Whilst some varieties will have ceased uptake and leaf growth, it could now mean prolonged green leaf retention on some crops that may prove more difficult to desiccate at harvest.

Split dose desiccation programmes are likely to be the norm, with more extensive use of flails if it's dry. Growers are going to need to be more adept at tailoring desiccation programme to specific crops and situations to avoid excessive costs.

"Stem tissue will be slower to desiccate than leaves, and therefore remain a source of innoculum for tuber infection for longer.

"Where re-growth occurs, it may be necessary to apply more desiccant, and further fungicide treatments will be required. Blighted re-growth greatly increases the risk of tuber infection," he warns. Re-growth is more likely where crops were immature when first burnt off, if stem kill was incomplete, where too much fertiliser was applied to the growing crop or if there is a delay in harvesting in wet weather.

Stem tissue that is slow to desiccate is a breeding ground for blight

Mr Collins highlights that to achieve good stem kill, and hence reduce risks of blight spread, growers should adopt techniques that can speed up desiccation. "Sequences of desiccation treatments allow more desiccant to be targeted onto the stem after the initial application removes the top layer of leaves," he advises. Furthermore, application technology, such as angled nozzles, can also improve distribution of desiccant within the crop canopy.

For more information download Potato Council Growers' Advice notes at www.potato.org.uk/blight

Mr Anderson warns that seed tubers that are immature at harvest are the most prone to damage and scuffing during the lifting process, which could allow disease pathogens to get into tubers. A single rotting tuber can contaminate over 100kg of potatoes. He reports the highest incidences of Pit Rot that he's encountered have been where crops have been planted late and consequently seed not harvested until mid-September, when soil conditions were cooler and wetter that is also conducive to disease spread.

He highlights the problem was more severe in seed crops in the north of Scotland harvested in the wet September of 2009, but was far less serious in drier conditions last year. Ware crops that have set skins are largely unaffected, and even varieties that appear susceptible to Pit Rot as seed crops show little or no symptoms as mature ware crops, which indicates some physiological factors too, adds Mr Anderson. Few, if any, crops in England have suffered.

He believes using acid for desiccation did have a useful side effect as it initially killed bacteria present on the leaf surface and acted as a sterilant, to reduce the level of storage disease pathogens. "Since we no longer have the acid option, now the speed of complete haulm kill, including rapid stem desiccation and avoidance of regrowth, is important to minimise the risk of infection, whilst harvesting damage and subsequent storage are also important factors.

"We certainly appear to be seeing greater incidences of storage diseases since the loss of acid, with some varieties more susceptible than others," he reported. "In the last three years there has been a resurgence of gangrene. Whilst some of the pathogens involved have relatively low pathogenicity, many factors such as cultivar, incubation temperature, wound size and tuber age may affect infection."

"Providing there is sufficient moisture five cm below the deepest tuber to create a ball of soil when squeezed gently in the hand, growers can use up to the full rate," he reports. "But if it is too dry and the soil crumbles, the initial dose should be limited to one l/ha."

The initial low-dose application will trigger senescence, initiate skin set and start to halt further tuber bulking. A second application after five to seven days, once senescence has started, can be at two to four l/ha as required to complete the crop desiccation. The second application should be applied once the canopy has opened up to allow good coverage of the stems.

An initial low dose of Reglone will burn off leaf canopy and open up the stems for further spray or faster flailing

"At the very least, an initial one or two l/ha Reglone treatment a week before the planned flail operation will remove the leaf – enabling far faster, cleaner and more cost effective flailing, with less risk of spreading diseased green leaf material," he advised.

Trials by Scottish Agronomy have shown fuel costs can be reduced by up to 33% with the pre-flail treatment. That equates to over 80 litres a day – worth £45 – on a 180-hp tractor typically using 250 litres a day*. When the costs of machinery and labour are included, he calculates, that adds up to a saving of over £14/ha.

Flail and spray with Reglone (right) will burn off remaining stems to stop blight and aide easier lifting

"For potato growers frequently battling against time the other big factor is increased work rates," adds Mr Ogborn. "Removing the bulk of the leaf and reducing green matter going through the flail can double output and ensure the harvester can be kept moving when conditions and tuber size is right."

Plots at the East Anglian Potato Event highlighted 75% desiccation of a crop from a single Reglone application at one l/ha nine days after treatment with the angled Syngenta Potato Nozzle, compared to 60% from the equivalent application with a flat fan nozzle.

Syngenta Application Specialist, Tom Robinson, says sprayer operators have reported excellent results with the angled nozzles facing rearwards to compensate for the forward speed of the sprayer and deliver droplets into the crop canopy. Alternating the nozzles to face forwards and backwards along the spray boom ensures more droplets move down through the canopy to enhance leaf and stem desiccation, and results in less drift.

Use angled sprays to ensure better canopy penetration with desiccation sprays

He advocates a water volume of 300 to 400 l/ha in dense crops or vigorous haulm on more indeterminate varieties that are harder to kill. Once the crops have been opened up with an initial application, or are already senescing, however, water volumes can be to 200 l/ha for more effective targeting and retention on the stems. The Syngenta Potato Nozzle is now manufactured from a harder wearing plastic for a longer life, and is available as an 04 version for spraying at eight to 10 km/hr, or an 05 for faster application at 12 km/hr.

Where growers are spraying with a conventional sprayer after a flail operation, he says the angled nozzles are essential to hit the remaining stems. In these situations the maximum water volume required would be 200 l/ha, to avoid run off and wastage.

1) Removing foliage with a pre-flail desiccant can reduce fuel costs by up to:

• 10%
• 25%
• 33%

2) The recommended water volume for desiccating vigorous crops is:

• 100–200 litres/ha
• 300–400 litres/ha
• 600–800 litres/ha

3) What is the advantage of angled nozzles for desiccating potatoes:

• They hold more spray in the top of the leaf canopy
• Spray reaches down to the soil surface
• Spray is targeted at stems and lower leaves

4) The blight risk for tuber infection is caused by:

• Skin damage
• Zoospores
• Dry soils