Latest Edition
(November/December 2008)
Automatic bunker keeps crop moving
A working demonstration at Dunsten Fen near Lincoln was billed by Grimme UK as a chance to see ‘the next generation’ of potato harvesters at work. Alongside the company’s top selling GT170 trailed machine was the latest incarnation of the two-row, self-propelled Varitron.
It’s been a record year for Grimme’s British subsidiary with sales of the two-row GT trailed harvester guaranteeing a healthy (and dominant) market share. Two versions were on show at the company’s demonstration in October – one fitted with twin MultiSep cleaners, the other with a single MultiSep unit and RS rollers. Annual UK sales of Grimme’s self-propelled machines are counted in single figures but it was the new Varitron 220 which seemed to be attracting most interest from visitors.
This latest variant neatly occupies the middle ground between the standard elevator machine, the 200, and the full-blown 270 bunker version which was introduced last season. The clever bit is the crop buffering system which comes into its own when the accompanying trailer is full. Operation of this facility is totally automated and operator-friendly.
During normal harvesting the ‘bunker’ acts as a bridge between the picking table and the cart elevator. The floor is made up of a full-width transfer web towards the rear and shorter one at the front which helps to spread the crop evenly across the elevator pockets. The floor and side panels are raised or lowered hydraulically.
The bunker is located just behind the cab giving the driver a clear view of its contents – not that there is much he needs to concentrate on when the system is in use. Once he has disengaged the cart elevator the electronic controls take over, slowly dropping the rear of the bunker section as it fills and raising the front of the picking-off web when holding capacity is reached. At this point an audible warning signal is sounded and loading stops. When the elevator and transfer rollers are started up again the bunker floor is raised slowly, allowing the crop to be discharged into the trailer.
The concept is simple, though undoubtedly some fairly sophisticated engineering has been employed in its execution. The bunker holds up to two tonnes of potatoes, giving the operator plenty of time to turn the machine and make a start on a new row, get through a wet hole, or change trailers – all without having to stop and interrupt the flow of the crop through the machine.
www.grimme.de/en/
Would you class this as a minor rot?
Cambridge plant pathologist Paul Gans wonders if levels of watery wound rot might be underestimated. He points out that without tests to back up a diagnosis it can be difficult to confirm the principal cause of breakdown.
If you look at some text book pictures of watery wound rot you might think that this is a condition which would be easily recognised. The centre of the tuber often appears grey and wet and the North American name, shell rot, accurately describes the way in which the condition hollows out the flesh.
My impression of this disease changed radically when rapid detection kits became available. I found that many rots which were difficult to identify tested positive for Pythium spp., a group of moulds, one of which (P. ultimum) causes watery wound rot. Often the symptoms did not look at all like the text book images.
One of the reasons why we see these complications is because once a tuber starts to rot, other organisms which are lying dormant also become active. Among the principal culprits is Pectobacterium atroseptica, which causes blackleg, but there are several other bacteria which can give rise to tuber wet rot.
The lessons to be learned from this are, firstly, that we need to carry out tests to corroborate our observations and, secondly, that watery wound rot may be more widespread than we realise.
In North America, pink rot (Phytophthora erythroseptica) and watery wound rot are often lumped together as ‘water rots’. This reflects a similarity between the symptoms and the circumstances under which both diseases occur. Like pink rot (described in Potato Review, September 2008), watery wound rot is associated with warm weather and damage caused during harvest, though the critical temperature is a little higher. The advice for growers in the US is not to harvest potatoes until the temperature is below 21°C.
Infection appears to be dependent on the presence of tuber damage and this has a consequence for fungicide protection. US growers have found that metalaxyl is better at controlling pink rot than watery wound rot. The explanation for this lies in the fact that much initial pink rot infection arises from invading zoospores prior to harvest while watery wound rot appears to be caused by soil contamination of wounds. In this way the disease appears to bypass the fungicide barrier. (As with pink rot, metalaxyl resistance has been reported.)
P. ultimum can cause damping-off of seedlings and rotting of roots, stems and fruits in a wide range of crops. As far as we know there are no special strains which prefer one crop to another. The organism forms oospores, sporangia and zoospores and these may all be instrumental in its survival. It appears to have developed its own special strategy for competing with other soil organisms by germinating whenever there are nutrients available in the soil and then retracting again to a form in which it can survive long term.
This strategy may also be the key to managing the disease if only we could understand the factors which affect the process – some soils may simply be unkind to P. ultimum. There are varietal differences which offer scope for breeding resistance in the future though there is little in the way of variety choice to avoid the disease today.
Watery wound rot, on a scale that is reported in North America, is unlikely to be an issue in the UK but the number of cases where it has been confirmed raises the suspicion that P. ultimum is present and may be causing more harm than we realise – even at below threshold temperatures.
Sorting out what has gone wrong once you are confronted with serious rotting is a complicated business. Fortunately, methods for identifying the different organisms which cause rots are improving. For some time now there have been test kits specific to Pythium spp and Phytophthora spp. This means that they can be used in combination to separate suspected pink rot and watery wound rot infections, though they will not differentiate between P. infestans (blight) and P. erythroseptica (pink rot). The kits* are based on the same principle as the lateral flow tests used for virus diseases.
Measures which are essential to manage watery wound rot are similar to those advised for pink rot; they consist generally of damage prevention at harvest and ensuring optimal conditions during the early storage period. Additionally, when things go wrong, it is well worth spending some money either on test kits or on sending a sample to a reputable lab.
*Pythium and Phytophthora kits are available from Neogen Europe Ltd, Ayr, Scotland, KA6 5HW. www.neogeneurope.com
Phytophthora kits are available from Forsite Diagnostics Ltd, Sand Hutton, York YO41 1LZ. www.forsitediagnostics.com
Plan ahead for timely harvesting
It may be an old cliché but the 2008 season has been like a game of two halves, says UAP’s potato specialist Barrie Florendine. Growers in the north and west have had it tough, with wet weather leading up to (and during) harvest which caused huge difficulties for lifting. Many in the south-east, by comparison, have had it relatively easy with a dry end to the season and good harvesting conditions. As a result, yields and quality have been hugely variable across the country.
Barrie Florendine says the difficult conditions experienced by some will help focus their minds on the selection of potato land for next season.
‘A year like this will bring home the question of growing the crop on marginal soils. Site selection will be very important this coming spring and whereas in the past growers may have got away with growing crops on marginal, heavy land, this season’s troubles may affect that decision in 2009.’
The key message when choosing land for potatoes is to ensure that growers know how good the field is, that the soil type is suitable – and reliable enough to get the crop off the field when and how they want to.
‘It’s not so much an issue for those in the south-east, who generally have greater opportunity to utilise more suitable soils, but further west and north – from north Lincolnshire upwards – site selection will be key to getting the maximum return from the crop,’ Mr Florendine advises. Growers taking on land must also be sure of its nutrient requirements, he adds. ‘Last season we saw too much nitrogen going on in late planted crops. This delayed dry matter and bulking up, and exposed some sites to later lifting.’
Disease legacy?
With a lot of wet crop gone into store after harvest in the north and west of the country he reckons the signs are that a difficult storage season could be ahead. Blight was nowhere near as big a problem as it was last year, he observes. ‘We didn’t have a really wet early summer as we did in 2007, and so infection didn’t really get going. That said, growers did a really good job in applying the right chemistry this season which just about kept the disease at bay. But blight will still have left its legacy – when it did come in, it came into the crop late and we are certainly seeing it in stores in the north.’
He concedes that black dot, which can be critical in depressing the value of prepack potatoes, has been an issue all season. But he reminds growers that good integrated management can avoid the worst of the problems. ‘Technology has moved on considerably in terms of black dot and soil testing systems offered by SAC can provide growers with a clear picture of the disease pressure likely to be found during the season,’ he says. ‘Sensible variety choice can also avoid many of the problems associated with black dot.’
Late harvested crops that have recently gone into store should be properly cured prior to treating with sprout suppressants, he advises, citing late-harvested King Edward as a variety that is susceptible to skin spot, a condition which can be exacerbated by early CIPC treatment.
Good ventilation and blowing through the crop will be vital for temperature control in addition to drying wet tubers, he continues. ‘Without the control, the combination of in-store blight, soft rots and high temperatures can result in sudden collapses in the stored crop.
‘How we look after our stored crops over the coming winter, and how we plan for a new season next year, will become even more important as the current financial crisis begins to affect demand.
‘Supermarkets will have to react as they come under pressure from the consumer to deliver economy pack products. In the long run, this may well effect farm gate prices,’ warns Mr Florendine. ‘This is something we are already seeing in Ireland and producers in the UK may well have to watch out for it in the near future.’
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