5 February 1999

Sulphur scab control Dutch and

By Edward Long

POTATO growers could halve common scab levels on tubers by applying elemental sulphur to ridges after planting, according to Robert Boothman of Boothman Agriculture, Bourne, Lincs, after extensive trials on a local farm last season.

He expects at least 240ha (600 acres) will be treated in 1999. "It was 18 months ago that I heard potato growers in Holland and Idaho in the US were successfully reducing scab by putting on elemental sulphur," he says.

"Scab is a massive threat to UK crops every season. It spoils skin finish to reduce saleable yield of a pre-pack crop, and increases waste for a chipper. So its economic impact is growing as modern baker and pre-pack markets demand even greater tuber quality and skin finish."

Scab is caused by the fungus Streptomyces scabies and is particularly troublesome where soil pH is high. Many Lincs crops are grown on alkaline silt. Until now the only control option, apart from avoiding susceptible varieties, has been to irrigate at tuber initiation. But few siltland producers have access to suitable water supplies.

"This is why I was keen to see what effect the sulphur prill Tiger 90, which is a trace element and not a pesticide, would have because it lowers the pH by about 1 point. With a band application it offers the first-ever chance to influence soil pH at a target-specific, localised level," says Mr Boothman.

The product contains 90% sulphur in a slow release form and is normally used on cereals and rape. But it is also recommended for brassicas and potatoes. It is activated by moisture when soil temperatures rise, and converted into sulphate which degrades to produce weak sulphuric acid.

A 5cm (2in) wide band on a ridge top provides a cone of reduced pH down to seed level at the critical tuber initiation stage. Unlike fertiliser sulphate which is easily leached, elemental sulphur remains in the soil longer.

"In last seasons trial on scab-susceptible Maris Piper, application rates of 50kg/ha reduced scab by 50%. There was also less corky scab and tubers were brighter than the untreated. With bagged Piper worth about £200/t and a pre-pack sample £260/t, a 50% scab reduction from treatment costing £17/acre could be worth £50/t. For a 20t/acre crop that totals £1000/acre.

"With less trimming and a higher per-bag yield of chips, growers supplying fish and chip shops can expect more repeat orders," says Mr Boothman.

Prototype built

A prototype applicator has been designed and built by Mr Boothman with the assistance of local farmer John Ward. Based on a 3m (10ft) wide scrapped potato planter with folding arms, it has three Microband hoppers each supplying four 14mm (0.5in) fluted rollers feeding to the ridges. It can be adjusted to span six 75cm (30in) or 90cm (36in) wide ridges.

Joe Bratley intends to treat most of his crop at Eaudyke, near Spalding, Lincs, with scab-reducing sulphur this season.

Last years treatment (above) helped a lot in keeping tubers free from scab (below).

Prepack and chip grower won over

AFTER hosting the trials last year and seeing the benefits, local grower R Bratley (Quadring) plans to treat 95% of the 88ha (218 acres) of Maris Piper and Pentland Squire for pre-packing and chipping this year. The only areas left untreated will be comparison strips.

"Our silt is alkaline with a pH of 7.5 or higher, so scab is a constant threat," says Joe Bratley. "In the three years we have been able to apply water at tuber initiation we have sold a higher proportion of crop to the value-added pre-pack market. But we can only irrigate 20% of our crop."

Mr Bratley had not heard of Tiger 90 until asked to host the 28ha (70 acres) of trials, but the idea of reducing scab was highly appealing.

"At lifting I was encouraged to see a difference in tuber skin quality from treated unirrigated areas. If this is repeated this year it will give us the chance to sell the bulk of our crop for pre-packing, and improve quality of tubers going for chipping."

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