23 August 1996

Early lifters best all round

Damaged tubers are costing growers a small fortune. John Allan and Edward Long find out how earlier lifting can help reduce the problem

EXCESSIVE internal bruising and/or external damage of potatoes intended for premium markets can render tubers unsaleable.

By adopting the right agronomy to promote earlier harvesting, and taking more care at harvest, such losses are easy to avoid, says Paul Coleman, potato agronomist with merchant group A &#42 Worth, Fleet, near Spalding in Lincs.

Varieties differ in their susceptibility to damage. Marfona, Maris Piper, and Cara can be particularly prone to bruising, especially when tuber temperature drops below 6-7C.

Physiologically aged seed can bring harvest forward so crops can be lifted before tubers cool, he says.

Roger Ward, consultant agronomist for Cambridgeshire-based Fenmarc, agrees. Early lifted maincrop potatoes store better than late harvested ones. They have brighter, cleaner skins and less sign of mechanical damage.

As more of the UK crop is washed after storage for value-added ware markets, skin blemishes and impaired appearance have assumed huge importance, he adds.

Once skins are set tubers should be taken up when soil and weather are most favourable, usually early to mid-September with maincrop, says Mr Ward.

For a timely harvest use well-chitted seed of the right variety, he advises. Balance fertiliser inputs to crop needs, and avoid stop/start growth from moisture stress.

Nitrogen rates can affect maturity, and should be targeted according to variety growth habit, says Mr Coleman. "Varieties like Estima, Marfona, Romano and Pentland Squire can take 40-50kg/ha (32-40 units/acre) more nitrogen than ones like Cara and Fianna, which grow on and on."

At harvest, a good skin set is vital to avoid external tuber damage at lifting time. Early maturing varieties are easy to manage. But later ones need careful burning off, preferably with sulphuric acid, says Mr Coleman.

Fenmarc likes growers to aim for a succession of maturities. Crops are burnt off in sequence and lifted as skin set is achieved. "It is important not to delay lifting longer than three weeks after burning off. Serious tuber deterioration could occur with silver scurf and black dot gaining a toe-hold," says Mr Ward. Soil staining is also a problem, he adds.

As harvest approaches, Mr Ward advises growers to check skin set about every five days. Skin movement between the thumb and forefinger means the crop is not ready to lift as tubers are prone to mechanical damage, disease, and moisture loss in store.

"The ideal temperature for harvesting tubers going into store is between 15C and 18C," he says. Any warmer and high temperature bruising can occur; below 15C and harvesting damage rises.

Stores should be ventilated to remove field heat then reduced to about 15C to minimise rotting risk. Tubers coming in at 18C can soon reach 21C or 22C, a level at which wet rot is a real threat.

&#8226 Choose right variety.

&#8226 Feed crop correctly.

&#8226 Avoid erratic growth.

&#8226 Assess skin set.

&#8226 Lift early.

&#8226 Train operators.

&#8226 Maintain equipment.

Early-lifted maincrop potatoes have brighter, cleaner skins and store better, says Fenmarcs Roger Ward. But good skin set is vital.