POTATO farmers that normally rely on the herbicide diquat to burn down their potato crops before harvesting, have been urged to consider the alternatives.

The EU banned diquat and withdrew licences to use anything containing it in 2019 – and the EU also set a final use date of existing stocks as February 4, 2020.

This led to a major management dilemma for potato growers and Leigh McClean, a development adviser at the College of Agriculture, Food and Rural Enterprise (CAFRE) in Northern Ireland, has advised potato growers to mix and match the alternative options for burn down this season.

Leigh said: “A number of alternative desiccation methods exist. Some such as root cutting, haulm pulling, haulm electrocution and applying salt solution have been trialled and shown varying degrees of success at defoliating the potato crop but are either too costly, too slow or raise other issues which mean they are not currently viable methods for local growers.

“Chemical alternatives still exist in the form of a group of herbicides called PPO inhibitors of which there are two products, Spotlight Plus (Carfentrazone) and Gozai (Pyraflufen-ethyl) registered for use in potatoes.

“These products have been around for a number of years and experience shows they work well on stems and are effective at promoting stolon detachment.

“Some growers may have previously used them in part of a burn-down programme following an initial application of diquat to take down the leaves and open up the canopy, exposing the stalks for a follow up application of a PPO inhibitor to work effectively,” he said.

Using mechanical flailing is a method that has been used in other parts of the UK, but there are a number of challenges that arise from this system.“Mechanical flailing is one alternative commonly used in England and Scotland to replace the role of Diquat in removing foliage,” Leigh said.

“Growers there have successfully been using this practice with a follow up application of a PPO inhibitor 24 to 48 hours later, once stems have started to wilt. This instantly stops passive bulking giving growers more control of tuber size important for higher value markets such as salads and seed.

“Flailing, however, poses a number of challenges. A single bed machine requires a pass over every bed. Triple bed machines reduce wheeling,s but need a larger, heavier tractor to drive them. For either configuration to work successfully and leave the field in good condition for harvest, ground conditions need to be good at the time of flailing. This has been difficult to achieve thus far this summer.”

With wet ground conditions restricting the ability to work in fields and limited availability of machines, flailing will not be a realistic option for many this year. This leaves the final alternative of multiple applications of PPO inhibitors.

This is challenging as these products are less effective than diquat at taking down foliage, working best when natural senescence has already started.

Factors such as earlier planting, nitrogen rates at the lower end of the recommended range and earlier nitrogen applied in the seedbed should all help canopies begin senescence earlier and improve the probability of a successful burndown using PPO Inhibitors.

With those decisions taken months ago there are still a number of things growers can do to maximise desiccant performance. He added: “PPO inhibitors can take longer to give the same effect as Diquat so plan to make the first application seven to ten days earlier than usual. Trials have shown both products work best in bright, sunny and warm conditions.

“Yes, these kind of days are at a premium but prioritise burndown sprays on bright days as close to the middle of the day when sunlight is strongest. Good application to maximise spray penetration into the canopy is key.

“Slow forward speeds, high water volumes, forward and backward facing nozzles applying a medium quality spray will all help maximise product efficacy.

“Gozai should be applied with the adjuvant Toil; Spotlight Plus has an adjuvant in its formulation. Both products are compatible with the fungicide Ranman Top which can improve the desiccants efficacy whilst also having good activity on tuber blight.

“Desiccation without the option of diquat will be a new approach for many growers. It is important to check the crop regularly for tuber size, plan to start at an earlier stage than usual if using PPO inhibitors and be ready to spray should the perfect weather conditions arise,” he said.