15 March 2002

Tuber blight prevention is top priority

Potato agronomy – get it right and this most responsive

crop will reward you with high output and high returns. Get

it wrong and you may end up with a worthless pile of

stockfeed. This special feature aims to help you achieve the

former with articles on crop nutrition, seed management,

slug and weed control and computer aided agronomy. But

first we dedicate a page to that most devastating of potato

pathogens, blight. Edited by Andrew Swallow

FAIL to control tuber blight and total crop loss can occur. Yet many growers still do not spot the warning signs or take control measures early enough, says ADAS.

"Tuber blight is what you must not get – it is the end of the world for a potato crop," says national potato specialist Denis Buckley. "Everything should be focused on making sure this doesnt happen."

Keeping foliar and stem blight out of the growing crop should be growers first aim and that starts with eliminating sources of inoculum. Seed, volunteers and even oospores are blamed by growers for primary infections but one source remains responsible for the vast majority of blight outbreaks, he maintains.

"Dumps, dumps and more dumps are the sources of blight infection, year after year."

Two or three applications of glyphosate is the most reliable method of killing off growth on a dump, with the first application when the dump starts to green over.

"It wants to be on before any crops start to emerge, and come back a month later," stresses Dr Buckley.

In-crop blight sprays should aim to be kept to seven-day intervals, starting at 15-20% ground cover and continuing through to burndown, he advises.

"Interval is one of the most important things in blight control – if it is stretched you will be forced to use more expensive products."

Similarly, blight forecasts should be used as a guide what to put in the tank. If a risk period has occurred then a translaminar product should be considered, especially on blight susceptible varieties such as Maris Piper.

But on more resistant Cara or Nadine sticking with cheap and cheerful mixes should be possible, provided intervals are not stretched as well, he comments.

As soon as blight is spotted in the crop it must be assumed that tubers are at risk from infection and the product should be switched to something with activity against tuber blight.

"If you are serious about tuber blight then you should be using fentin or fluazinam. But these are protectant-only products so they may need mixing to increase foliar activity – I wouldnt use either as a straight," he comments.

Dimethoate and zoxium (as in Electis) have some tuber blight activity and cyazofamid (as in Ranman) has high activity claimed, he adds.

However, for many growers and some others in the industry spotting infection is the problem.

"Most growers recognise foliar blight in the crop but fail to recognise stem blight. This is trouble waiting to happen in terms of tuber blight," he warns.

Whatever the product choice, growers need to move on from the myth that a couple of fentin sprays at the end of season will prevent tuber blight infection, he says.

"You need to be focusing on tuber blight much earlier in the season, for the latter third at least and earlier if foliage blight is seen," he concludes. &#42

Can you spot stem blight? Too many miss it on flower stems and leaf petioles, says ADASs Denis Buckley (inset).

&#8226 Destroy dumps.

&#8226 Seven day sprays.

&#8226 Strengthen if stretched.

&#8226 Target tuber blight at first sign of leaf or stem infection.