20 June 1998


Dont allow potato blight the slightest toehold. Adopt a three-dimensional defence, reports Tia Rund.

ALL the indications are that growers could be facing the prospect of another big year for blight stoked by an abundance of inoculum provided by infected groundkeepers and storage dumps.

Last year, however, illustrated that epidemics can be severe even without a high carry-over from the previous crop.

The biggest lesson to be learnt from last season, says Andrew Wells of ADAS Gleadthorpe, is that any areas, however small, that cant be reached with the sprayer, should be killed off now. "Noticeably, in many cases last year blight infection established along the edges of fields, where the canopy had flopped over beyond the reach of the boom.

"Similarly, if you cant back into a corner with the sprayer, it shouldnt be planted. Any inaccessible patch, even around telegraph poles, should be burnt off. Dont provide any unprotected crop for spores to survive on," he warns.

"Once blight is in a crop, you cant get rid of it – you only delay the epidemic. You need to be in the fields regularly. Early identification gives you the chance to burn out if there are foci, and to move to something more appropriate than purely protectant chemicals."

Buffer strips present another potential entry route for blight. Many of the products approved for buffer zones are contact-acting only, so spray intervals may need to be shortened, especially during early growth, advises Mr Wells.

Another common toehold for infection is presented when ponding at the bottom of slopes prevents spray protection from being applied to these areas. By creating barriers to the potential run-off, tied ridges can stop this from happening. "Theres still time in some crops to tie-ridge," he says. "Until the plants meet between the beds, you can put the ridger through. The soil will be quite damp in a lot of cases so should hold the ridges quite well."

If covering the crop spatially is important, protecting it in the third dimension – through the depth of the canopy – is equally so. There are only two ways of doing this, states Mr Wells – using either a conventional hydraulic sprayer with water rates up to 400 litres/ha, or else a sleeve boom to open the canopy which will allow rates down to 200 litres/ha. "Skimping on volume when your workload is putting you under pressure is dangerous."

Keith Dawson of CSC Cropcare agrees that delivering chemical through the canopy is the key to control, particularly given the increasing incidence of stem blight. "This change in the pattern of blight might reflect that not enough coverage is going to the base of the crop. Theres also limited evidence that oospores are surviving in soil to transfer blight from one potato crop to the next."

Dr Dawson notes that a number of crops last year suffered blight despite growers sticking rigidly to a good control programme and an anti-resistance strategy. Quite a few of these instances, he believes, stemmed from the use of inappropriate nozzles. "Low drift nozzles produce too coarse a spray, particularly for contact materials." The new generation of low drift nozzles are an improvement, he adds, but still arent the best choice for potato blight.

"A medium spray quality should be the aim. Adjuvants – for example Bond and Banka – can offer a definite advantage, helping to balance uptake from the systemic ingredient and coverage from the contact component."

Air-assistance is a further benefit in delivering material down into the canopy bottom, but needs using properly to be a real improvement. "Keeping to one setting for the season isnt the way to benefit from the technology," he says.

Early identification of blight gives you the chance to burn out.

GROWERS have been warned that late-planted crops will be emerging when air temperatures are higher than normal and therefore more inoculum is airborne. They are advised to apply the first blight spray at an earlier growth stage than normal.

Higher temperatures at emergence will also encourage more rapid haulm growth than usual, so maintaining appropriate spray intervals is critical. The systemic phenylamide component of fungicides such as Fubol or Trustan is particularly valuable in protecting new growth in this case.

The alternative is to use non-systemics such as Shirlan, but this relies on regular, short spray intervals being maintained.

THE Fungicide Resistance Action Committee (FRAC), which aims to prolong the usefulness of phenylamides by managing resistance issues, recommends their use only as a protectant, and only until the end of active growth, certainly no later than the end of August. It also advises that crops be treated with a total of no more than five sprays of, for example, Fubol 75WP, Galben M, Ripost Pepite, Tairel and Trustan. The Scottish Agricultural College, on the other hand, suggests resistance risk is minimised by using a maximum of three phenylamide applications.

The FRAC-sponsored monitor of phenylamide sensitivity shows that the resistance situation is stable. Last year, 58% of the 70 blight samples tested had some level of resistance. This is roughly the same as in recent years and less than the peak in 1987-91 despite the higher disease pressure and the extensive use of these sprays.