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Haulm Destruction without Diquat

20 August 2020

With many seed potato crops around Scotland coming up to size rapidly, potato growers are getting to grips with haulm destruction in the post-diquat era.

The good news is that there are still viable options to bring down even vigorous canopies, but a change of mindset is needed.

The industry standard is now to “flail-and-spray”; the canopy is destroyed with a pulveriser, and one or more follow-up desiccant sprays are used to finish the job and to prevent regrowth. Two products are approved for haulm destruction in potato crops: Gozai and Spotlight Plus. Both contain active ingredients from the protoporphyrinogen oxidase (PPO) inhibitor class, and as such are contact herbicides.

When using a flail, it’s important to pay attention to detail. Topping during wet and windy weather risks the spread of disease-causing organisms such as blackleg bacteria. It is also important to leave 15 – 20 cm (6 – 8 in) of stem as a target for the follow up contact herbicide: shaving the top of the drills will make controlling regrowth difficult! It is also worth waiting 2 – 3 days after flailing before the follow-up dessicant pass, as this gives time for trash to fall away from the remaining stem sections.

PPOs are slower acting that diquat and are at their best in combination with sunlight and warm temperatures, so if possible use them during the middle of the day. The wet conditions in Autumn 2019 also demonstrated that the use of a pulveriser isn’t always possible. A sequence of PPO sprays will bring most crops down, but it will take longer than diquat, so plan accordingly and start around five days earlier than you would have done using diquat. The labels of both PPO products also state that the onset of senescence is the best time to apply.

There is a growing body of evidence which suggests, perhaps surprisingly, that rates of skin set and stolon detachment are similar for flailing and PPO desiccated crops.  Less passive bulking is said to occur with flailing compared with a chemical sequence, but this is an area where a bit more evidence is needed to say confidently one way or the other.

If plant tissue is still green it needs protecting from late blight (and virus in seed crops), so crop protection programmes need to continue until leaf and stem death is complete. In practice, this may mean an additional blight spray.

Growers have adapted well in the past to changing approaches to haulm destruction, and will do so again now. Research into the best ways to use alternative methods such as brine, haulm pulling, burners or pelargonic acid is also ongoing.

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