UNCERTAINTY IN STORE

With most of this year’s crop lifted and stored, potato growers have been waiting for a decision on UK authorisation for DMN, a sprout suppressant already widely used in Europe

Words by: Spence Gunn

Good news: virtually all the UK’s potato crop is now safely in store. Bad news: much of it is at risk of going to waste, through no fault of growers.

This situation, which will puzzle anyone interested in food security or saving waste, let alone in helping growers to do their job, is thanks to the continuing uncertainty surrounding UK authorisation of a storage sprout suppressant, long-used on imported crops that are routinely sold to processing outlets in Britain.

DMN (1,4-dimethylnaphthalene) has been available to growers in the USA, Canada and New Zealand since the late 1990s and is authorised in more than 20 EU countries. The manufacturer, DormFresh, has been working to obtain UK authorisation for more than six years and had been expecting a decision by October. An emergency authorisation (EA) was granted for the 2020/21 season, when growers were unable to use CIPC for the first time – but it came late in the season and was restricted to a limited volume of processing crops.

“Growers are confused and angry,” says NFU Potato Forum chairman Alex Godfrey. “It’s hard to understand why DMN is already authorised and in use in countries with similar growing and market conditions to our own, yet still has not been authorised here.

“We and our colleagues in the other potato supply chain bodies are trying to establish with the Chemicals Regulation Division (CRD) exactly what the issues are and look at possible ways of going forward.”

Neither the industry nor the manufacturer felt it appropriate to ask for an emergency use for a second year, while an application for full authorisation was in progress.

DMN is a synthetic copy of a volatile compound that occurs naturally in potatoes and inhibits sprouting by maintaining the tuber’s metabolism in a dormant state.

AHDB trials over three seasons at its Sutton Bridge crop storage research station showed it is as effective as, and potentially better than, CIPC. None of the currently available alternatives are as effective, risking unacceptable levels of wastage in stored crops.

Imports from the EU of potatoes for processing which have been treated with DMN are already believed to account for 20-25% of processing volume, which the NFU says distorts the market and risks allowing in pests and diseases such as Epitrix species, ring rot and brown rot.

“Apart from the difficulty this situation is creating for storage of UK crops, what’s particularly galling is there is an maximum residue level (MRL) set for the UK which allows imports of treated tubers from the EU,” adds Mr Godfrey.

Growers of processing crops for chips and crisps – which can be in store longer than tubers for other markets but can’t be cold-stored because of the effect on sugar content and hence fry-colour – were particularly hard-hit by the withdrawal of CIPC.

“Not having access to DMN in time for storage this year is going to affect us big time,” says Tim Rooke, Potato Forum vice-chair and a major grower for the processing sector – around half of his 10,000 tonnes of production was historically treated with CIPC.

“To my mind it’s a nonsense to stop growers here from using it, but to allow treated potatoes to be imported. The industry needs to come together and talk to the regulators. Some common sense needs to be applied.”

Mr Rooke says growers producing for frozen-chip processors, such as McCain, would have to rely on ethylene. “It does result in a small rise in sugar content but experience from last year suggests the effects on fry colour are manageable,” he says. “It’s even more difficult with potatoes for crisps, where the impact of ethylene on fry colour is too great.”

He says growers had used spearmint oil as an in-store treatment last year with ‘reasonable success’ where it had followed a growing-season application of maleic hydrazide.

“From my experience, it’s the maleic hydrazide that’s doing a lot of the job,” says Mr Rooke. “Not all of my stores are set up to fog with spearmint oil so the tubers in them only received the in-field maleic hydrazide and were still sprout-free in February.”

Maleic hydrazide is difficult to use, however. Its timing for application is restricted as it needs to go on about a month before senescence, but while the smallest tubers are about 25mm in diameter. It also needs particular weather for best results, not too warm and a 24-hour dry period following spraying. On top of all that, it’s slow and expensive to spray at the required rate of 400 litres per ha.

“Conditions in 2020 were almost perfect for application,” he says. “But this year was a lot trickier. My gut feeling is that it’s not going to be as good, but we won’t know until January or February.”

The maleic hydrazide and spearmint oil regime might work, when conditions are right, for potatoes for chip processing, which can be stored at 6-7ºC, but it’s not effective at the 8-10ºC storage temperature necessary to avoid fry colour issues for those destined to become crisps. Cold storage at 3.5-4ºC is an option for fresh and pre-pack growers.

“Cold storage uses energy,” points

NFU Potato Forum chairman out Mr Rooke. “That’s an issue this year because of the exceptionally high energy costs. In any case, given that we should be working seriously to reduce emissions, we surely need to be less reliant on cold storage – another key reason access to DMN is so important.

“There is nothing else as effective on the horizon; we need an authorisation if we are to remain competitive. For processing growers, in particular, it may be the only solution in years when maleic hydrazide can’t do the job.

“If we end up with processing tubers in shortage because they have not been stored for long enough, our customers will simply turn to imports.” P

An orange oil product, Argos, was authorised in September for UK use as a sprout suppressant in potato stores.

It joins spearmint oil, ethylene and growing-season application of maleic hydrazide as the only sprout-suppressant treatments currently available in the UK.

The active compound is a terpene, d-limonene, extracted from orange peel. It acts on the growth points of newly-emerging sprouts, killing the tissue and preventing regrowth.

It can be applied as soon as sprouts are seen, and treatment can be repeated up to nine times in a season at the maximum permitted dose of 100ml per tonne of tubers. Treated tubers can be removed from store for sale or processing 48 hours after the last application.

AHDB reports of trials in Belgium say orange oil initially provided similar sprout control to spearmint oil. It had no detrimental impact on fry colour.

The authorisation runs until January 2027.