancozeb) as all having demonstrated very similar levels of activity, giving very good control of foliar blight, considering the severity of infection present in the trials,” he reports.

“Our approach this year will see us base the pre-T1 rosette stage spray upon fungicides with activity which can protect against leaf, stem blight and also zoospores, the cause of tuber blight,” he advises. 

“Some may question why at this stage we need to stop zoospores from reaching the soil and contributing to stem blight, but why not? Using product such as Tanos (cymoxanil + famoxadone) gives very good early season protection against leaf and stem blight and also zoospore production, which makes sense.  

“An application of Shirlan (fluazinam) and/or Revus (mandipropamid) or Roxam (zoxamide + mancozeb) and Curzate (cymoxanil) may also come in to the programme at this timing.”

Then, at the stage of rapid haulm expansion, he advocates switching away from, the previously popular, phenylamides as the faster, fitter blight strains have resulted in  almost complete  resistance to this group of chemicals.

“Instead, we advise applying two or three sprays at this stage that offer ‘kick-back’ amongst the product attributes. This helps during the rapid plant growth phase, with the ‘kick-back’ providing retrospective control of incoming infection threatening new growth.”
 
“Infinito (fluopicolide + propamocarb), Percos, Ranman (cyazofamid), Revus and Valbon are the products of choice here,” he advises. 

“In France, Valbon demonstrated its consistent position amongst new chemistry, working extremely well, and continuing to be a key product that we’ll be using across the Agrii group this season.”

Then, alternating protectant activity, we go from Ranman or Roxam, plus cymoxanil if pressure is high, to those with systemicity and curative action, explains Malcolm. “During the first half of stable canopy this will see us applying Valbon or Percos.

“Then in the second half of stable canopy, alternating back to the protectant products and those with a degree of curative activity – Infinito, Revus or Percos.”  

Malcolm reports that although Valbon offers good  curative activity and is suitable for the later stage of stable canopy, it fits more logically early on in the first half of stable canopy, to make the most of its good systemicity against foliar blight.

“We will then tend to use Revus in the second half if Valbon is used earlier because they are from the same chemical group, so it’s important to alternate for resistance management purposes.”

“Finally, into senescence, at the end of growth we will then switch back to Ranman, as the preferable treatment against tuber blight, or Shirlan,” adds Malcolm.

Application advice

Malcolm urges that throughout the entire blight spray programme this season it’s very important to sustain application rates and keep spray intervals tight. “There is no reason to go on with less than the maximum, full recommended rate or to stretch application intervals, he advises. “It is not enough to prevent today’s current blight population establishing in crops.

And regardless of weather conditions he points out that irrigated crops will continue to provide ideal blight conducive conditions which demand a robust and sustained programme with a high level of application efficacy.

The role of adjuvants alongside blight sprays this season offers significant benefits, believes both Malcolm Smith and Dr Tudor Dawkins, Certis’ technical manager.

“We are now recommending that all blight sprays are applied with an adjuvant this season. Banka in particular has shown good trials data and improvement in control and yields achieved  with all blight fungicides,” notes Malcolm. 

“Increasingly the role of adjuvants in blight spray programmes is recognised against today’s blight threat,” believes Dr Dawkins. “Particularly where complete spray coverage isn’t possible. It’s an eventuality that comes with dire consequence,” he believes.  

“In recent trials Valbon has been performing consistently as a top player, even where set against new chemistry now entering the market, with no fall off in activity.  “The addition of adjuvant ZinZan to Valbon applications is a recommendation which has been explored by Certis ahead of this season to ensure the most robust treatment during severe blight pressure,” notes Dr Dawkins. The work follows initial observations of the tank mixture made at Eurofins trials in 2010, indicating enhanced protection to new foliage, controlling blight development.

Plant pathologist and Associate Professor at the University of Nottingham, Dr Stephen Rossall was tasked with conducting independent trials work with the fungicide-adjuvant mix to explore the initial findings observed at Eurofins and substantiate any tank mix benefits. 

The aim was to determine the full impact of ZinZan on redistributing Valbon to unprotected growth and to evaluate the systemicity of this blight fungicide treatment with other industry standards and an untreated control.

He explains that three and five days after applying the various fungicide treatments, upper leaves immediately above the sprayed foliage were inoculated with a virulent culture of 13-A2 blight spores. “Then seven days after inoculation, measurements of the blight development in terms of the diameter of blight lesions, were used to determine if there had been any fungicide movement into the untreated growth.

“Significantly less blight was recorded in the unsprayed upper canopy leaves for those plants that had been treated with Valbon plus ZinZan,” he noted. 

The addition of ZinZan to the Valbon fungicide formulation appeared to have enhanced the systemic movement. “With ZinZan thus delivering a beneficial effect on improving systemic movement and uptake of Valbon, giving protection to newly emerging leaves.”

“Findings corroborated field performance, with Valbon + ZinZan acting to improve the window of crop protection, with a degree of control on newly emerged foliage,” comments Dr Rossall

“A preventative test, comparing sprayed leaves which were inoculated with blight spores also revealed Valbon plus ZinZan as achieving the lowest level of blight development alongside one other treatment.

“The addition and subsequent effect of ZinZan is an important development,” notes Dr Dawkins, adding to Valbon’s complementary ‘kick-back’ activity against leaf and stem blight. “And helping to get the blight fungicide material in to the plant and up the stems.”

ZinZan is approved for use alongside Valbon at all labelled growth stages of application, right up until seven days before harvest. This differs from other adjuvants that are only permitted until tuber initiation.

“The findings have been impressive, further illustrating the product competence – particularly under high blight pressure conditions where spray intervals have been stretched or rainfall is imminent,” believes Dr Dawkins.