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30 Jun 2021

Post-diquat potato desiccation messages emerging

As featured in Arable Farming Magazine

Post-diquat potato desiccation messages emerging

by Arable Farming

Two years of AHDB-funded research* show there is plenty still to learn about how to best use the alternatives to the recently withdrawn Reglone (diquat). Andrew Blake reports.

In the absence of diquat, work led by potato consultant Dr Mark Stalham, former head of NIAB CUF, has been exploring the effects of different sequences of replacement desiccants (see panel) with and without mechanical flailing and comparing them with undefoliated controls.

Brine products currently have no approval as desiccants, but this treatment was included for research purposes.

The research used a range of varieties and targeted vigorous, complete canopies close to commercial defoliation timings.

Speed of leaf and stem desiccation, skin set, yield effects, internal defects and disease incidence on both ware and seed crops were all assessed.

Skin set Also tested was a reduced rate of nitrogen fertiliser (RB209 recommendation less 15%) to examine its impact on skin set.

Achieving adequate skin set to avoid harvest damage is important, says Dr Stalham.

One of the surprises has been that differences in the rate of foliage desiccation between treatments did not correlate well with skin set, he adds.

But when averaged over all experiments and demonstrations, skin set from the replacement treatments was reached only slightly later (one to two days) than when using diquat or flailing.

Industry wisdom would expect greater differences, says Dr Stalham.

The work, conducted mostly on Strategic Potato (SPot) Farms, did highlight some significant differences in the time needed to reach skin set.

Two crops were skin set just two weeks after treatment.

Two others, a Daisy seed crop in Scotland and Royal crops in Shropshire and Suffolk, were still not set after four weeks.

There was even some reversing or ‘unsetting of skins in the Royal crop in Shropshire between three and four weeks after treatment.

Anything already showing signs of active senescence at the first treatment timing responds rapidly to chemical desiccation.

How we get there is the key target.

Flailing ahead of chemical treatment resulted in instant canopy death in all the ware crops and there was no regrowth within four weeks of T1.

However, as we saw in 2019, the seed crop in Scotland had some regrowth from basal nodes on flailed stems.

Aim for early- to mid-morning application of PPO desiccants to give the chemical maximum time to kill cells, he says.

The application timing for Spotlight and Gozai can be more crucial later in the season when its cooler and duller. Treatments Part of future work should focus on pre-flail chemical treatments, at reduced rate, to determine whether this prevents regrowth following flailing.

The effects of water volumes, spray additives and certain types of nozzles could all merit further investigation, Dr Stalham adds.

Not defoliating slowed skin set in most of the experiments, but encouragingly there was no effect of defoliation method, chemical or timing on vascular browning or stem-end necrosis.

There were also no problems with stolon detachment in most crops. There was some stolon plug removal in Daisy and Royal, but it was unrelated to chemical or mechanical defoliation, and the method of haulm destruction had no impact on rotting or black dot diseases at harvest.

Finalsan was the slowest acting of the tested actives on foliage but was equal to Spotlight/Gozai combinations in time to skin set.

Reducing nitrogen fertiliser rate by 15% from the recommended RB209 rate had little effect on skin set, advancing it, on average, by less than a day.

But the practice is worth pursuing, Dr Stalham adds.

In crops or at sites which have difficulty in achieving skin set owing to active green canopies at desiccation, 10-20% less nitrogen than the RB209 amount should be tested to try to advance canopy senescence.

Skin set depends on a combination of factors, not just leaf death.

Stem desiccation in Royal and Daisy delayed skin set despite the fairly rapid loss of leaf cover after desiccation or flailing.

Mechanical methods stop carbohydrate assimilation immediately, but theres an opportunity for flailed crops with active root systems in wet soil to take up water, reduce tuber dry matter concentration and increase fresh weight yield.

This can result in a yield increase of three to five tonnes per hectare, with more of the crop becoming oversize, which can be crucial to crop value in seed or salad crops.

However, there was no evidence this ‘passive bulking differed across chemical treatments.

Its also important to kill all leaves and prevent regrowth for the control of tuber blight and virus infection in seed, he says

In the field Jim Reid, Aberdeenshire

Research to find the best way to desiccate potato crops in the absence of diquat is certainly worthwhile, but particularly so for seed producers, according to Jim Reid, who grows about 70 hectares of seed on medium loam at Milton of Mathers, St Cyrus, Scotland.

He says: From a seed producers perspective, desiccation is now even more critical because of the new rule which permits only one insecticide treatment of ware crops.

That means over-sized tubers from a seed crop, which can get two or more insecticides, may no longer be sold as ware, he adds.

So, desiccation timing is crucial to get the right seed fraction.

As well as hosting AHDB work, Mr Reid has conducted trials with Scottish Agronomys Eric Anderson since 2012.

Mechanical flailing is best avoided, especially in Scotland where crops are slow to senesce naturally, he believes.

It just tells them to regrow and risks spreading blackleg.

On our soil type the compaction it causes on the rows also slows harvest.

I feel that where a flail is to be used, a chemical treatment seven days before needs to be applied to shut the crop down. Pruning any plant encourages fresh growth and the potato plant is no different.

Mr Reid offers some pointers from the 2020 campaign.

Group 1 and 2 (determinate) varieties are much easier to desiccate than Groups 3 and 4 (indeterminate).

Chemical treatment pre-flailing avoided regrowth and stolon detachment at harvest was better; and while PPO inhibitors were slower to act without flailing, stolon detachment was better at harvest.

As with any contact-acting material, application is very important.

The water volume should be at least 400 litres/ ha and nozzle choice is crucial, he says.

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