Attention to Detail Essential for Successful Potato Desiccation Following Diquat Withdrawal

August 13, 2020

For a number of years diquat has been widely used to desiccate local potato crops.  Following the withdrawal of diquat from market in 2019 and a final use up date of 4th February 2020 for stocks left in store, potato growers will have to consider alternative options for burn down this season.

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) & 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.

One alternative commonly used in England and Scotland to replace the role of diquat in removing foliage is mechanical flailing.  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 approach 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 most areas of Northern Ireland recording over 200mm of rainfall in the period mid-June until the first week in August.

With ability to work in wet ground conditions restricted 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 too is challenging as these products are less effective than diquat at taking down foliage and tend to work 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 dessicant performance.

These products can take longer to give the same effect as diquat so plan to make the first application 7 to 10 days earlier than usual.  Trails 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 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.  Consult manufacturers for product specific queries (FMC manufacture Spotlight Plus and Belchim produce Gozai) or your local CAFRE crops development adviser.