Learning lessons to beat blight

4 June 1999




Learning lessons to beat blight

BEATING potato blight demands a good understanding of the disease and how best to use fungicides. With severe pressure forecast this summer growers need to exploit the lessons learnt in recent seasons.

Those include the need for close attention to start date, spray intervals and product choice, targeting stem and tuber blight as well as the classic foliage disease, says ADAS root crops specialist Denis Buckley.

"The key to successful blight control is risk assessment and matching the risk with adequate fungicide cover. With large reservoirs of blight lingering from last year, the potential is for another severe blight year."

Risk factors include potato dumps, volunteer potatoes in adjacent crops and infected seed producing early primary infectors, especially under plastic or fleece.

Blight forecasting systems should not be considered infallible, he adds. Smiths Periods remain the best, but only if measured in the approved Met Office way, he says. "Applying Smiths Period criteria to enclosed crop situations will give false positives."

When to start spraying is an imprecise science, he continues. "If you start early spray is wasted because there is more ground than crop. Start late and you will be chasing blight with expensive products for the rest of the season. Its a balance between risk and expense."

Late-planted crops this year will be emerging into a potentially high-risk situation from day one, so early treatment should be considered, he advises.

Spray intervals are very important. "Some growers still believe a 14-day spray interval is adequate whatever. Even in a low-risk season growers shouldnt make that assumption. When spray intervals slip it is very easy for blight to come in.

"When blight risk is high growers should aim at seven-day intervals, product labels permitting. Any delay is then not as disastrous as aiming at 10-14 days and being delayed."

Systemic or locally systemic fungicides should be used initially and right through the rapid growth period which can be well beyond flowering into mid-July. Label restrictions on the number of applications of any one product must be observed.

Stem blight, which is more common than thought, also needs watching for. It is the result of spores washing from leaves onto the stems. Crops with leaves free of blight, but with stem blight beneath are not uncommon, he says.

"Parting the crop to check for stem blight is absolutely essential. Stem blight can easily lead to tuber blight as spores are washed further down the stem to the soil."

Once blight has established in a crop use mixtures of active ingredients where at least one is tackles tuber blight, Mr Buckley advises. Such materials include fluazinam, dimethomorph or tin.

He also warns growers not to rely too heavily on tin products alone. "There is a myth in the industry that tin-based products will completely prevent tuber blight. They may do in a favourable season, but I would not bet money on them doing so." &#42

BEATINGBLIGHT

&#8226 Start early.

&#8226 Tighten intervals as needed.

&#8226 Take care with forecasts.

&#8226 Check for stem blight.

&#8226 Maximise tuber blight products.

TUBERBLIGHT WEAPONS

&#8226 Fluazinam products:

&#8226 Barclay Clobber.

&#8226 Landgold Fluazinam.

&#8226 Legacy.

&#8226 Salvo.

&#8226 Shirlan.

&#8226 Dimethomorph products:

&#8226 Invader.

&#8226 Tin-based products:

&#8226 Brestan 60 SP.

&#8226 Ashlade Flotin.

&#8226 Barclay Fentin Flow.

&#8226 Farmatin 560.

&#8226 MSS Flotin 480.

&#8226 Super-Tin 4L.

&#8226 Super-Tin 80 WP.


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