DONT BE BEATEN BY
Potato growers are losing £30m a year due to bruising. So the scientists are on the attack, as
Gilly Johnson reports.
MAJOR problems with bruised crops last year led to rejections and price penalties when they came out of store. Finding out why is a priority for the British Potato Council (BPC).
"Bruising and damage are high on our agenda for research funding; quality aspects take about a third of our budget," says Dr Mike Storey of the British Potato Council (BPC).
The plan of attack, funded by growers levy cash, covers all fronts: from investigations into tuber physiology and an exploration of what happens when cells are damaged, through to husbandry practices, turgor, the effect of skin set, variety differences, and the set-up of harvesting and handling machinery.
At a recent BPC brainstorming session, scientists explained to leading potato agronomists how gene technology could be used to warn growers if a crop was heading for problems.
Particular genes are expressed in the bruised portion of potato tubers, explained Dr Ron Croy of the University of Durham. In theory it should be possible to construct a "DNA probe", which when inserted into a potato, would reveal within hours which crops were most susceptible to bruising damage by checking out the genetic profile of the growing tubers, he said.
Armed with this knowledge, producers could then take precautions with vulnerable crops. It would also provide evidence as to which husbandry and storage practices might affect bruising.
Also being investigated is a chemical test as an alternative to the hotbox technique for bringing out bruising damage, said Professor Cobb.
Back to the present. Crop husbandry tactics to reduce bruising include:
• lifting when the soil has warmed up. An experiment at ADAS Terrington showed 10% severe bruising, and 50% minor bruising when Pentland Dell was lifted in the morning at 11íC. But there was no bruising on tubers harvested later in the afternoon, at 18íC.
• ensuring soil potassium (K) is not limiting, because deficiency is linked to susceptibility. However, the relationship between potash and bruising is not straightforward -potash applied at rates higher than that necessary to achieve maximum yield will not reduce bruising, and some crops adequately provided with K have been susceptible.
• watching tuber turgor. If this is high, then the crop is more prone to "shatter bruising", resulting from internal cracking. Certain processing varieties have shown susceptibility.
Irrigation and nitrogen fertiliser do not have a clear effect on bruising risk, said Dr Chris Cole of HRI Wellesbourne.
MOST bruising damage is done to potato tubers at harvesting – yet crop handling characteristics are at the bottom of the checklist when growers are choosing new harvesting equipment, said Fraser Milne of the Scottish Agricultural College (SAC), Edinburgh.
"Growers want a high work rate first and foremost," he said. "Theyre looking for a machine that can lift in all conditions, is reliable and produces a clean sample. Sadly, damage is not a priority."
With even a low damage rate of 1% typically costing about £24/hour, this is a mistake. Part of the problem is the lack of an independent trialling system for assessing harvester damage, he argued. "Theres no way for growers to know how individual makes of harvester perform in terms of crop handling – perhaps the BPC could consider this."
Mr Milne called for a prescription for equipment handling standards, which might include maximum drop heights and the specification of materials used. "The industry needs a low damage harvester – the manufacturers must take on this challenge."
Key danger points on the harvester are at the point of haulm removal and elevator discharge. A trial using visibly sensitive objects – eggs – passed over a typical harvester shows the vulnerable points clearly (fig 1).
Growers could do more to help themselves, said Mr Milne. For example, a fall breaker, which reduces damage at elevator discharge is a simple solution, designed by the SAC. But few have been sold (at about £385) despite the backing of the BPC.
Varietal differences exist in bruising susceptibility, but the relationship is not simple because some varieties may show less bruising, but more shatter and other damage. For example Record can resist internal cracking but the energy is dissipated and appears as bruising damage instead.
Commercial results from growers supplying the Lincolnshire group Branston Potatoes in the 1997 season show significant different variation between varieties (fig 2).
Tuber size and shape, and speed of impact have a big effect on damage level, added Mr Milne. "Large tubers increase the risk of damage. Certain areas, such as the stolon end, are most susceptible to impact."
ETHYLENE treatment reduces severe bruising, according to Dr Ewen Brierley of the BPC. (some results).
As yet, it isnt clear why ethylene has this effect – Dr Brierley suggests it might be due to a boost in cell division, which reduces the severity of bruising symptoms.
However, the commercial potential of ethylene is limited. Treatment is only effective prior to the mechanical impact which causes bruising. If tubers are given ethylene after an impact, the bruising symptoms may be exacerbated.
Most impact damage happens at harvesting, and not when the crop goes in or out of store. So to be effective, ethylene would need to be applied prior to harvesting, as a conditioning field treatment which would be translocated below ground to penetrate the tuber.
Professor Andy Cobb of Harper Adams University College did not rule out the development of a spray which when applied to the crop could generate ethylene in the tuber, but even if this proves possible it might affect sprouting and perhaps fry colour.
5% 8% 15% 20%
Crop price £120/t:
£/hour £120 £192 £360 £480
Crop price £200/t:
£/hour £35 £56 £105 £140
£/ha £79 £126 £237 £316
Assumptions: 20t/hour harvesting rate. Higher value crop, 45t/ha: 80% crop saleable. 40% grade 1; 40% grade 2; and grade 2 price = 40% of grade 1 price. For every 1% damage, 0.75 grade 1 and 0.25 of grade 2 is affected.