WITH the current potato market battling oversupply, leaving growers facing price challenges, ensuring good crop quality could improve end marketability.
Potato storage revolves around having a good skin, as Adrian Cunnington from Potato Storage Insight is keen to point out.
“When it comes to storing potatoes, having intact skin is crucial for preventing excessive moisture loss and preserving the potatoes from external threats,” he said. “If you don’t have good skin set, then you end up with the potatoes having a high weight loss in store, which you sell by. So, if you get excessive shrinkage, it also affects potato quality, and if disease comes in, that will also affect the quality of the potatoes. At worst, it could also cause rotting.”
A potato is made up of 80% water therefore growers should aim to preserve as much of that as possible during storage, whether that be two months or 10 months, particularly as a potato is a living organism and generates heat from respiration, said Adrian.
“We must use ventilation and, in some cases, refrigeration, to cool the crop down. But whenever we’re putting air through the potatoes to do that, there’s always the risk that air removes moisture from the skin through evaporation. Even if you get an intact skin, moisture still evaporates through it. So, if your skin set is not good enough, that rate of evaporation is higher.”
To achieve a good skin set, growers can act pre-harvest.
“The main thing to do pre-harvest is to make sure that you’re getting the crop to reach skin set before you want to harvest. So, you need to desiccate the crop in time to get a strong skin before you try to lift the potatoes.
“If you don’t get the foliage down to the right level, you won’t get enough skin set, and you’ll have problems with the crop being damaged as you lift it, with more moisture loss in store,” said Adrian.
The loss of diquat from potato desiccation programmes has reshaped how growers approach the end of crop management. What was once a relatively simple “burn-down” exercise has evolved into a more considered and integrated process, where chemistry, timing and technique all play a defining role. In this new landscape, protoporphyrinogen oxidase (PPO) inhibitors such as Nichino’s Pyraflufen-ethyl, marketed as Gozai, and FMC’s Spotlight Plus, have become central to desiccation strategies.
Maximising performance of PPO inhibitors starts with a good understanding of how this chemistry works. Knowing its behaviour in relation to the environmental conditions, in particular ultraviolet wavelengths of sunlight, and how the active should be applied in a wider system, will significantly increase the efficacy and lead to the most cost-effective result, according to Nichino’s Commercial Technical Manager Oliver Johnson.
PPO inhibitors behave very differently from systemic actives. Activity is entirely contact-based, with negligible movement within the plant itself. This means that only those tissues directly contacted by the spray will be desiccated, placing greater emphasis on application technique than was historically required.
At the biochemical level, PPO inhibitors act by disrupting chlorophyll synthesis through inhibition of the enzyme protoporphyrinogen oxidase. This leads to the accumulation of photodynamic compounds which, in the presence of light, generate reactive oxygen species. These free radicals go on to rapidly degrade cell membranes, causing structural breakdown and desiccation of plant tissue. In the field, this can often be observed within hours of application, as leaves take on a wet, scorched appearance, a visual indication that the outer cell layers are collapsing.
The key here is that PPO inhibitors are “photodynamic compounds” meaning they need UV light to create the free radicals which cause the leaf tissue degradation.
A PPO inhibitor will not compensate for poor spray deposition, leaving untreated tissue green and capable of regrowth and blight infection. While canopy collapse may appear slightly slower than with legacy diquat-based programmes, well-executed applications can deliver comparable results within a matter of days under favourable conditions, Oliver adds.
Effective desiccation is essential to halt tuber bulking at the target size, promote robust skin set, and encourage stolon detachment ahead of harvest, he advises.
“Failure to remove canopy quickly and completely can delay the lifting window, increase the risk of damage at harvest, and push tubers beyond desirable market specifications. As a result, desiccation continues to be one of the most important management steps in the potato production cycle,” said Oliver.
The absence of diquat has pushed growers towards more staged programmes, often combining mechanical and chemical approaches. Rather than relying on a single pass, desiccation is increasingly a sequence of actions designed to open the canopy, expose the stems, and complete the kill in a controlled manner.
Only those tissues directly contacted by the spray will be desiccated, placing far greater emphasis on application technique than was historically required, Oliver said.
Desiccation timing is crucial for growers to ensure that potato quality reaches its peak for packers and processing contracts. By desiccating at the right time under optimal conditions, growers can improve the chances of cleaning and storing the product more effectively, according to Commercial Technical Manager for the north of England and Scotland at FMC, Antonia Walker.
“The art of desiccation is incredibly important, because it affects the overall product. For example, if you’re lifting in wetter conditions having desiccated, all these factors have a knock-on effect as to how that product ends up being marketed and stored,” said Antonia.
When planning desiccation, Antonia notes that application is needed after flailing, at least 21 days before lifting dates, to ensure the correct skin set growers aim for and to prevent additional blight infections from entering the tubers.
A Protoporphyrinogen Oxidase (PPO) inhibitor produces highly reactive molecules that attack and destroy lipid and protein membranes, leading to cell disintegration, breakdown of plant material, and ultimately killing the plan, Antonia said.
“One thing to remember, with this mode of action, is that you can often see the results within one or two hours of application, which looks like some water-soaked marks on the foliage, and then you end up with the eventual necrosis of the actual tissues,” said Antonia.
Oliver also emphasised the importance of timing and environmental conditions, stating that they are just as critical as the dose rate.
While canopy collapse may appear slightly slower than with diquat-based programmes, well-executed applications can deliver comparable results within a matter of days under favourable conditions, they state. However, stem destruction can be more challenging. In dense or actively growing crops, leaf material can shield stems from spray penetration.
For this reason, a multi-stage approach is often required, either using initial applications to open up the canopy and/or integrating a flailing pass to expose stems ahead of follow-up treatments, he said.
“Timing should align with the onset of natural senescence, when canopy vigour is beginning to decline but plants remain physiologically active enough to support uptake. Spraying too early into a lush, actively-growing crop can reduce efficacy and extend the overall programme duration.”
Whether a determinate or indeterminate variety of potatoes has been planted, a benchmark date for application is useful but needs to align with the correct conditions, said Antonia.
“When it comes to determinate and indeterminate varieties, we would often be looking at some form of decision tree in terms of when we’d be programming how to desiccate the potato crop in question. The key things to remember with desiccation are that you must ensure you’ve got the right conditions, which is critical,” said Antonia.
When talking about desiccation and applying a PPO inhibitor, Antonia recommends focusing on essential conditions and the timing of applications. For example, she advises applying after flailing when the sun is shining brightly, and temperatures are at their peak, since PPOs are most effective under these conditions. This allows the active ingredients to perform optimally.
“We prefer to flail the crops first and then apply Spotlight Plus, with two days being the absolute maximum latest timing after flailing, and we always advise using at least 300 litres of water. We recommend monitoring the crop post the first application to ensure that if any regrowth does occur, this can be followed up with another application at 0.6 litres.”
By optimising conditions, growers can achieve better skin set and storage, thereby enhancing marketability, she said.
Oliver added: “Adequate soil moisture supports plant metabolism and improves uptake, while warm temperatures enhance reaction rates. As such, applications made early in the day under favourable weather typically deliver the most consistent results.”
A new strategy is the integration of a PPO ahead of a flail-and-spray program which is increasingly common outside of the UK. Applied two to three days ahead of flailing, it can begin to weaken the canopy and initiate senescence. Alternatively, post-flail applications benefit from improved access to stems, allowing for more targeted and effective destruction. Maintaining sufficient stem height after flailing is important to ensure there is adequate target material for follow-up sprays, typically 15-20cm, both experts add.
Oliver recommends an early application of Gozai at a rate of 0.8 L/ha, with up to two treatments spaced at least seven days apart. Sequential applications with other PPO inhibitors such as Carfentrazone are typically required to fully complete stem kill, particularly in vigorous crops or challenging conditions.
An additional consideration is the ongoing risk of late blight during desiccation. As long as green tissue remains within the canopy, infection risk persists, making it essential to maintain fungicide protection throughout the programme. Tank-mixing with products such as cyazofamid is a common approach to managing this risk.
If using Spotlight Plus, this can be tank-mixed with a fungicide and does not require any wetters, Antonia said.
“It is also important to note that using another desiccant in the same tank mix doesn’t give any added benefit and is a waste of money. You either use one or the other. That’s the important thing to remember,” said Antonia. “The key benefit here is that growers can use it on its own or in sequence, at seven-day intervals, and this can be done with other approved desiccants.
