Avoid soft rots in early storage – Potato Review

As the weather around harvest time can be unpredictable, Andrew advises growers to create flexible plans around which field to prepare first for harvest.

“It is a good idea to pair one field which tends to be wet and has heavier soils, with another which tends to be dry. Then if you have wet weather, you can opt to lift the dry one first, and get it out of the way. Of course, if the weather is dry they you go into the wetter soils. This helps keep options open so you can take decisions when weather windows appear.”

He adds that fields which are known to be infested with PCN should be harvested last, to reduce the risk of carrying the pathogen from field to field on the machinery.

Decisions on whether to windrow should also depend on field conditions. While putting the lifted tubers on to the next ridge can help skinset because they cure more quickly, if conditions are very dry it may raise the risk of bruising, he warns.

Soil type also affects bruising levels. Clay and silt soils provide better protection at harvest than lighter sandy soils because they cushion tubers on the primary web as long as they are free of clods.

“Different conditions across the field can influence tuber damage levels, and as soils can be more cloddy around the tramlines, and it is a good idea to slow down in these areas and take extra care to avoid bruising.”

At the time of writing, summer weather conditions are very dry, and if they remain dry until lifting, the potatoes are unlikely to be as turgid as Andrew would ideally like. As a result, dry matter (DM) levels are likely to be high, increasing the tubers’ susceptibility to bruising.

Even small changes in tissue elasticity and turgor influence bruising susceptibility, so Andrew notes that when soils are dry, irrigation prior to lifting can help to keep the soil on the web for longer, and reduce mechanical damage and subsequent bruising.

“Bruising occurs when a mechanical impact damages the tuber skin and can penetrate into the flesh resulting in black pigments,” he said, adding that level of damage can be assessed by the number of peels to remove the bruise.

If irrigation water /infrastructure is limited, fields with the more susceptible varieties, such as Russet Burbank or crisping variety Taurus (both of which have a Bruising score of four), may need to be given priority over varieties which are more tolerant such as Melody, Innovator or Sante (Bruising score eight). Most of the other popular commercial varieties have a score of around six.

Andrew said: “Before going in with the harvester, if you are growing in a hilly area, it is a good idea to step back and think about which direction you are going to harvest in. For example, if you are lifting as you go up the slope, the tubers are taken up the web. “But if you are going downhill, they can roll excessively on the web and bounce more, causing damage.”

Soil temperatures at lifting can also affect the risk of bruising. If they are above 10oC, the likelihood of damage is much lower than if they are 8oC, so harvesting in September can be quite different to harvesting in October.

However, growers always have to wait until optimum yield and skin set regardless of date.
Keeping soil damage and compaction to a minimum is also key, particularly when conditions are wet at harvesting and Andrew highlights the importance of having correctly fitted low pressure tyres on the trailer.

“Also, once the trailer is full, rather than turning off to go at a right angle to the rows, the driver should follow the tramlines.”

One of the challenges many growers are facing this year is a lack of experienced harvest operators. Andrew recommends spending time training new operators up to speed on the nuances of the machinery is time well spent.

“Machine operator and trailer drivers should be fully briefed on expectations and the need to minimise bruising.”

Resistance to damage depends on a fine line of balance between too much and too little water. While delayed skin set can become a problem in wet soils, he acknowledges that, conversely when very dry, skin set can be slower in non-irrigated crops than those which have been kept under irrigation.

“As a result, extra care needs to be taken to make sure that a good skinset is achieved to help keep potential damage as low as possible.”

Soil moisture levels in the field at harvesting can make a big difference to tuber damage, and the ideal soil moisture deficit (SMD) is around 50mm. However, this is di cult to control; if conditions remain dry irrigation can help by increasing turgidity and help cushion potatoes as they go up the first stage of the harvester; but if rain arrives in sufficient quantity to cause such problems, the soil can quickly become too wet.

Modern harvesters are both speedy and efficient, and modern harvesters can lift up to 60/t an hour. This means that attention to detail is ever more crucial, says Andrew.

Taking time to check the machinery over for any roller damage that can cause pinching or slicing helps keep damage levels down, he says, pointing out that it is also useful to check there are no sharp points on the trailer or elevator.

“Prior to setting up your machinery, it is important to know how deep the discs should go; some varieties set the daughter tubers below the mother, whereas others are around the mother.

“Of course, as you move round the field and the topography changes, you may find you need tweak your set-up a number of times during the day.”

The share height needs to be set correctly, so the potatoes are not damaged by the share. and haulm rollers should not be set too aggressively as this is one of the most highly potential areas for damage on the machine.

Gentle handling of tubers and minimising drop heights are vital to keep bruising to a minimum, and many people are now using mats or cushions.

“If you think about it, a one per cent increase in damaged tubers on a harvester lifting 25t/hour could reduce your saleable tubers by up to 2.5t/day. Such losses quickly mount up.”

Some growers use on harvester spray systems to spray water on the tubers to reduce bruising. This can also be done on top of the grader hopper before the coils or rollers.

“With care to minimise damage, potato growers could save themselves thousands of pounds a year by reducing the amount of tuber bruising that occurs during and after harvest,” said Andrew.

“Care needs to be taken at all stages of the process, from harvester set-up, to how the trailer is driven and tipped, and careful grading into and out of store,” Andrew said.

Regular samples taken at every stage and hot boxed at 32-34 C for 12 hours will help understand where any problems are, so they can be reduced, he advises.

“We find taking five or six samples each of 25-30 tubers from each harvester at different times of the day will help identify what is happening. If your sample comes back with a lot of bruising, you want to know what steps can be taken to correct any problem before it gets out of hand – and a lot of tubers can be lifted in six hours.”

If bruising is slight, it will mean an extra peel on processing potatoes, and if it is severe, it can take up to four peels.

“Look carefully at the damaged potato; most of the damage on the harvester is caused by haulm separation systems and operators not tweaking the extraction roller settings correctly. But it can also imply that desiccation and haulm separation have not been given time to complete, in which case you need to slow down or ideally move on to another field.

“If your principal problem is bruising, again, you should hold back and give the crop a little more time to cure in the ground.”

Ensuring a smooth transition into store is crucial to maintaining quality, emphasises Andrew. Prior to loading, he encourages attention to detail to ensure that any rots are graded out to prevent the infection passing to others.

Rots such as pithium (watery wound rot) are caused by warm conditions and poor skinset, and the tubers start to liquify. “Ensuring effective ventilation will help keep incidence down.”

Pink rot is another soil-borne pathogen, which can be present in over-wet soils and tubers are not dried properly before storage. It gets its name from the colour it goes when cut open.

Drying is important for all crops going into store, but particularly so for seed crops, which are often passed through drying tents. Despite the dry conditions this year, Andrew has found some varieties have blackleg, these tubers should be removed at grading.

“You really want to avoid bringing pathogens such as silver scurf or black dot into store. While you can use fungicides to reduce incidence they can be tricky to apply accurately and many protocols do not allow treatment.

Once loaded into store, the tubers need time to cure, which for processing and packing potatoes is often done at 12oC for a week, after which temperatures are brought down by 0.5 deg.C each day until they are at the desired temperatures for each category.

“The trick is to set temperatures at 2-3 deg.C less than the incoming temperature of the tubers and reduce moisture levels because increasing humidity at the early stages raises the threat of black dot or silver scurf developing.”

This calls for good ventilation for air movement and a constant temperature throughout the store to avoid wet spots. Ventilation is also key to effective sprout control, adds Andrew.

“Since the withdrawal of CIPC, growers have been applying newer sprout suppressants such as orange oil and mint oil, both of which are more expensive, so they need to ensure they get the
maximum efficacy.”