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Defoliation – which technique are you using?

27 July 2011

The latest thinking on defoliation programmes to help growers manage the most vigorous of crops attracted plenty of interest at the recent AHDB Potatoes East Midlands Potato Day.

Since sulphuric acid was revoked as a desiccant in 2009, growers have been looking for an effective alternative to kill off vigorous haulm, rapidly and totally.

Sequences and mixes of chemistry can now offer growers similar performance on the greenest of seed crops and hard-to-kill indeterminate varieties, says Frontier Agriculture’s Stuart Maltby.

“Quite a few techniques have been developed, based on sequencing of desiccants, sometimes in combination with flailing. Getting the technique right is crucial – you need to stop the crop at the right time so it meets customer specifications for tuber size, dry matter and skin finish.”

Several different treatments (see table) were demonstrated and compared in a crop of Marfona at the event, held on July 5 at QV Foods, Holbeach Hurn, Lincolnshire.

“All the treatments worked well – although the crop was growing vigorously – and Marfona is relatively easy to kill. But the demonstration provided a great starting point for discussion,” says Stuart.
 
Flailing and spraying is proving to be a very effective alternative to acid on very vigorous crops. This can be done in combination, using an all-in-one flailing/spraying machine often operated by a contractor.

Straight diquat can be used at this timing, but adding Spotlight (carfentrazone) is gaining popularity. “Spotlight is better at desiccating stems and it also prevents regrowth from green stems, which can act as a conduit for blight infections,” Stuart explains.

“Regrowth also slows stolon detachment and reduces skin set, leaving tubers more prone to damage at harvest.”

Flails should leave about 8in (20cm) of stem to allow uptake of desiccant, and they should move flailed haulm off stems and into the furrows to allow spray to hit the stems more effectively.

“Some growers are now using a pre-flailing spray of 1litre/ha of diquat to open up the crop and reduce the physiological shock which can lead to an increase in bruise susceptibility,” he notes. “Others prefer to flail and then follow with desiccant – if tops are not moved effectively, allow two to three days wilting before spraying.”

Desiccant sequences alone are effective on less vigorous crops, especially with supermarket packers where rapid burn-down is needed to avoid development of black dot and other diseases.

Programmes may start with a low dose of diquat to open the crop, helping to increase the efficacy of the main application. Again, the addition of Spotlight as shown in the table is proving effective on more vigorous crops to reduce regrowth. “However, diquat is more effective at desiccating leaves, so you still need to keep it in the mix.”

Sequences also remove the risks associated with a single high dose of diquat. “A single application at 3 l/ha can be very effective at finishing off senescing canopies.

“However, it does carry a risk of vascular browning, especially when soils are dry. Growers will be going through the crop anyway with blight fungicides, so the split dose doesn’t mean extra passes through the crop,” says Stuart.

 

Defoliation – treatment and timings

Treatment/Date

Tues 14 June

Tues 21 June Tues 28 June
Untreated nil nil nil
Flail only nil Flail nil
Flail and spray nil Flail +  Spotlight Plus 1.0L nil
One diquat nil Diquat 4.0L nil
Diquat sequence Diquat 1.0L Diquat 2.0L Diquat 2.0L
Spotlight Plus sequence Diquat 1.0L Diquat 1.0L + Spotlight Plus 1.0L Spotlight Plus 0.6L

 

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