Sprout suppression starts in the field – Potato Review

He reiterated that activities to set the crop up for storage start in the field, with maleic hydrazide (MH), which is applied in the ­field three to ­five weeks before desiccation. MH is ‘the cornerstone’ of sprout suppressing, Andrew said.

“This cost-effective active not only initiates dormancy and thereby inhibits sprouting, but also stops secondary growth and prevents volunteers growing in following crops.”

The strategy is particularly useful if potatoes are to be followed by maize or other crops destined to be used in an anaerobic digester (AD) because Shield Pro (clopyralid) is not permitted for this use. In addition, it is very useful for action against volunteers if the next crop is vining peas, as there is little choice of herbicides for use with them.

Although MH is approved for certain tank mixes it is more effective when applied alone because it is taken into the crop slowly, and therefore does not wish anything to inhibit it, he finds.

Attention to detail and keeping strictly to the label rate of 5kg/ha and the water rate should be between 350-500l/ha, he stressed.

“The operator should go at a slow forward speed (8-10 kph) with slightly larger droplets to ensure coverage. Be aware – sometimes MH can foam during tank filling with some water sources, an anti-foam product can be added.

“Application should preferably be made on a dull, dry day with humidity below 50%, temperatures of between 22-25 deg.C with no rain in the following 24 hours. Tubers should be larger than 25mm and the crop must have ­finished flowering, as this can affect the last few weeks of growth.

“Last year many growers took the decision not to apply MH because the crops were stressed and they were concerned that it might affect the canopies and slow down tuber bulking whilst others thought crops might not uptake it properly.

“The problem with making these decisions is that you have to balance these issues with putting all your eggs in one basket, by relying entirely on in-store sprout suppressants. This can work out to be much more expensive because in-store sprout suppressants are more expensive per tonne.”

Last year Andy saw cases of early dormancy breaks, which meant earlier than usual fogging of stores. This was particularly evident in crops where MH had not been used and ambient store temperatures were higher than ideal.

“These cases were very variety dependent, as some varieties are more dormant than others,” he said.

For example, crisping variety Lady Claire is very good for long term storage, and breaking dormancy would have been very unusual. The downside of the variety is that it is often difficult to grow, and has lower yields than other varieties.

While ambient stores for processing potatoes are between 7.5 – 9 C, those destined for the packing industry are placed in different storage conditions, he adds, noting that they are usually kept at around 3C, and the cold temperatures induce dormancy so MH is not necessary.

“These lower temperatures reduce the incidence of unsightly blemishing diseases such as silver scurf but if chipping or crisping crops were stored at low temperatures this would lead to an increase in sugar levels which would not be acceptable in the processed sector, as fry colours would be a­ffected and increase the risk of acrylamide.”

Sometimes things have gone wrong, particularly as temperatures were so high at lifting last year, and Andrew notes that some of his growers had identified a small number of cases of internal sprouting.

“This can happen where varieties with short dormancy and vigorous growth are stored in warm conditions, or when timing of applications has been too close to dormancy break, or insufficient sprout suppressant has reached the tubers. The risk of it happening are higher in seasons where growing conditions promote early dormancy break, such as we had last year.”

Reflecting on the 2023 crop, he notes that the stress of last year has impacted on the seed potatoes.

“We have seen quite a few uneven ‘hen and chick’ crops this year, and this is often because the seed tubers were physiologically more aged than normal, and so they had started chitting. Also, since planting was delayed some seed had sprouts up to 40mm long, some of these were de-sprouted before being planted, which a­ffects vigour and stem numbers.

“Belt planters do not cope well with excess chits, causing some misses. This was made worse because the cold, wet weather meant that we were three to four weeks late planting in the spring.

“The crops remain behind, and at the end of July some of them have not yet closed canopies across the rows before flowering. They are unlikely to be able to make up this time and we are expecting lower yields.”

If possible, avoid placing low and high dormancy varieties together in a store, recommends Andrew.

“This goes back to the planning stage, when you should think through the varieties you will be growing, and where they will be stored, and how much can be put into each store.”

He also advises growers with box stores to think about airflow, planning layout so that the sprout suppressants have the best possible conditions in which to work.

“Box stores should have about 40% air space, which needs to include above and in front of the boxes. When the fog is introduced, it is important to avoid it hitting the potatoes immediately, so you need some empty space for the gas to spread once it enters the store.”

This is not new, he explains, adding that it was learned with CIPC, because if the gas settled on the potatoes it left a silver crystalline layer. Also, air always takes the path of least resistance, so if boxes are stacked tightly on either side of an alley, the air will take your expensive sprout suppressant down the alley and not across the boxes.”

As the previous year’s crop moves out of store, Andrew recommends servicing ventilation, cleaning and ensuring store integrity by checking for leaks and ensuring they are closed.

“Dust and debris can harbour pathogens and fungal spores which can create problems later in storage, particularly when there is movement of air which can transport the pathogens onto the stored potatoes.”

He emphasises the importance of filling the store as quickly as possible, switching on the fans for continuous air circulation as loading starts, and stacking boxes across the direction of air movement so they get maximum exposure to the air.

“Ventilation controls will help minimise condensation, and once the store is loaded the doors should be closed and internal air/temperatures managed to keep crops dry and at the correct temperature.”

“Attention to detail and monitoring crops can help minimise disease coming in from the fi­eld and assess levels of treatable disease so best practice can be used to control them.”

He recommends discarding at grading those with poor skin set, symptoms of blackleg or spraing, and any blighted tubers to minimise rots occurring later in the season.

Once the store is loaded, store fans are switched to dry and cure any wounds at about 10-12C, for two weeks, before bringing temperatures down.

Stores can be ready for the ­first application of a sprout suppressant three or four weeks later.

“Since the withdrawal of chlorpropham (CIPC) we have had to get to grips with the management of new actives, some of which have been used elsewhere for a number of years but have only recently been approved for use in GB. We still have more to learn about them to get the best results.”

“Reports show that the main reasons for variable results from sprout suppressants include instore temperature variation. The ideal temperature is about 8.5C for fogging so a sealed, insulated store can help maintain the required stable temperature, although there are times when ambient ventilation is necessary.

Excess moisture can result in condensation, and subsequently diseases. In addition, excess soil on the tubers can affect how well the product works as it restricts air flow and coverage of the product on the tubers.”

However, he recognises, it is hard to avoid in a wet lifting season. When it comes to sprout suppressants, it is not usually the agronomist making the decision, but an agreement with the grower, fi­nal customer, and discussion with the fogging contractor.

For the crisping industry, there is a choice of 1,4-DMN, orange oil (ARGOS) and mint oil (Biox-M).

Andrew explains that 1,4-DMN works by inhibiting protein production and the metabolism of the tuber, effectively bringing on dormancy.

“This is a preventative measure rather than curative, so is often used earlier in the storage calendar. It has a lower application rate than the others, so although it is applied slowly, it takes less time to apply than other products which have a higher application rate, so time efficiency for the operator is greater – the maximum volume of fog for the best results is 50 l/hour.”

Follow-up applications depend on regular monitoring for the ‘eyes’ peeping, signifying sprout emergence, and knowledge of when the store will be unloaded.

“People like 1,4-DMN; in a difficult season it has proved itself to work very well,” he said, adding that mint oil and orange oil have not disappointed either. The two oils work on contact and are used to burn off the tiniest pinheads of sprouts, and should be applied when 20 – 25% of the tubers have sprouts of less than 1mm.

“Industry experts have reported better results for orange oil in bulk stores, while mint oil works better in box stores. Timing and application of sprout suppressants are key – in this they are very much like plant protection products; you need to choose the right one for the circumstances and the environment you are operating in.”