Potatoes SPot Farm desiccation trials (2019-2020)

Key findings

• Within individual experiments, Spotlight/Gozai combinations were no different to flail or Saltex in terms of skinset at 3 weeks post-T1, but when averaged over all experiments and demonstrations, there was a small (1‑2 day) advantage in reaching skinset for flail and Saltex methods compared with Spotlight, Gozai and Finalsan chemical methods.

• Flailing ahead of chemical treatment resulted in instant canopy death in all the ware crops and there was no regrowth within 4 weeks of T1. However, in both years the seed crop in Scotland had some regrowth from basal nodes on flailed stems.  

• Saltex was the most effective chemical in removing leaves in 2020. This result differed from that in 2019 where Saltex efficacy varied between sites, depending on atmospheric conditions immediately following spraying.

• Spotlight/Gozai treatments used in 2019 were all similar in terms of leaf death, but were only 2-4 days slower in killing leaves than diqquat [Reglone]. Spotlight only, Gozai only or Spotlight/Gozai combinations all worked similarly in terms of leaf death and skinset.

• Finalsan was generally slower in killing leaves than other chemical treatments but was equal to Spotlight/Gozai combinations in time to skinset.

• Stems were much harder to kill chemically than leaves. There were variable rates of desiccation across experiments, but a similar ranking in rate of desiccation between treatments was found in both stem and leaf desiccation.

• Differences in the rate of foliage desiccation between treatments did not correlate well with skinset, the key criterion for the ability to harvest without damage.

• Reducing nitrogen (N) fertilizer rate by 15 % from the recommended RB209 rate had little effect on skinset, advancing it, on average, by less than 1 day.

• Plots that were left to grow on without defoliation and maintained close to complete cover increased in yield by c. 7 t/ha in 3 weeks compared with plots which were desiccated or flailed. There was no evidence to support differences in ‘passive bulking’ (yield increase after T1 applications) between chemical or mechanical treatments.

• Not defoliating slowed skinset in most experiments.

• There was no effect of defoliation method, chemical or timing on vascular browning or stem-end necrosis.

• There were no effects of chemical or mechanical method of haulm destruction on rotting or black dot diseases at harvest.

Practical recommendations

• The aim should be for early- to mid-morning application of PPO desiccants to give the chemical maximum time to kill cells. Time of application for Spotlight/Gozai can be more crucial later in the season when it is cooler.

• In dry soils, skinset is faster.  The timing of the last irrigation prior to desiccation (particularly of salad crops), would influence skinset.  Aim to stop irrigation for 7 days prior to desiccation, but no earlier owing to the risk of tuber dehydration increasing the risk of bruising.

• There was, on average, only a small (1-2 day) delay in skinset for Spotlight/Gozai compared to flail or Saltex, but this should be factored into any harvesting schedule.

• Crops which were indicating signs of active senescence (ground cover less than 98 %, lodging, brittle leaf texture and paler green colouration) responded rapidly in terms of leaf death when chemical desiccation took place. Where crops did not demonstrate these symptoms and were actively growing at T1 timing, leaf death was more prolonged.

• In crops or at sites which have difficulty in achieving skinset owing to active green canopies at desiccation, 10-20 % less nitrogen than the RB209 amount should be tested to try and advance canopy senescence, despite the lack of evidence that this had an effect in 2020. Growers or agronomists should re-visit RB209 to calculate nitrogen rates and compare with commercial practice.

• It is important to kill all leaves and prevent regrowth for control of tuber blight or virus infection in seed.

To provide independent information to help inform guidance to potato growers on the best methods to desiccate crops in the absence of diquat.  The emphasis was on the ‘hard to stop’ situations, with long-season, indeterminate varieties and actively growing seed crops.  This research was closely aligned with the AHDB potatoes Knowledge Exchange programme, so most of the sites were on existing or former Strategic Potato (SPot) Farms or demonstration sites.

Visit our SPot Farm page

The trials compared chemical desiccants and flail-and-spray approaches. The work assessed different sequences of protoporphyrinogen oxidase (PPO) inhibitors (pyraflufen-ethyl [Gozai, Belchim] and carfentrazone-ethyl Spotlight Plus, FMC]) and compared them with undefoliated control treatments.  The desiccants were also combined with flailing and in 2019 diquat was included for comparison. The use-up period for diquat ended on 4 February 2020 and it was not included in the 2020 trials.

Some experimental  treatments (Finalsan [pelargonic acid, Certis] and Saltex [concentrated brine solution, Omex] were assessed in both years.   The trials included vigorous, complete canopies at close to commercial defoliation timings and assessed the speed of leaf and stem desiccation, skinset, and effects on yields and internal defects on both ware and seed crops.