14 September 1999

Getting a leg-up on blight

It has been a challenge to keep blight at bay this year. Could better application hold the key?

KEEPING on top of blight in the potato crop generally centres on deciding which fungicide to use and in what sequence. But the method of application holds the key to greatly improved levels of control, according to trials carried out over five years by seven independent organisations, including ADAS, Harper Adams and SAC Auchincruive.

Their work shows how alternative sprayers could dramatically decrease the blight burden within the crop, while still using conventional chemistry.

"When the concept is explained, people understand why there has to be a more efficient way of applying sprays," says Graham Basil, technical services agronomist with the sprayer manufacturer, Benest. "Crops are generally divisible into two groups – those with a dense canopy and those where the canopy is relatively open.

"Now imagine a sprayer travelling over the dense crop. Spray particles dont reach deep into the canopy. In a crop like potatoes, the foliage at the bottom of the canopy might receive just 6% of the spray deposition on the upper canopy. If were talking about blight sprays, thats unlikely to be enough active ingredient to have the intended effect.

Spray target

"Conversely, in a very open crop like onions, the aerial target for spray droplets is minuscule. Most particles fall on the ground and are wasted. Sprayed horizontally though, the target area is much larger."

Benest sprayers have one dominant feature – a unique drop-leg attachment descending from the sprayer boom. These reach deep into the canopy and eject a 360-degree corona of spray into the heart of the crop, covering all surfaces – undersides of leaves, stems – that otherwise would be missed. But the legs can also be easily removed for the sprayer to function in a conventional manner.

"For contact products such as mancozeb, its really important to achieve maximum deposition," stresses Mr Basil. "Some horizontal movement of the spray can be achieved using cone nozzles and angled sprays but, compared to the drop-leg system, they also generate more drift."

He acknowledges the advances that have been made with systemic products. "Theyre efficient, and the crop benefits greatly from their use. Notwithstanding that, theres a greater benefit if the active ingredient is applied to more surfaces in the first place; uptake will be increased and quicker."

Benests claims are borne out by a trial results, covering all crops including potatoes (blight, aphids), swedes (cabbage root fly), and wheat (septoria) as well as assessments for other field crops, including vegetables and soft fruit. But it is potatoes that provide some of the best illustrations.

A 1997 SAC analysis (see graph) shows how the drop-leg sprayer provided a dramatic increase in control over the conventional approach. Seven days after treatment at full canopy, crops treated with the Benest saw only a fraction of the infection experienced by the conventionally treated crop.

Choice of chemistry is still a key factor in the final outcome. Plots treated with Curzate M (cymoxanil + mancozeb) reduced levels of stem infection to 23.1% at 7 DAT (52.1% with conventional sprayer). Those treated with Invader (mancozeb + dimethomorph) displayed infection in only 11.8% of stems (49.1%).

"Generally, the results indicate that improved deposition, and hence protection, can be afforded with the alternative technique," says Graeme Ligertwood, SAC Auchincruive, who conducted the study. "

Robust

"Interestingly, although the weakness of conventional application was exposed, its robustness also became evident," adds Mr Ligertwood.

Mr Basil adds: "Stem blight is at its worst in the crops latter stages and, although tin-based sprays can be applied to dry up any lesions that appear, reducing its incidence beforehand is preferred. Thats why getting spray inside the canopy becomes so important.

"Also, the fact that the nozzles generate spray below the canopy surface means that there is less drift of chemical. Not only does that make more efficient use of the active ingredient, but it enables applications to be targeted purely to the crop," says Mr Basil.

Confirmation of this comes from a 1994 SAC trial, where the same three types of sprayer had their respective spray drift measured at three heights within and above the crop. Fluorimetric analysis was used to measure the amount of spray drift. Both the Benest Drop-leg and the Hardi air-assisted produced similar amounts of drift, but the total drift from both was less than half that produced by the conventional sprayer.