Unique challenges


01 June 2022
Scottish agronomist responds to the challenges of potato haulm desiccation in the North-East Highlands

ANNUALLY advising on 500ha of potatoes across Moray, Inverness-shire, Black Isle and Easter Ross, Agrii agronomist Ed Scaman is quick to acknowledge the unique challenges he faces each season when working with potato growers based in Northern Scotland, where sustained dry weather windows are both rare and often difficult to predict.

Of course, adverse weather and soil temperatures dictate all areas of potato production, from planting dates through to both weed and blight control. But, as the growing season ends, crop desiccation - in particular, can be the toughest challenge when so much of the strategy planning is based around establishing a dry weather window confirms Ed.   

“Colder early season soil temperatures and shorter summers mean we plant later and tend to harvest much earlier and it’s this compressed season that dictates every aspect of crop management. Inclement early season weather can often lead to a wide variation in emergence times and because of our summer humidity combined with generally higher rainfall, both late blight (Phytophthora Infestans) and desiccation pose significant challenges.

“In 2018 we fully committed to a flail and spray approach, advising customers to adopt a new desiccation approach before the diquat safety net was removed. In hindsight I believe this was the right strategy, giving us time to assess the merits of key PPO inhibitors carfentrazone and Gozai (pyraflufen-ethyl).

“Critical factors for successful desiccation must include flailing well to ensure clear separation of stems and foliage. Stems need to be 15-25cms in length and PPOs should be applied in the morning – ideally in bright, sunny conditions to maximise their efficacy.

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“If an indeterminate variety is being grown then it is advisable to apply a PPO pre-flail to create a “false” senescence in the crop. However, with a determinate variety you’re more likely to get a natural stop on the canopy size, allowing you to look for a dry weather window to flail and then apply the first PPO. Given our shorter season I generally begin talking to customers about starting desiccation in late August - given weather and temperatures can often deteriorate quickly into September.

“Optimum weather conditions for desiccation within my territory are rare, however – if a crop has begun to senesce then I will still actively look for a 3–4-day dry weather window. Beginning with 1l/ha of carfentrazone to open the crop, we then flail and apply Gozai at its full rate of 0.8l/ha with methylated seed oil (MSO) alongside an application of Ranman Top (cyazofamid) to give the crop on-going tuber blight protection through desiccation whilst killing any remaining zoospores post-flailing. To ensure good product coverage I’d always advise applying Gozai in water volumes of not less than 300l/ha and making sure that the PPO is applied no later than 48-72 hours after the crop was flailed.

“On seed crops, once tubers have reached their target size then you need to be decisive and, if the dry weather window is there, go into the crop first with a PPO to create that “false” senescence before flailing and then applying up to 2 sprays of Gozai at a rate of 0.8l/ha per application post-flail.

“Seed crops can be very tricky, given that they want to keep growing, so the key is to follow up quickly with the first application of Gozai post-flail to minimise the potential problem of stem regrowth, and to kill off the crop before any other disease can take hold in the crop.

“Due to our geographical position and weather patterns, I rarely advise a chemical-only desiccation strategy to a customer. However, if the customer cannot flail – and if a determinate variety is being grown and has naturally begun to senesce then I would generally advise 2 applications of Gozai at 0.8l/ha + MSO followed by 1 application of carfentrazone at 1.0l/ha at 7-day spray intervals until the crop is desiccated. Growers must bear in mind that a chemical-only strategy is less economical and will take longer to achieve a successful outcome than a flail and spray approach” he concludes.

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