11 September 1998

Keeping quality

high in storage

By Andrew Swallow

STORED potatoes win high prices, but only if they are top quality. Blight pressure and late planting this year mean extra vigilance is required, says the British Potato Council.

All crops should be lifted by Oct 20, following a complete blight control programme, effective defoliation, and enough time for full skin-set.

"Growers must not be tempted to go for extra yield by lifting later, even if crops were planted late," warns Adrian Cunnington, operations manager at Sutton Bridge Experimental Unit in Lincs. "Yield in preference to quality rarely pays, particularly with ware."

Studies show early harvest in Sept and Oct greatly reduces store losses, due to lower disease risk, less damage and a drier cleaner crop entering store.

High blight pressure demands comprehensive control and continued vigilance right up to lifting. "A breakdown to blight or other soft rots of only 1% in store means growers have a serious problem," warns Mr Cunnington.

Many crops are less mature too, so a longer skin set period is probable, making timely and effective defoliation even more important.

Defoliation should be as rapid and effective as possible too minimise the skin set period. "The longer the crop is in the ground, and the slower the skin set, the more susceptible tubers are to the invasion of disease."

Good crop cleaning, preferably in the field is important, as excess soil prevents fungicides or sprout-suppressants reaching the tuber, and interrupts air-flows in store. If soil is reaching the store, use a moving head on elevators or box-fillers to avoid soil cones.

First priority loading the store is to ensure the crop is dry. This means immediate ventilation at the highest possible flow rate if field conditions are wet.

Drying may form the first part of the curing period, an important and often underestimated phase of storage. Stores should be held at high temperature (10-15C) for 10-20 days for wound healing so moisture loss and disease invasion is reduced.

During curing, stores can become humid, favouring diseases such as soft rot, silver scurf and skin spot. Afternoon aeration for 2-3 hours is advised to lower the humidity, without lowering temperature.

Only if there is significant risk of breakdown to blight, blackleg, or similar infection should curing be omitted. In such cases storage life is very limited anyway, Mr Cunnington adds.

After curing, crop temperature should be dropped as quickly as practicable. For most refrigeration systems 0.5C/day (0.9F/day) should be possible, until the target holding temperature is reached. Using air below 6C (43F) can damage fry colours, crucial in processing crops.

Having successfully looked after the potatoes for six months, dont fall at the final hurdle, warns Mr Cunnington. Warm tubers gradually to at least 8C (46F), so they are more resistant to bruising when loading out of store.

SPUD STORE STEPS

&#8226 Comprehensive blight control.

&#8226 Timely defoliation for full skin set.

&#8226 Plan to finish lifting by Oct 20.

&#8226 Leave soil in the field.

&#8226 Dry crops into store.

&#8226 10-20 day curing period.

&#8226 Ventilate for low humidity during curing.

&#8226 Automate for most effective use of ambient air.

&#8226 Tuber temp. at 8C (46F) min before out-loading.

Lift early for best quality even if it means a slightly lower yield, urges BPC. Then store crops well to protect prices.