Expert tips on managing potato volunteers

Milder autumns and winters are giving potato volunteers a helping hand. Two experts offer the latest advice for keeping on top of them.

From black dot to late blight and potato cyst nematode, potato volunteers add significant pest and disease pressure to fields.

And with unharvested tubers able to remain viable in the soil for several years, controlling them should be a focus point across the whole rotation, advises Nick Winmill, principal agronomist at Potato Agronomy & Technology.

While developments in technology – such as spot spraying with glyphosate – have offered advantages in control, attention to detail is key to good field hygiene, says Mr Winmill.

“Looking at desiccation, make sure you have got good stolon detachment at harvest, so you’re not pulling tubers through the harvester webs and leaving them behind.

“Make sure the harvester is set up properly to maximise the retention of whole tubers rather than cutting them. Part of a tuber can be just as viable as a whole tuber if it’s left behind, creating a volunteer challenge.

“Jack Frost is our best non-chemical intervention, particularly if you can ensure tubers are not buried through post-harvest cultivations. That also gives the chance for natural predation from rodents and rooks.”

Sprout control

A holistic approach is important, but Fazor MH (maleic hydrazide) has become the foundation of sprout control programmes, offering a reduction in volunteers of 70-80% since the loss of chlorpropham (CIPC).

“However, much of its success is down to the operator and the conditions it is applied to,” says Mr Winmill. Humidity after application is perhaps the number one key to success, he adds.

“I have had some really good results from Fazor MH – even last season when we were uncertain about the weather and whether we could maximise its uptake. But we were selling King Edwards out of the store in January without additional sprout suppressant.

“Ideally, you want a minimum humidity of 75%. Look at applying it in the early evening with a high water volume of at least 400 litres/ha, so you have the whole of the night to allow for the MH to slowly dry on the leaf and therefore be taken up by the crop.

“It’s all about extending the latent time of the material in liquid form being able to dry slowly on the leaf to maximise uptake into the plant.”

Herbicide timings

Other important factors influencing Fazor MH translocation include application timing and method, the health of the crop and weather conditions, says Geoff Hailstone, UPL potato crop lead for the UK and Ireland.

He has been working alongside potato consultant Mark Stalham to establish updated guidance for the product to maximise its value.

“There is a lot to get right for the best Fazor MH uptake in tubers,” says Mr Hailstone.

“When we had that dry season in 2022, there were so many factors growers couldn’t fully adhere to that some did not bother applying it, and that was a bit of a disaster because it made sprout control in store really difficult.”

Considerations

“The first thing to consider is application timing. The marketable tubers need to be 25mm across or bigger because that is when cell division is complete. This means you won’t be restricting tuber size.”

Fazor MH should be applied 3-5 weeks ahead of desiccation, depending on the variety and growth stage. This is more commonly now 4-5 weeks rather than 3-4 weeks because the crop will then be more vigorous.

“It t will be translocating down to the tubers more effectively than if you leave it right up to three weeks. You don’t want the haulm to be senescing – you want to get Fazor MH on before that when the canopy is nice and green and still healthy.”

Extra care should be taken, particularly with determinate varieties, because they can die off quickly. Regular inspections are therefore needed to ensure it’s not left too late, adds Geoff. Crops should also be well hydrated.

Water stress

“Low to moderate water stress is okay, but not anything worse than that. If it’s a fully irrigated crop with good moisture levels, you can delay the application until 3-4 weeks before harvest,” he says.

“If it’s unirrigated or stressed, then you need to go a bit earlier, more like the 4-5 week mark. A stressed crop won’t translocate Fazor MH so well, and stomata will close up.”

Fazor MH can also help with preventing secondary growth in crops that have experienced stress from hot, dry periods. This can lead to plants producing a lot of deformed tubers, which affects crop quality.

“This includes loss of starch and glassy tubers. You get more reducing sugars produced, affecting fry colour. If you apply Fazor 2-3 days after a stress period has finished, that’s when it can have this useful additional benefit.”

Fazor should be applied at 5 kg/ha for potatoes, with a high water volume of 400-500 litres/ha, and a slow forward speed. It’s all about keeping the product on the surface of the leaf as long as possible.

“You don’t want a small water volume that will dry up very quickly,” says Mr Hailstone. “You want larger drops that stay in the liquid phase as long as possible on the leaf to help with translocation.”

Fazor MH should not be tank-mixed with other products, particularly mancozeb, as it may affect uptake. It should also not be used with adjuvants because some can cause leaf runoff when used with high water volumes.

“You only apply Fazor MH once, so it makes sense to focus on getting that right. In addition to higher humidity, daytime temperatures should be below 26 °C, and there should be no rain forecast or irrigation planned for at least 24 hours after application.

Rapid uptake

Part of the work carried out with Dr Stalham highlighted that maleic hydrazide uptake in tubers is much more rapid than first thought.

“We used to think it took a long time for MH to accumulate in tubers, but in actual fact, the maximum amount is reached in tubers after about seven days.”

Growers should aim to achieve a benchmark of 12 parts per million of Fazor MH in tubers to achieve good sprout control. “The maximum residue level is 60 ppm, so this figure is a good way below that.”

Fazor MH accumulates less in smaller tubers, so if a farm has a lot of tubers below 40mm in size, Mr Hailstone suggests grading them out before going into the store to reduce the need for early fogging to accommodate smaller sprouting tubers.

“Generally, the smallest tubers may have four times less Fazor MH uptake than the big ones. Although it’s good to have 10-12 ppm, even if you get 5-6 ppm that can still give you 50-60% control of sprouting.

“It’s still really useful in a season when conditions aren’t ideal.”