30 January 1999

scientists are ro oting for profit

Potato testing is fatally flawed

TWO fatal flaws are attached to soil mapping for potato cyst nematode (PCN), warns Dr Bill Parker of ADAS Wolverhampton. First, most commercial mapping techniques are not accurate enough to be used as a basis for patch nematicide applications. Second, and potentially more serious, is that PCN build-up in untreated areas of infested fields could be extremely rapid. This could put long-term potato production at risk.

Rapid advances in global positioning technology have made PCN sampling easier and faster, and this has encouraged the vogue for mapping and patch treatment. But the industry has taken a wrong turning, insists Dr Parker. His warnings are backed by a number of nematicide scientists.

"Sadly, little account has been taken of the biological impact on PCN of management based on patch treatments. This is potentially dangerous." The fact that many growers are finding PCN problems increasing over the rotation adds weight to his warnings.

First, Dr Parkers criticisms of mapping techniques. He does not deny that knowing more about the location and level of PCN infestation in any given field is a good idea. But the cheapest and most common sampling technique – taking points on a field using a 100m intersecting grid – is potentially the most inaccurate, he says.

"Such simple point maps have no practical value in the tactical management of PCN. However, if repeated over the years they could be useful in tracking trends in population multiplication or decline, but only if the sample points are accurately re-located in subsequent years."

Block maps, where the field is divided into equal units (usually 1ha) and a standard W sampling pattern used within each, are a more accurate way of crudely establishing high and low areas of infestation within a field, says Dr Parker. However, the results are not good enough to be used as a basis for deciding patch treatment with non-fumigant nematicides.

"That said, the block approach may be sufficient for identifying areas for fumigation, because the high cost of treatment means that only areas definitely showing very high infestation are likely to be treated."

Second, the long-term implications of patch treatment. "By definition, patch applications will leave areas of low or undetected infestation untreated. But where a susceptible variety is grown, this approach will lead to higher PCN populations than if the whole field area had been uniformly treated."

Where a PCN resistant variety is used, then some protection is afforded against build-up of the Globodera rostochiensis (golden cyst nematode) species of PCN, although yield loss may still occur in the absence of treatment. But there is a heightened threat from the more persistent and difficult species G pallida (pale cyst nematode), because only partial resistance to this species is available. In these circumstances, patch treatment is also a dangerous strategy because it would increase selection pressure for G pallida build-up.

Dr Parkers arguments are backed by Dr Ken Evans of IACR Rothamsted. "For long-term control, the effectiveness of nematicides is much greater at low population densities. This contradicts patch treatment principles. But growers tend to use products only when they expect a yield response in that particular crop; this is a big mistake for long-term control."

The most cost-effective way of exploiting patch treatment is with soil fumigants, used in the autumn on areas of very high infestation, and then following up with blanket granular nematicides over the whole field in the spring.

Dr Parker warns against low rates of nematicide. "With PCN, its highly questionable whether lower infestation requires lower rates of treatment. Full doses are required to cover the extended hatching period, particularly with G pallida. Applying a reduced rate will simply reduce the level of control and allow a higher rate of PCN multiplication and possibly reducing yield."

DNA TESTING

ANYONE relying on resistant potato varieties as their only defence against potential potato cyst nematode (PCN) infestation should beware. They could be creating a serious control problem for the future, according to research at the Scottish Agricultural College.

A common strategy in whats thought of as low risk circumstances has been to plant PCN-resistant Maris Piper. But this potato is only resistant to Globodera rostochiensis – the golden cyst nematode. Maris Piper is not resistant to pallida, which is able to thrive and multiply. Selection pressure could quickly lead to a build-up of G pallida, the pale cyst nematode, which is the more difficult species to control.

G pallida is now widespread covering 67% of the English ware-growing area, either on its own or with G Rostochiensis. It has crept into Scotland – 23% of fields tested this year by the SAC had PCN and in nearly half of these G pallida was identified. Scientists believe this is a result of the industry relying on resistant varieties.

To combat this problem, a test which distinguishes between PCN types is required. However, current testing systems for PCN depend solely on egg counts in soil samples. A new DNA testing scheme, which identifies the two PCN species, is now part of the Scottish Seed Potato Classification Scheme. This DNA diagnostic test will also be offered to ware growers throughout the UK this year – contact the SAC for details.

FUNGAL COCKTAIL

PARASITIC fungi could be used to combat potato cyst nematode (PCN) in the future.

Glasshouse trials at IACR Rothamsted have shown that cocktails of certain fungal species can kill PCN hosts. This effect may be already happening in certain field locations; one site in Spalding has shown a decline in PCN populations following repeated cropping with potatoes and brassicas. Fungal parasites are the likely cause, according to Dr David Crump.

Further research will explore the commercial potential for applications of fungal spores as biological control agents for PCN.

GM resistanCE

ON THE horizon are potato varieties fully resistant to both PCN strains. This would offer major environmental benefits, due to the questionable toxicological profile of some nematicide treatments.

But theres a snag. The technology used to produce the resistant varieties includes genetic modification – a fact which might adversely affect public acceptance. This would be ironic given the potential benefits, says geneticist Peter Urwin.

Pioneering work done at Leeds University has established that the PCN problem could be solved by biotechnology. However, the initial research has used genes from chicken egg whites. To be more acceptable to the public, a plant-sourced gene would be preferable, says Mr Urwin. "We must address the publics sociological and ecological concerns before PCN resistant varieties could be commercialised. But we have now shown it can be done."

New PotatoES

THE consumer now expects new potatoes year-round. It is a tough objective currently met by imports, but Keith Mawson of the Sutton Bridge Experimental Unit has good news for UK growers. While the market for UK new potatoes has been extended to October by second cropping, controlled atmosphere storage can stretch it to Christmas, he says.

Low temperatures with high humidity can slow down deterioration in appearance of new potatoes, but increase accumulation of sugar which can make the crop unpalatable.

Mr Mawsons trial showed very promising results with the with two test varieties, Carlingford and Maris Peer. Low oxygen, and low oxygen/high carbon dioxide regimes, all at about 3.5íC and 98% humidity, were tested on three tonnes of each variety. The results were then compared with potatoes stored under ambient stored conditions.

Skin set was not affected by gas composition in the atmosphere, but to keep the taste quality acceptable to consumers the sugar balance must also be kept.

New potatoes have low levels of sugars glucose and fructose, and high levels of sucrose. Accumulation of glucose and fructose was reduced in the low oxygen and low oxygen/high carbon dioxide atmospheres compared with ambient. Sucrose accumulation increased, but when the potatoes were taste-tested, the change was only detected in Carlingford stored under the low oxygen/high carbon dioxide treatment.

NUTRITION

UNTIL now, the emphasis when measuring potato quality has been on visual characteristics and disease resistance. But potatoes are such an important staple that Dr Finlay Dale and others at the Scottish Crop Research Institute say we should be looking at nutritional quality much more.

Conventional breeding as well as the process of creating transgenic varieties can introduce unexpected variation, so that progeny can have very different nutritional profiles to parent varieties, says Dr Dale.

Potatoes supply about a quarter of daily vitamin C needs in the UK, and nearly half in the USA, so varietal differences could make a substantial difference to nutritional dietary intake.

Crab dressing to the aid of beet

CHITIN HELP

STRANGE but true: ground up crab shells liberally applied as a top dressing and incorporated can dramatically reduce nematode problems and hence Docking disorder in sugar beet. Crab waste also has a helpful effect against other soil pests such as collembola (springtails), and improves seedling emergence by reducing the fungal disease Rhizoctonia solani.

Theres nothing magical about why crab shells have this effect; theres a scientific explanation, says entomologist Steve Ellis of ADAS High Mowthorpe.

"Crab shells contain a substance called chitin, which also happens to be a component of insect exo-skeletons, the egg cases of nematodes and is in some fungal hyphae." Putting extra chitin on the soil encourages the growth of micro-organisms which feed off chitin. The micro-organisms dont confine themselves to the crab waste – they also munch away at chitin in insect skeletons, fungal hyphae and nematode eggs.

Crab waste has been used as an organic fertiliser in certain areas, particularly for the home and garden market. As the crab waste degrades, it also releases ammonia which acts as a fumigant and a fertiliser. The encouragement of chitin-eating organisms could explain why yield effects have been noted.

Funded by the beet growers levy cash via the SBREF, research with crab waste is now under way in field trials following preliminary studies in glasshouses.

"Im surprised how effective crab waste appears to be against nematodes – its just been phenomenal," says Dr Ellis. "The effect appears to be there with PCN in potatoes, too, reducing egg viability. Wed like to do more work on this crop."

The drawback? It takes a huge amount of ground up crab shell, equivalent to about 13t/ha (5t/acre) to generate the highest response. This raises the question as to whether it would be both commercially practicable and cost effective, says Dr Ellis.

However, there may be a way round this. Researchers have seen encouraging results on crop emergence from pelleted seed incorporating a small proportion of chitin. And for those concerned about the sustainability of UK crustaceans if demand were to explode, crabs are not the only source of chitin; it could be manufactured from fungi. Research is continuing this year.

LIQUID BENEFITS

CONVENTIONAL wisdom has it that beet crops dont benefit from starter fertiliser placed with the seed. Not so. New research shows a startling 15% increase in crop ground cover in May when a dose of liquid nutrient, diammonium phosphate, was placed 4cm below the seed at drilling. Thats not all. Root yield went up by about 7t/ha (2.8t/acre).

Another benefit is that plants are bigger at the time when herbicides are applied. This is particularly helpful on peat soils where weeds are competitive and herbicide strategies need to be tough.

The liquid fertiliser had no adverse effects on growth, says Pete Saunders of NIAB. This is probably thanks to the safe distance between seed and nutrient. "The tines on modern equipment are narrow, and this may also have has an effect."

The effect of starter fertiliser was most advantageous on primed beet seed (Advantage seed); the conventional crop showed less benefit, points out Mr Saunders.

The trial has thrown up some puzzling interactions. Even where no nutrient was applied, the action of the injection tines alone increased amino nitrogen content to a "worrying" extent, says Mr Saunders.

"Light and fluffy seedbed conditions may have played a role. Work continues this year. "Wed like to explore which nutrient mix might be best for beet," says Mr Saunders.

Dutch harvest

IF THE wet autumn caused you problems when harvesting root crops in the UK this year – spare a thought for Dutch producers, who have endured the worst root harvest in living memory.

Constant rain and frost have forced growers to abandon all hope of lifting 3% of the Dutch national beet crop, says Jan Wevers of the Institute of Sugar Beet Research, the Netherlands. It is hoped that A and B quota requirements will be met – just. However, C beet will not feature in Holland this year.

"We have heard of one large northern producer who has had to leave all his beet quota – over 1,000ha worth – unharvested," says Mr Wevers. Some compensation is available from the Dutch government, as a disaster fund, for growers in designated areas.

Beet and potatoes left in the ground create future problems for the rotation. Crops cannot now be sprayed off, because conditions are too wet to support any machinery. "It could be difficult when it comes to creating a spring seedbed."

Attempts have been made to modify harvesting machinery, using tracks and stilt boards, which might allow equipment to operate in waterlogged fields. "Growers have been using their ingenuity to try and salvage some beet," says Mr Wevers.

The difficult harvest comes as the final straw in a season when both beet and potatoes have struggled from the start. A wet spring delayed drilling of many crops by up to a month, potato cyst nematode problems were acute, and rhizomania symptoms developed in sugar beet earlier than ever before, says Mr Wevers.

September brought floods. Then sharp autumn frosts sent temperatures down to record levels and damaged crops in the ground. Yield forecasts were down even before the rain started at harvesting. "Some rhizomania trials have been completely washed out," says Mr Wevers.

Problems in Holland will boost the UKs ranking on the European sugar production league table. This year the UK is heading for either second or third position. Last season British growers were in fifth position, behind France, Holland, Benelux and Austria. The comparison is made on the basis of total sugar production per hectare.

British Sugars Clive Francis wants British growers to be at the top of the league table within 10 years. A tough target, but one that is possible, he told the conference. The new contract now being negotiated between the NFU and British Sugar – the IPA agreement – will improve the potential for improvement, he says.

Morley nozzles

BEET growers not wanting to risk spray drift against water courses may be nervous of using the fine spray, flat fan nozzles that are the traditional choice in repeat low dose programmes. The alternative – low drift, air-entrainment nozzles such as Bubblejet types – can be just as effective, says Libby Powell of Morley Research Centre.

"These work as well against broad-leaved weeds. But there could be a drop in control if fine grass weeds such as Poa annua are the target."

Beet tramlining

THERES a revival of interest in tramlining for sugar beet, as growers try to make the most of high output, large scale machinery – particularly in the beet heartland of Lincolnshire where perhaps half the large estates are making the switch. It looks to be a smart move – trials show no sign of any yield or quality penalties when using 12m tramlines, according to Philip Ecclestone of British Sugar. "In 1997, tramlined crops even did slightly better than conventional ones."

Some drills will allow seed rate compensation, adding 20% more seed in rows alongside the tramline. However, results so far show no indication that this practice helps either yield or quality – but more work is continuing.

Volunteer potatO

ONE fifth of the sugar beet area in the UK has a problem with volunteer potatoes. A mix of clopyralid and ethofumesate (Dow Shield and Nortron) proved an effective control in a Norfolk trial by Martin Lainsbury of the Morley Research Centre. Control of volunteer potatoes is important, not only because they reduce sugar beet yields, but to reduce the volunteers in subsequent crops.

In Mr Lainsburys trial, volunteer Desiree tubers were hand-planted at densities of 0, 5, 10, and 20/m2. Volunteer densities usually seen are about 0-10/m2. The potatoes took about three weeks to emerge and the fields had to be sprayed with phenmedipham and metamitron to control other broad-leafed weeds.

Fields were sprayed with two applications of 0.5litres/ha clopyralid and 1.5litres/ha ethofumesate. The vigour of treated beet stayed steady, but crop vigour on untreated areas declined due to competition from the volunteers. The effect was long-lasting; the beet didnt recover even after blight killed all the potatoes halfway through the season. At the higher potato densities the potatoes competed with each other, as well as reducing sugar content in the beet.

Yields were hit hard: five potatoes/m2 reduced untreated beet yield on the sandy loam by 15.9t/ha (6.4t/acre), compared with only 1.3t/ha (0.5/acre) in treated beet. Volunteer control saved £581/ha (£235/acre) in 1997, and the saving on the loamy sand was £770/ha (£312/acre). Even at a price of £70/ha (£28/acre), the treatment is cost-effective and should be part of strategic herbicide planning for beet growers, says Mr Lainsbury.

Rhizo

EUROPEAN rhizomania-resistant sugar beet varieties are not necessarily as resistant in the UK. The first cultivar, Ballerina, to be recommended by NIAB for use in the UK, performed well in Dutch trials on silty polder soils.

However, in the UK the virus predominates on light sandy soils, and when Dr Mike Asher of IACR Brooms Barn tested Ballerina he found that root yields were reduced by up to 24% on severely infested trial plots. The difference is likely to be because shallow rooting caused by the rhizomania further limited each plants ability to take up water and nutrients. The research highlights the need for new varieties to be tested under UK conditions.

Views on GM cropping

IF THE consumer isnt happy about the safety of GM crops, despite their improved quality and storage characteristics, will there be a market for them?

Professor John Beringer, chairman of the Advisory Committee on Releases to the Environment, believes that the public are at present ill-informed about the true risks of introduction of GM crops into the UK.

Even with conventional plant breeding, an unknown number of genes for toxic products and weediness are transferred to progeny, especially where crosses involve exotic plants. But unlike genetic modification, traditional breeding doesnt require the same stringent regulation and approval. In the UK there is essentially already a moratorium on GM introductions, he says.

But public opinion reflects that of vociferous pressure groups. One common fallacy, says Prof Beringer, is that the UK environment is entirely natural and should not be changed. But this environment has adapted to live with human interference – are existing agricultural systems better than any future ones? he asks. Whats needed is sensible debate about how agriculture and wildlife should be managed in the future. Prof Beringer says that the public have to overcome their fears not only about genetically modified crops being released, but put in the mouth and eaten.

Clive Francis of British Sugar insists that they wont take any GM beet until the public are willing to accept products manufactured from it. It seems certain that the public will ultimately decide which GM crops are grown – manufacturers give the market what it wants.