Sulphur puts paid to scab

17 February 2001

Sulphur puts paid to scab

Scab can spoil a good pre-pack potato sample. Sulphur is one solution, as Gilly Johnson reports.

CAN sulphur reduce common scab? Although the scientific jury is out, theres a growing number of potato producers signing up for sulphur. Theyre convinced it is a cost-effective treatment.

Sulphur can be supplied in different forms. Trials have included ammonium sulphate, calcium sulphate or gypsum, and foliar applied sulphur products such as Thiovit-type materials. But no product has produced consistent results with potato scab – until now, says Lincs-based distributor Robert Boothman, of Boothmans (Agriculture) Ltd. He believes he has found the answer, with a sulphur product called Brimstone 80, which is placed in bands along the potato ridge just under the soil surface.

The formulation is based on a combination of 10% calcium sulphate with 80% elemental sulphur, in a clay carrier. The sulphate supplies the instant hit, while the elemental sulphur releases soluble sulphur slowly. Its this dual action which makes the difference, and which is why this formulation works better than any other, he says – particularly when application is targeted near the tuber.

The theory behind sulphur as a scab control agent revolves around pH, claims Mr Boothman. It is accepted that at high pH, scab levels rise. So achieving the opposite scenario – acidifying the soil around the tuber – could inhibit the common scab organism, streptomyces scabies. Sulphur dissolves to form dilute sulphuric acid in the soil moisture, so reducing pH. There may also be a direct anti-bacterial effect from the sulphur, but this is uncertain.

Commercial attempts to exploit the sulphur effect on soil pH have had mixed success. Sulphate fertilisers, including ammonium sulphate, are not the answer, says Mr Boothman, and this opinion is shared by some scientists and other specialists.

"There are three reasons why these dont work," argues Mr Boothman. "First, theres not enough sulphur in them to start with. By comparison, Brimstone 80 is providing a massive hit of sulphur. Second, the sulphate part is highly soluble, so it leaches out rapidly, and the useful effect vanishes. Third, fertiliser applications are usually overall treatments, and not targeted to where the pH effect is needed."

Mr Boothmans sulphur "pastilles" – lentil-like, small, semi-hemispherical prills – win out because the elemental sulphur is released slowly, which prolongs the pH effect. A company trial in Cambridgeshire, analysed independently, recorded a season-long reduction in pH (fig 1), after treatment with Brimstone 80. The elemental sulphur is degraded by a combination of moisture, warm temperatures and microbial action.

Application is critical. Company trials show that best results come from a continuous, concentrated narrow band of pastilles placed along the top of the ridge just under the soil surface at, or immediately post planting. Its not a difficult operation; existing planters or power-ridgers can be easily adapted. The company offers advice on how to convert equipment, will sell an applicator, or instead will apply the product as a contract service.

Recommended application rate is 75-100kg/ha, depending on the soil type and scab risk. Interest has been great, says Mr Boothman. "We started four years ago and did 30ha; results were excellent. The second year we went commercial, the following season we treated 1% of the national acreage and last year this doubled to 2%."

The scab effect is complemented by improvements in tuber size, with one trial showing 28% more tubers in the 50-85mm fraction, claims Mr Boothman – even in non-sulphur deficient soils. This indicates a nutrient effect, he says.

Cost of the Brimstone 80 is between £12.10-13.80p for each 25kg bag, depending on quantity ordered and time of ordering. Contract application adds another £11.15/ha; its available within a 50m radius of Bourne, Lincs.

There is US trials work which points to sulphur improving scab control and skin finish, says potato specialist Barrie Florendine of UAP. "And we have anecdotal evidence for cost-effectiveness in the UK. However, water remains the major factor in scab reduction."

He adds a caution: "Better scab control is most welcome, but it doesnt guarantee an increase in crop value. Reducing scab from 50% down to 25% is unlikely to improve value – but reducing it from 5% to 2.5% certainly would."

Yorks-based Keith Lonsborough, also with UAP, has been working with sulphur applications for the past seven years. He uses Canadian product Tiger 90, which is a sulphur pastille in clay, like Brimstone 80. Price is about £12.50/25kg; application rate is 40-50kg/ha depending on soil type.

"Of the growers using sulphur, Ive never had anyone say they wouldnt do it again," says Mr Lonsborough. "We do see better skin – treated crops have a bright clean finish that points to a fungicidal effect."

&#8226 PSD regulations require that any application of elemental sulphur should be made to remedy nutrient deficiencies only, and not used as a pesticide. Any benefits with scab control are therefore not claimed by the supplier.

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