Potato growers are starting to see much more cracking and internal defects in their crops, with tuber cracking being the third biggest quality problem in potatoes last year, according to one expert.

Mark Stalham of Cambridge University Farms believes understanding and acting on tuber cracking will be particularly pertinent following the year just experienced and considering the strong likelihood of seeing a similar one soon.

Talking to major packers, Dr Stalham notes two of the key issues seen every year are greening and bruising. Last year, however, they were closely followed by tuber cracking, particularly in Scotland, where the situation was described as “horrendous”.

SAC senior potato consultant Matthew Smallwood agrees it was a tough year and these are issues that need to be addressed. “The waterlogged soils and lack of sunshine not only prevented bulking for long periods, but also resulted in growth cracking in many crops and greening through tubers,” he says.

The prolonged wet weather has been one of the major factors in generating this widespread problem. However, despite this uncontrollable element, Dr Stalham believes there are practical options available to growers to mitigate the risk.

He says that understanding the problem will be just as important as being proactive in trying to avoid it in the future.

“We all know it’s been a tough year and for many there was nothing that could have mitigated the trouble. However, there are still things you can do in such conditions and it’s all about being prepared for them again,” he says.

Cracking types

Dr Stalham says there are three types of cracking that growers need to be aware of: shatter cracking, turgor-induced cracking and secondary-growth cracking.

Shatter cracking occurs on a very limited number of varieties. If a tuber is really pumped up with water (turgid), when it is dropped the shockwave goes right through the tuber very quickly without dissipating.

“The simplest example you would see is a crush injury, resulting in a crack that radiates around the tuber,” he says.

There are some measures that growers can adopt, however, as Mr Stalham points out that slightly dehydrating the tuber so it is able to cushion the force will help dissipate the energy.

This problem usually occurs when growers are harvesting susceptible varieties very early, when the tuber is in a turgid state.

Potatoes generally become more dehydrated the closer you get to a “normal” harvest time.

“If you know you’ve got a susceptible variety, irrigation will be your biggest tool and controlling this will be important,” he says.

Mr Stalham highlights research showing that drying out the soil for three days to a week – if the opportunity arises – by stopping irrigation prior to harvest, significantly reduces the effect of cracking.

“If you irrigate or it rains right up until the harvest day, you are liable to have very severe issues.

“It’s all about controlling water and if you don’t irrigate, you can’t do that. You can’t control the weather, but you can control how you react to it,” he says.

Dr Stalham suggests only harvesting at certain times of the day, particularly on hot days, will help reduce the problem. A normal tuber will dehydrate during the day and rehydrate at night, so the tuber is likely to be more dehydrated (less turgid) and suffer from less cracking at the end of the day or during mid-afternoon than in the morning.

“This is a practical thing you can do. If it’s a hot, bright day, wait until later. Don’t harvest when it’s cold, wait for things to warm up.”

Drainage

Dr Stalham notes that drainage will once again prove vital and growers must use every factor they control to their advantage.

“There is nothing you can do about the weather, but you can try to make sure those crops are not sitting in wet soil, so drainage is key to avoiding this.”

If there is excess water in the soil, roots suffer from a lack of oxygen and plant growth slows down. Once this happens there is a risk of the tuber taking up extra water, even though it is in a very turgid state.

The real problem then occurs when the crop starts growing again and the skin of the tuber (periderm) cannot contain the internal pressure.

“Effectively the tuber is like a really pumped-up balloon. All you need to do is knock it and it will fracture. In some cases just removing the soil makes it split.”

These turgor forces can build up later in the season, but the big problem lies earlier in the season when the periderm isn’t thick enough to retain the forces. By August it will be thicker, with much greater elasticity, so it retains the forces better.

This early season effect, two or three weeks after emergence at tuber initiation, is when Dr Stalham has seen the most extensive cracking, particularly when the soil is wet.

“The key time growers need to start looking out for this is the first week after tuber initiation.”

What happened last year, however, was that crops didn’t dehydrate during the day, because evaporation was slow.

There is a belief that it is rapid growth rates that cause tubers to split, but splitting can occur even at slow growth rates. It’s simply to do with the physical pressure – if the skin can’t retain it, it will burst.

In helping to avoid cracking, Dr Stalham admits growers have some tough choices to make and will often have to find the right balance.

“Irrigation at tuber initiation will help common scab control, but it’s about assessing where the worst damage will come from and also selecting the relevant varieties,” he says.

Planting decisions will also be key. Selecting fields with the correct soil types will immediately give you a head start.

Free-draining soils obviously fared much better following the season we just had, but this can act as a lesson for the future when deciding where your crops will go.

Cultivations

With drainage being so important, even simple steps taken to improve it will help, according to Dr Stalham.

“It really comes down to making sure you don’t have anything that impedes the natural drainage of the soil and this includes using the right cultivation techniques,” he explains.

Cultivating the right soil at the right wetness to ensure you don’t create compaction during planting will be key. Recognising compaction from previous years and using this in the decision-making progress will also help.

Last year also saw tubers suffering from secondary growth cracking, which is physically very different.

This requires a complete reverse in weather conditions. A very slow growth rate caused by dry soils, followed by a prolonged or large input of water causes the crop to begin growing again very quickly.

This encourages the reinitiation of cells where they have previously stopped growing, along with the creation of new cells, thus leading to differential growth in the tuber. Therefore, the tuber becomes malformed and misshapen (as shown left), which can be particularly severe in some varieties.

“This again comes back to having control of irrigation, as growers can help mitigate these problems.

“This is why many packers and processors are getting growers under contracts where they have to irrigate, because, if you get in a dry spell and then it rains, you can end up with a very-low value crop,” comments Dr Stalham.

More on this topic

Luke Casswell on G+