14 August 1999

Organics potential ripe for

harvesting

The UK should be able to meet the swelling demand for organically produced potatoes, argues Branston Potatoes.

THE amount of land farmed organically in the UK is predicted to more than double in the next three years, following a marked increase in consumer demand. But there is a shortage of home-grown produce with the UK importing more organic produce than any other EU member state, according to a recent report by Corporate Intelligence on Retailing.

Although our ability to satisfy demand for some organic foods may be constrained by climate, the potato should be the exception. Growers are not the only people to have recognised this possibility: Branston Potatoes, with packhouses in Lincolnshire and Somerset, are in the process of being accredited by the Soil Association for packing organically grown potatoes.

While the area under production is set to increase, there is also scope for raising production by agronomic techniques which not only improve yields but also reduce wastage, according to Branston agronomist Dr David Nelson, who has been increasingly advising organic growers in recent years.

Ensuring adequate soil fertility by use of manures, legumes and tapping into soil nutrient reserves is the key to a successful organic system. Potatoes are generally regarded as having a high nutrient demand, but this assumption has been questioned over recent years. "For phosphate, potash and magnesium, soil indices above 2.5 are generally sufficient to allow maximum yield," says Dr Nelson. "Only when soils are below these levels can additional manures or permitted mineral fertilisers be justified."

By comparison, nitrogen is a far more mobile nutrient, and potatoes require an uptake of 80-160kg/ha, depending on variety and length of growing season. Most soils would be incapable of supplying this quantity without supplementation from manures or previous cropping with legumes. Growing techniques which minimise overwinter leaching and encourage a strong root system will assist an efficient nitrogen uptake.

It should not be forgotten that no matter how good your husbandry or soil fertility, potatoes are particularly susceptible to either a shortage or surplus of water. In eastern England, the low rainfall will constrain yield on all but the most water retentive soils. In the west, excessive water not only limits rooting and leaf growth but encourages a number of diseases including bacterial rots, powdery scab and blight.

Disease risk

Dr Nelson stresses the importance for organic growers of starting with seed stocks carrying minimal disease burdens. However, diseases vary in significance. Rotting diseases such as soft-rot (Erwinia spp.) and dry-rot (Fusarium spp.) can both be catastrophic, resulting in plants failing to emerge or dying prematurely. Even some of the blemishing diseases such as skin-spot (Polyscytalum pustulans) and black scurf (Rhizoctonia spp.) can also delay emergence and increase wastage, either off the field, or out of store.

The good news for organic growers is that the techniques for minimising the disease burden within organic systems have much in common with those considered as best practice in conventional systems. Clean soils, clean input seed, monitoring of aphid populations, early harvesting, dry-curing and cold storage are all key factors.

Of these, Dr Nelson particularly stresses the value of early defoliation and harvesting. Defoliation can be achieved by several techniques including flailing, wind-rowing, root cutting and use of the Greenburner.

The Greenburner can also be used for weed control just before emergence, but more frequently a combination of harrowing and ridging can produce acceptable results. In addition, a number of simple rotary and tined weeders are now appearing on the market.

However, according to Dr Nelson, they need to be used with care. "All mechanical techniques are difficult to implement in wet weather," he says. "Conversely, in a dry spring, repeated deep cultivations can set crops back by bringing up fresh weed seeds, damaging roots and causing high moisture loss."

Once emerged, potato blight is one of the most serious threats to potato yield and quality. Apart from good hygiene – for example, avoiding re-growth on dumps and volunteers – varietal resistance should be used wherever possible.

Information on the blight susceptibility of most UK grown varieties is available from NIAB. The most popular blight resistors are Sante and Cara, but the latter can be very late maturing. More recently, the variety Cosmos from Agrico has also shown considerable promise in both Holland and the UK, says Dr Nelson.

While the use of copper-based blight sprays is currently permitted until 2003, there should be procedures in place to ensure treatments can be fully justified and kept to a minimum. The use of local weather data to assess blight risk is strongly recommended.

Other agronomic techniques that may influence the level of blight include seed ageing, ridge shape, use of mixed varieties and prompt defoliation of hot-spots with the Greenburner. Another avenue currently under investigation is the use of ellicitors to slow disease progress.

Yield threats

Other threats to crop yield and quality include nematodes, wireworm and slugs, and, again, there are organic control mechanisms available. Potato cyst nematodes can be controlled by a combination of a double resistor variety such as Sante and possibly trap cropping. Length of the rotation will also have a big impact – a minimum five-year gap will control nematode populations and also help reduce soil-borne diseases such as rhizoctonia and black dot.

On land with a history of potato production, Dr Nelson strongly advises organic growers to keep a close watch on changes in the eelworm population by soil tests in the winter after potato cropping.

Wireworm can also cause severe damage to tubers and are to be anticipated within the first five years of ploughing up long-term leys or permanent pastures. The use of green manure crops such as rye and mustard before potatoes may offer some reduction in damage, but early harvesting (before mid-August) is strongly advised on high risk sites.

Slug damage can also be kept low by lifting before early September and planting a resistant variety such as Sante or Desiree. However, even these practices are unlikely to eliminate all slug damage on high risk sites, particularly in a wet season. The high soil humus content often associated with organically farmed soils tends to increase slug numbers but over time, natural predators can help to stabilise the population.

After blight and slugs, Dr Nelson identifies poor storage as one of the main causes of organic potato losses. The benefits from dry-curing followed by a refrigerated holding are a reduction in rotting diseases, better retention of skin finish and control of sprouting, he points out. However, while these techniques can enable organic ware potatoes to be stored well into the following spring, such cold temperatures would increase sugars and damage the fry colour of potatoes destined for processing.

"There is no doubt that growing potatoes organically is a serious challenge to even the most skilled farmers," says Dr Nelson.