By Ken Fletcher

Potato growers are being urged not to risk applying any chlorpropham (CIPC) sprout suppressant to the 2020 crop – even though there may be time to do so before the final use-up date of October 8, 2020.

David Wilson, of AHDB Potatoes, said after the October cut-off, the maximum residue limit (MRL) will immediately drop to a new lower level, which any crop treated in that season is likely to exceed, therefore preventing it from being sold. “If you fog them, you can’t flog them,” is the AHDB slogan to warn growers of the risk.

Under current EU rules, the MRL could drop to the ‘limit of quantification’ of just 0.01ppm after CIPC is banned, however the industry is campaigning for this to be raised, given the background levels of CIPC in the fabric of many stores. AHDB tests of bulk and box stores with different histories of CIPC use found residues ranged from 0.052ppm to 0.36ppm.

“We’re hoping EFSA approve a temporary MRL of 0.4-0.5ppm, which will reduce over time, but should give time for background residues to diminish. This MRL won’t be approved until after the last sale date for CIPC (January 8, 2020),” said Mr Wilson.

“MRL is a real challenge for the industry. Don’t be tempted to squeeze one more CIPC application onto the 2020 crop, because the risk of exceeding the MRL is too high,” he warned.

What are the alternatives to CIPC?

CIPC has previously been used in over 85% of long term potato stores and approval to buy it withdrawn on January 8 2020, with the use-up period to follow.

Here are some of the available alternatives to CIPC and also some actives we don’t yet have approval for, but might do in the future.

Maleic hydrazide

MH is a popular plant growth regulator applied in the field and used to control growth of potatoes in store and those left behind in harvesting operations that might otherwise become volunteers.

It is marketed under a range of brand names, including Fazor, Crown MH, Itcan and Source II. As a sprout suppressant, it has been approved for use in the UK since 1985 and works by inhibiting tuber cell division.

Unlike other sprout suppressing chemicals applied at storage, MH is applied as a foliar treatment, usually about five weeks before defoliation or natural senescence and is absorbed by leaves before translocation to tubers. It has an MRL of 50 mg/kg.

Because it is used as a foliar spray, its efficacy as a volunteer control and sprout suppressant is determined by its level of uptake, which is ultimately governed by canopy and environmental conditions at the time of application. Crop should be actively growing.

The timing of MH application and its effect on sprout control in storage has been the subject of ongoing trials at two AHDB SPot Farms. Sutton Bridge Crop Storage Research is also investigating the minimum effective dose for sprout control in storage.

Trials have shown that MH as a stand-alone treatment does not always completely prevent sprouting but, providing it is applied correctly, it gives good control in situations when potatoes are stored for short periods and the risk of sprouting is low. It also works well for longer term storage when used sequentially with other treatments – for example, when followed by ethylene or an essential oil such as spearmint, or DMN.

Applied in the field

MH must be taken into the tuber and requires good growing conditions and timing

Can be used in combination with other sprout suppressants


This is a plant hormone that inhibits sprout elongation once sprouting has naturally initiated.

Research has shown that performance varies significantly between varieties, as some are more responsive to it than others. As a result, ethylene is more effective at lower temperatures (< 5 °C) and is, therefore, more widely used to control sprouting on potato varieties destined for the fresh pack market, such as Nectar and Melody.

Unlike CIPC, ethylene usage as a sprout suppressant is not governed by a MRL threshold and because it has minimal residual effect, sprouting can resume just a few days after removal from storage, which is a disadvantage for shelf life.

Controlling sprouting with ethylene has proved less popular with potatoes destined for processing to date. That is because, in addition to inhibiting sprout growth, ethylene can also temporarily increase respiration in tubers and the resulting accumulated sugars have an effect on fry colour, which varies significantly between varieties.

In AHDB trials, a gradual increase to 10 ppm of ethylene and subsequent maintenance at this concentration was sufficient to reduce sprouting to commercially acceptable levels for six months in Markies and Russet Burbank, but not Maris Piper. Ethylene treatment had an effect on fry colour but was within commercially acceptable limits for Maris Piper, Markies and Russet Burbank.

More recent research suggests that maleic hydrazide followed by ethylene in store can work quite well, but more research into ethylene is currently underway.

Applied or generated in store throughout the storage period from the first signs of sprouting

Ethylene levels must be built up over time to the holding level.

While reducing store leakage of ethylene is desired it is not as important as ensuring CO2 levels do not reach 0.5% or above.

Zero-day harvest interval

Can be used in combination with other treatments, eg MH

Spearmint oil

This has been part of the sprout suppression tool box since it received full UK registration (Biox-M, MAPP 16021) in 2012 .The active ingredient R-carvone, a terpenoid, is naturally found in spearmint, from which it is extracted.

It is marketed as Biox-M in the UK and comes as a volatile oily liquid, usually applied as a hot fog. Small-scale and commercial trials have demonstrated its efficacy for the fresh pack market and it has been used successfully in some pre-pack supply chains.

It can rapidly burn back existing sprouts and has been reported to have been used successfully for ‘recovering’ commercial packing crops in this way. The warmer temperatures of the processing sector encourage more vigorous sprouting than the cooler temperatures of pre-pack storage and repeat applications are required with consequent cost implications.

Recent work has shown spearmint oil reduced sprout growth in processing crops, particularly when used in combination with other sprout suppressants, for example a single low dose of maleic hydrazide.

And, while previous research at experimental and semi-commercial scale has shed more light on suitable storage conditions for effective control of sprouting with spearmint oil, further research is required, said AHDB.

Applied as hot fog in store, so use approved contractors

Applied when sprouts are first visible

Multiple treatments may be required – the ‘leakier’ your store, the more applications needed

Can be used with other sprout suppressants, like MH



DMN is a volatile emitted by potato tubers that naturally inhibits sprouting and induces tuber cells undergoing division to stay at a resting stage.

Marketed in synthetic form as 1,4Sight, it is EU Annex 1 listed and has recently received national registrations in the Netherlands (2015), Austria (2015), Belgium (2017) and Germany, France, Poland and Ireland in 2018.

In Europe it has been used to effectively control sprouting in commercial processing and pre-pack stores. DMN offers flexibility, both in terms of application equipment that can be used and treatment conditions (six doses at up to 20 ml/t each, over the season).

Because it is a volatile however, stores must remain sealed for 24 to 48 hours post-application. The active is awaiting registration in the UK.

Orange oil

This has EU Annex 1 listing and is currently undergoing registration in Europe. Naturally found in orange peel, it contains the active limonene, which reduces sprouting by burning back active sprouts.

Similar to spearmint oil, it is usually applied as a hot fog and stores must remain sealed for 24 hours after application.

A Belgian project investigating alternative sprout suppressant for the processing and fresh market has provided some of the latest insights into orange oil efficacy on European varieties. Efficacy trials have shown that orange oil initially provided similar sprout control to spearmint oil but that subsequent treatment applications became less effective over time. Fry colour was also tested and no negative effects were observed.

UK approval is anticipated.


This is the volatile oily liquid, available as SmartBlock from approval holders Amvac, it is currently only approved for use in USA, Canada and Israel. EU Annex 1 registration is underway.

It is already used as a flavouring additive in the food industry. As a sprout suppressant, it acts on actively growing cells and gives long periods of residual sprout control. It is best applied as a hot fog.

Efficacy trials at the French research institute, Arvalis, have shown that timing of application and temperature conditions in store have an effect on the efficacy of the active.

At the lower tested temperature (4 °C), a single application (100 ml/tonne) at dormancy break was sufficient to control sprouting over eight months. At the higher tested temperature (7.5 °C), three application at the same dosage were necessary.

Tubers with longer sprouts are likely to require an increased dosage. Trials at Sutton Bridge CSR have shown good efficacy at storage temperatures typical for crisping cultivars. In the USA, 50 % of the fresh pack market are treated with SmartBlock.