The loss of CIPC for potato sprout control in 2020 has required growers to consider a more integrated approach to long-term potato storage.

Before the ban in 2020, 80% of CIPC application in the UK was to stored potato crops and applied as a hot fog, it provided a cheap and effective means of controlling sprouting in storage since the late 1960s.

However, concerns about the potential toxicity of CIPC residues in food led to the European Commission recommending it for non-renewal.

See also: Four critical months to minimise OSR yield losses

We look at some of the alternative products to suppress sprouting in stored potato crops.

Issues with alternative products

In the fresh sector, lower storage temperatures of 2.5C-5C reduce sprouting pressure and alternative products – such as the spearmint oil-based Biox-M and ethylene gas – are accepted by the customer and are effective and widely used.

But there are limitations for the processing sector and in particular crisping, where you may have to store at higher temperatures [of 9C-10C].

There are also issues with length of storage for the processing sector where producers are storing from a September or October lift right through until July in some cases.

For processing stores spearmint oil would need to be applied several times, which may make the cost prohibitive.

1,4-dimethylnapthalene or DMN

Lincolnshire-based independent potato consultant Simon Faulkner believes the scale of the challenge faced by the processing sector rests on the approval of 1,4-dimethylnapthalene or DMN.

Already on sale on mainland Europe and the Republic of Ireland, British growers have been waiting on its arrival since 1999, when it was first touted as an alternative to CIPC.

In 2021 it was given emergency authorisation for limited and controlled use for potato storage, and although the active substance 1,4-dimethylnaphthalene (1,4-DMN) has been approved in the UK, there is no authorised product.

Spearmint oil

He adds that for packing potatoes, growers aren’t in such a hard place, as spearmint oil is approved and provides an effective alternative.

“If you have a good, efficient store and can cool and cure the crop quickly at harvest, it should be enough to keep things under control long term.”

This is echoed by Branston’s agronomy manager David Nelson, with the potato producer and packer having good results with spearmint oil in stored fresh potatoes for a number of years.

Biox-M is made by French company Xeda and marketed in the UK by Juno Plant Protection. The formulation is 100% spearmint oil and its active ingredient, L-carvone, burns soft tissue in the tuber’s eyes or on sprouts and disrupts growth. 

Branston has reduced reliance on any form of sprout suppression through investment in its box stores, enabling temperature to be controlled well, improving airflow and eliminating potential hotspots.

But where there are any issues, spearmint oil provides a good firefighting tool to halt sprouting and is also useful before packing, burning off any sprouts on the tubers and helping to extend shelf life for retailers.

There are concerns about taste taint when using spearmint oil, particularly from processors, but these concerns have been unfounded in the fresh sector and with about eight years of experience, the signs are good.

“You need to make any application at least three weeks before packing and using it in this way. We haven’t received any customer complaints so far,” adds Dr Nelson.

Ethylene gas

Another potential alternative to CIPC is ethylene, a naturally occurring gas that is introduced to the store using specialist equipment and is thought to disrupt tuber hormones relating to dormancy break.

AHDB Potatoes have been working with ethylene at SBCSR for many years and Mr Briddon says its mode of action can make it unpredictable, as results vary between varieties.

Fry colour is also known to have been damaged from its use on processing crops, but the more that is understood about its use, the more successful results have become.

“In the past couple of years, some processing stores have switched to ethylene. As there is scope for damaging fry colour, it has to be very carefully managed depending on the variety, but it’s working well,” he explains.

Mr Faulkner says much of the issues with ethylene came in the early days of use when understanding of correct use was lacking, with some treated crops suffering with blackheart, poor fry colour and an alleged reduction in cooking quality in fresh potatoes.

Ethylene increases the respiration rate of tubers, so if a crop goes into store when warm and respiring quickly and then treated with ethylene, the combination of factors quickly starves the store of oxygen and potentially exacerbates the risk of blackheart.

“From that, we’ve learned to get the respiration rate down before introducing ethylene to the store and when it is, it’s introduced at low levels and ramped up very slowly,” he adds.

Mr Faulkner says the evidence to suggest poor cookability and texture when fresh potatoes are treated with the gas is largely anecdotal, with work carried out in conjunction with the University of Lincoln at Holbeach suggesting no issues.

“A consumer panel did some taste tests with potatoes treated with ethylene and there was no significant difference between the treated and the untreated.”

Maleic hydrazide

A plant growth regulator used in potatoes for sprouting and volunteer control, maleic hydrazide – in products such as Fazor and Crown MH – is applied just before crop senescence and translocated into the tubers to provide its effect.

On its own, the product can prevent sprouting in a stored potato crop for a considerable length of time, but may need a follow-up treatment in store, so it isn’t a complete solution.

The two sprout suppression options currently approved – spearmint oil and ethylene – provide useful efficacy, but Mr Briddon says SBCSR work shows to optimise their efficacy they should be used in a sequence with maleic hydrazide (MH).

This will also be the case for DMN and orange oil, if the two sprout suppressants are approved in the UK.

“We like MH, because it allows integrated sprout control where two or more varieties with contrasting dormancies are held in the same store. It also reduces the need for in-store application and a crop that has had MH correctly applied will be much easier to handle in store,” adds Mr Briddon.

This puts the active at the heart of future sprout control programmes, but a recent complication has seen a stock feeding restriction placed on treated potato waste, such as outgrades and peelings, due to lack of data on its effect in the stock food chain.

This may limit its used in the short term, with packers and processors practically or economically unable to keep treated and untreated waste separate, so end users are likely to refuse treated crops.

Approval holders Arysta LifeScience and Kreglinger Europe are currently conducting trials to fill in data gaps and Arysta’s technical manager for UK and Eire, Don Pendergrast, tells Farmers Weekly the results should be submitted to regulators later this year.

As a result, it is unclear whether the restriction will be in place for harvest 2020: “It’s difficult to say, but we are hopeful it will be lifted in the medium term,” he adds.

Mr Briddon adds that a continued restriction would seriously affect the future of maleic hydrazide if end users refused to accept any treated crops.

“Growers will need to talk to their customers and see whether the factory is able to accept MH-treated crops or otherwise,” he adds.

Increased costs

CIPC cost less than £1/t for each treatment, which was extremely cost effective.

Perhaps the price efficient after CIPC is ethylene gas. It is supplied by two companies in the UK, Biofresh and Restrain, and the two have slightly different cost structures.

With Biofresh, growers can lease the application hardware for about £4,000/year or buy the hardware for about £15,000. After which, gas is supplied on demand by a third party for little cost.

With each unit able to run any size store, or multiple stores on one site, costs are diluted the higher the stored tonnage serviced.

Restrain lease the equipment for each store and supply the ethylene, with a small service charge applied from year two onwards. This covers maintenance, such as replacement of in-store sensors.

The company quotes a cost of £3-£3.50/t for a 6-8 month storage period, which is slightly above three CIPC applications.

The costliest is spearmint oil, which is about £4-£6/t for each treatment, depending on the rate used. A treatment is typically required every 4-6 weeks across the storage period. 

Potential new actives such as DMN and orange oil are likely to enter the market at a similar level, reflecting the considerable cost of registration.

CIPC, due to its low cost and efficacy, has often been used a sticking plaster for poor storage facilities or practices.

Without it, Mr Briddon says growers now have to consider a number of non-chemical actions that will help maintain sprout suppression in an “integrated” approach.

“I think that’s going to be key in the future, no matter what chemicals we have available to us, because it’s a way of controlling costs,” Mr Briddon adds.

Assess storage

Dr Nelson agrees that strides have been made on the quality of stores, but it is a good idea to look again at how stores are performing and address any issues.

Key areas include insulation, temperature control, ventilation and airflow, with the aim of reducing hotspots and the risk of sprouting.

“In box stores, can you store 5% less? I go to many stores and they are so full that airflow is restricted, and the crop is essentially being cooled from the outside in. Improving stacking patterns can also help enhance airflow,” adds Mr Nelson.

Mr Faulkner agrees that stores need to be better and ensuring they are well sealed and have adequate ventilation is key to getting efficacy from CIPC alternatives.

“Both mint oil and DMN won’t stay around in store long if they are leaky. They are very volatile vapours and for them to be effective, you have to seal the store up for a good 24 hours after treatment.

“If the store is relatively new and sealed well, it should work well, but where they aren’t sealed well, or you start fans up too early, then it won’t work as effectively,” he adds.

Variety selection

An integrated sprout control strategy will start at planting with the right variety selection, with tuber dormancy periods varying hugely between varieties, affecting storability.

AHDB have already started to trial all the main varieties, with one Yorkshire-based experiment carried out last year and two being planted this year to increase the validity of results.

Mr Briddon says dormancy results will be published as soon as possible and will help inform variety selection for long term storage.

It is well known that King Edward and Maris Piper break dormancy early and often drive sprout suppression programmes, but very little is known about a number of other varieties.

“Some of them, particularly in the processing sector, have very long dormancy periods and can be stored for several months without chemical. Markies for processing and Taurus and Brooke for crisping are examples,” he explains.

Although it is not possible for most growers to suddenly switch varieties on the same scale, it will be prudent to start thinking about future contracts to ensure long dormancy is used to reduce the need for sprout suppression in 2020.

Segregation is also a key issue, with short-dormancy varieties such as King Edward sometimes prompting the unnecessary treatment of a whole store, when other varieties lay dormant.

“That is one area we could see maleic hydrazide being particularly useful. If we put it on the short-dormancy variety, could we leave the rest of the store untreated with sprout suppressant?” asks Mr Briddon.