A slow start to the growing season through a cold wet May this year means many potato growers will be tackling big crop haulms that are still vigorously growing come desiccation day, warns Masstock Arable potato specialist, Andrew Farley. The Devon and Somerset advisor advocates a split-dose desiccation programme to stop tuber bulking and initiate skin set, in order to secure premiums in a year when quality will be paramount for good prices.

Mr Farley advises growers not to be tempted to leave things too long before desiccation and to get tubers safely out of the ground and stored in good conditions for quality premiums. Delays in lifting if weather turns bad will only increase the risk of Black Dot and other skin diseases developing, he adds.

Scottish Agronomy’s desiccation trials have shown that for speed and skin set, a split-dose Reglone will be equally good, if not better, than two acid applications – but at half the cost and putting desiccation completely in the control of the grower.

Agronomist Eric Anderson advises not to stint on the initial application, particularly in vigorous crops - where at least 1.5l/ha is recommended to remove enough haulm to allow a second application, up to a total dose of 5 l/ha, to target the lower stems and complete the desiccation process. The initial hit triggers skin set in the tubers.

“Growers need to look at what’s going on under the ground, rather than just the leaf canopy; the speed of skin set is far more crucial than the rate of haulm death,” according to Mr Anderson. “It is the skin set that dictates harvesting date and reduced levels of Black Dot or Black Scurf. Scottish Agronomy studies have shown there is no difference between Reglone and acid.”

Spiraling fuel costs will hit growers using power hungry flail desiccation and gas burners especially hard this season. A 170-hp tractor pulling a two-bed flail could use over 200 litres per day – at a cost of over £140, up 70% on last year. With tractor typical machinery ring rates costing £42/ha for flailing plus fuel costs of over £12/ha , trials have shown the cost of the flail power unit alone could be more than £54/ha.

Flailing green top crop consumes vast amounts of power and fuel; pre-flail treatment would give significant cost savings. Reglone application during flailing can avoid the need for follow up application

A Reglone application a week before flail operation significantly increased the speed of operation, achieved a far cleaner haulm removal and reduced the risk of vascular browning tuber damage. The trials by Scottish Agronomy have consistently shown flail work rates increase by an average 30% when the crop has been treated with Reglone at 0.5 l/ha five days before flailing, and over 50% faster following a 1.0 l/ha application.

Tractor costs alone would be reduced, from £26/ha in the untreated, to £20 and £17 respectively with 0.5 and 1.0l/ha pre-treatment desiccation applications.

Defoliation trials at Cambridge University Farms have highlighted the potential risk of rapid defoliation increasing levels of vascular browning and stem end-rot.

Syngenta Technical Manager, Jon Ogborn, advises the greatest risk of tuber damage is for crops that are being stopped early when haulm growth is still vigorous, typically where growers are aiming to meet specific tuber size requirements, such as early bakers or salads.

“The problem is caused when the stolon dies quickly and detaches from the tuber, so the tuber skin doesn’t get time to develop a scar of suberized tissue.” The result of rapid stolon detachment could leave a wound that allows soil bacteria to enter and encourage rotting.

An application of Reglone at 0.5 to 1.0 l/ha prior to rapid defoliation techniques, such as flailing, has proven sufficient to reduce tuber damage. Mr Ogborn reports the initial low dose is adequate to trick the plant into commencing early senescence and skin set, without a shock to the crop that may cause tuber damage. “Even vigorous crops will die back to 50% ground cover within a few days of application, when further desiccation or flail is unlikely to cause damage,” he adds.

Cooling autumn conditions trigger blight to increase production of infective tuber blight zoospores. Results of trials and field experience have shown the highest levels of tuber blight infection often occur in fields with relatively low levels of foliar blight.

Research has identified that blight spores may remain viable in the soil for several weeks. To minimise soil inoculum and potential infection by viable spores on the foliage at lifting, it is essential that foliar blight protection is maintained as long as there is any green material present. Where foliar blight has been observed in the crop an interval of at least 14 days should be observed between desiccation and lifting, reports Jon Ogborn of Syngenta.

He advocates growers use Reglone, to achieve rapid and complete desiccation of leaves and stems, in conjunction with Shirlan to maintain a high level of protection against zoospore infection.