Tackling storage disease in spuds starts with seed

21 July 2000

Tackling storage disease in spuds starts with seed

By Edward Long

POTATO growers are losing £12.5m a year to storage diseases that spoil the quality of the pre-packed crop and impair fry colour for processors.

Yet much of that cash loss could be avoided by paying more attention to detail throughout the production process, from seed selection through to store unloading, says the British Potato Council.

"Disease always threatens so it is impossible to eliminate all losses. But there is ample scope to reduce the drain on profits," says Adrian Cunnington, operations manager for the BPCs Sutton Bridge Experimental Unit in Lincs.

"Markets are becoming increasingly quality conscious, so the goalposts are moving. But growers are managing the crop better and capable of producing what is required, although this is not always achieved."

This summer the BPC has been hosting workshops around the country to help growers and store managers meet the needs of pre-packers and processors.

Pre-packing currently accounts for two-thirds of the fresh crop, with supermarkets dominating the sector. "To achieve the quality demanded growers must do everything right from sourcing seed to grading out of store," Mr Cunnington says.

"Good quality, disease-free seed is essential to get the crop off to a good start. The aim must be to produce a quality product. This must take priority over yield. The next priority must be to get the irrigation right because effective scab control is vital and a stop/start growth cycle should be avoided."

Assuming a sensible blight control programme is implemented, the next vital job is to plan the harvest so skins are set at lifting, without leaving tubers exposed to soil diseases for too long after defoliation.

Mr Cunnington says growers should aim to finish lifting by Oct 20 to provide flexibility to cope with delays in a wet autumn. Lifting priority should be determined and crops burnt off to allow a two to three week gap between defoliation and lifting.

A wider gap leaves tubers exposed to diseases such as black scurf and black dot. A narrower gap increases the chance of poor skin set, leaving the crop vulnerable to silver scurf and skin spot, and increasing the risk of moisture loss. "It is a balancing act, so regular sample digging to check skin setting is important."

Once lifting starts stores should be filled rapidly. A 10-12 day period is ideal. If store fill is extended potatoes are still coming in when the first loads ought to be cooling down, he says.

As soon as the crop is safely in tuber wounds need sealing. For early lifting that is usually secondary to cooling to 3-3.5C for blemish disease control. For later liftings when disease pressure is higher curing is more important. The temperature reduction in store is usually sufficient for the curing process in a low disease/low damage season. If not the store should be held at 10C for seven to 10 days before temperatures are dropped.

Although CIPC remains the main chemical for sprout suppression, growers aiming at pre-pack markets are increasingly relying on temperature control alone. Whilst possible with most varieties, it is not with early dormancy breakers such as King Edward which need a chemical suppressant.

For processing, fry colour is the main quality consideration. Crops need the same in-field management as for pre-packing, and tubers should be mature before the harvester moves in.

"As the crop is stored at a higher temperature, between 6.5-10C, it is vital skins are properly set or an unacceptably high moisture loss could occur. There is less pressure to cool the crop during the early storage phase, but with higher temperatures chemical sprout suppressant is vital, there is no other option. &#42

Getting to grips with stored tuber quality… Adrian Cunnington urges growers to focus on quality throughout the production cycle.

CO2 advice

Research suggests that high levels of CO2 in a store holding a processing crop can adversely affect fry colour. BPC says this could be avoided with daily aeration for 5-10 minutes to flush out the gas. When not blowing with ambient air for temperature control.

STEPSTOQUALITYCROP

&#8226 Select good quality disease-free seed.

&#8226 Match nutrition to crop needs.

&#8226 Irrigate for scab control and to avoid stop/start growth.

&#8226 Adopt a gap-free season-long blight control programme.

&#8226 Decide lifting priority.

&#8226 Burn off to leave a 2-3 week gap before lifting.

&#8226 Lift early to minimise disease risk, fix an Oct 20 harvest deadline.

&#8226 Load into store rapidly and carefully.

&#8226 Cool the crop as soon as possible for blemish disease control.