GETTING blight fungicide where it is needed at full canopy has always been a source of anguish for potato growers no matter how favourable the weather.
Trials both in England and Scotland now suggest that stem blight could be more easily targeted by fitting dropleg attachments during the blight spraying season.
The agrochemical manufacturer Cyanamid commissioned research at the Scottish Agricultural College, Morley Research Centre and at Harper Adams Agricultural College after users of its fungicide Invader (dimethomorph) asked for advice on application through air-assisted systems.
Conventional, air-assisted and dropleg systems were compared for spray coverage at differing canopy heights and for subsequent blight infection. Perhaps unsurprisingly, lower canopy and stem coverage was improved by using the dropleg spray system, and it resulted in less infection being found in the potatoes where blight fungicide was applied through droplegs.
"The air-assisted and conventional systems were very good at depositing droplets on top of the canopy," says Antony Goulds, of Cyanamid. "The dropleg put about two-thirds on top and about a third underneath."
The results (fig 1) from Harper Adams and Morley show air assistance placed the most fungicide on potato leaves, with slightly less through conventional application. Lowest penetration to the lower canopy leaves was through the conventional sprayer and the dropleg deposited more active ingredient in the lower canopy.
However, for getting fungicide on to the plant stems, the droplegs were better overall than the two more common alternatives (fig 2). Air-assistance outpaced the droplegs slightly for coverage of the upper stems but the dropleg had the clear advantage in getting active ingredient to the lower part of the stems.
The rate of active ingredient reaching the plant stems was, however, markedly lower than the fungicide successfully applied to leaves.
Nevertheless, at the SAC site at Auchencruive, Ayrshire, this extra deposition on the stem was reflected by a much lower incidence of stem infection at all heights in the dropleg plots sprayed at full plant ground cover.
Application through conventional hydraulic sprayers also had a slight edge on air-assistance for conrolling disease.
With only 75% ground cover, leaf infection was still curtailed most by dropleg application but air-assistance also performed well in aiding control in the top and middle leaves.
There was evidence in the Morley/Harper Adams work that air-assistance is responsible for more spray going on to the ridges rather than the target plants. Conventional spraying was the least wasteful in this respect, but the amount of active ingredient lost in this way was quite low in all cases.
"If stem blight is significant, the dropleg will provide the farmer with an opportunity to hit it at full canopy," Mr Goulds points out.
Plants used in the experiments were not fully assessed for damage which might be caused by the droplegs. With some varieties such as Cara, the risk of damage would be greater although commercial dropleg machines do now have a break-back capability if resistance is met in the canopy.
Its been a bad blight year. Could a few changes to the farm sprayer have helped? David Millar looks at evidence in support of the dropleg.