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Cut drift to keep potato blight sprays on target

Potato blight risk under dark skies

Drift reduction is extremely important with all applications. Firstly to get the maximum amount of product onto the crop, where it will do the best possible job, and to minimise any loss to the wider environment.

For blight protection, it’s even more crucial that operators achieve consistent coverage across leaf surfaces. 

One of the major issues with drift during application has been identified as gusty wind, which moves the spray pattern, even within the width of the spray boom – potentially leaving patches under-dosed and more exposed to infection.

Syngenta application trials have shown that on a ‘perfect spray day’, with an average wind speed of 1.2 metres/second at boom height (2.7 mph), the actual wind speed varied from 0.1 to 4.3 m/sec (0.22 to 9.6 mph).

Using new technologies, such as nozzle design, sprayer operation and the use of drift retardants, offer operators the chance to mitigate some of the effects of spray drift.

Results of new trials at Syngenta Jealott’s Hill International Research Centre have shown the use of Crusade drift retardant with Revus doesn’t just make droplets bigger per se, but has the potential to focus more of the droplets in the spectrum that is better optimised for potato application, .

Alan Cochran

Using laser diffraction technology to measure particle/droplet size, Syngenta application specialist, Alan Cochran (above), has shown that, by physically altering the properties of individual droplets, the drift retardant appears to reduce the number of ultra-fine droplets that are most susceptible to drift, but at the same time reduce the number of very coarse droplets that could be more susceptible to bounce or rolling off the leaf.

"The net effect is to reduce droplet numbers at either end of the spectrum, and thus producing more droplets into what would appear to be the ideal range for potato leaf blight applications."

The actual result depends on the nozzle being used, but trials would indicate that for nozzles that produce a higher proportion of fine droplets the average droplet size would increase, whilst for nozzles that typically create a higher proportion of coarse droplets, the average droplet size would decrease, reported Alan. In both instances, the proportion of droplets in the optimised size range for a combination of drift reduction and target leaf retention is being increased.  

Laser defraction to measure droplet sizing

The Jealotts Hill research showed that, with the industry standard Syngenta 05 Potato Nozzle, the addition of Crusade drift retardant with Revus decreased the proportion of <100µm droplets (the finest drifty droplets) by 47%.

At the same time the volume median diameter (VMD) of droplets – essentially the average droplet size – only increased by 22%, from 277μm with Revus alone to 338μm, thus maintaining good droplet numbers for coverage. That equated to an average 35% improvement in the nozzle’s drift reduction capability when operated at two to three bar, compared to a standard flat fan.

"Equally – or potentially more – importantly, the drift retardant tank mix showed it tightened the relative span of droplet sizes in the spray pattern – creating more droplets in the optimum sizing."

With the 05 Potato Nozzle, for example, the relative span was reduced by 15% when operated at both two bar and three bar pressure.

For other nozzles the effects were less marked. With 05 Amistar Nozzle, for example, which already reduces drift risk with its air-induction design, the controlled environment trial showed limited benefit from the addition of Crusade with Revus – just 2% improvement in DRT compared to Revus alone.

90% Drift Reduction Nozzles

However, with a categorised 90% Drift Reduction Technology Lechler IDTA 05 (above), the addition of Crusade appeared to reduce the number of extremely coarse droplets – showing an 11% lower VMD and an 8% tighter relative span. There was no negative impact on the drift reduction. 

Droplet mechanics 

Furthermore, with spray droplet mechanics, it is possible to influence where in the crop canopy the spray reaches, according to Syngenta Application Specialist, James Thomas (below). All nozzles produce a wide range of droplet sizes; the differences in nozzle type and design primarily alter the two extremes of large and small droplets and the proportion of droplets of each size category, he pointed out.

James Thomas

"Potato sprayer operators and agronomists often talk about requiring ‘small’ droplets, to permeate down through the potato crop canopy, and avoiding big heavy droplets that could bounce off the leaf or over wet and run off the canopy surface," reported James.

"However, nozzles producing more droplets on the smaller end of the spectrum almost inevitably increase the risk of drift."

The question is what defines ‘small’? In reality, the very small droplets (sub 100 micron) lose virtually all energy and momentum soon after they leave the nozzle tip, and are highly susceptible to drift. In many instances they do not even have sufficient energy to break through the surface tension of an air pocket that sits above the potato crop canopy; they can’t reach the leaf surface. 

That can be further exacerbated if the soil or crop is relatively warm, compared to the surrounding air temperature, when rising air creates a thermal inversion that will lift and carry away these very small droplets. 

At the other end of the spectrum are large, high-velocity droplets, typically categorised as over 500 micron. These have the momentum to more easily penetrate deep into the crop, but can have a tendency to be intercepted by the top canopy and potentially bounce or roll off the leaf.

Hairy covering of potato leaf surfaces

Potato leaves are relatively large and have a hairy surface, which does make them easier to wet and to retain droplets - which was confirmed in a further Jealott’s Hill study by using high speed video photography to visually examine the droplet impacts and retention. However, deeper in the canopy, often in higher humidity conditions after irrigation or rainfall, and where surfaces are already moist, large droplets can over wet the leaf to result in run off and less retention.

Furthermore, angling the nozzle spray pattern, such as the Syngenta Potato Nozzle, has been shown to enable more of the larger droplets to penetrate in under the canopy surface.

"For the 2019 blight season, our application advice would be to use the Syngenta Potato Nozzle where possible." 

"From results we have seen with the Jealotts Hill nozzle research, the addition of Crusade drift retardant in the tank mix with Revus would certainly appear to have a valuable role."

For the future, James belives new 90% DRT nozzle designs offer the opportunity to research better targeting of application in the field. "If the initial research results showing the effects of drift retardants can maximise result of blight fungicides, whilst reducing risk of drift, it’s an exciting potential," he added.