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 [T1] 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 it’s 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 there’s 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.
“It’s also important to kill all leaves and prevent regrowth for the control of tuber blight and virus infection in seed,” he says