Too many potato growers could be shutting down their irrigation schedules too soon, leaving soils over-dry and crops at risk of excessive bruising.
“Most growers take great care with their early-season watering for scab control, but often neglect their irrigation during August,” says Dr Mark Stalham of Cambridge University Farm (CUF).
“Many growers in the south regard mid-August as the cut-off date and northern growers even earlier. While it may rain more frequently during the second half of the month, when it doesn’t, many people believe that it will rain soon and don’t reconsider their decision to stop irrigating,” he explains.
Growth can suffer when irrigation is stopped prematurely. This can affect yield but, more importantly, Potato Council-funded work carried out by CUF from 2004-07 indicated that letting soil moisture deficits increase beyond the optimum for growth could lead to an increase in bruising.
In an attempt to quantify this, a three-year Potato Council-funded project is examining 12 commercial fields managed according to growers’ protocols in each year, as well as two experimental crops subjected to controlled irrigation treatments in the last four weeks of growth to build up different soil moisture deficits before desiccation.
So far, firm findings are proving elusive, Mark admits. “In 2011 especially, the late season watering sites have generally been wet enough not to restrict crop growth, so we saw little difference between the programmes.
“However, we know late-season management is critical, and the underlying feeling is that bruising has been a major issue this season in many areas. On both experimental sites we found 60 per cent incidence of bruising across all treatments.
“The theory is that the onset of blackspot bruising is largely related to senescence – the earlier and more rapidly this happens, as occured this year, the more severe bruising tends to be. The moral of this is to monitor irrigation throughout August if the crop continues to grow. This means maintaining a moderate deficit, for example 45mm for sandy soils, 55mm for a sandy loam and 70 mm for silts.”
In a more normal season he is convinced the findings will bear out earlier work. Growing conditions were relatively benign in 2009, but no rain fell during the last 10 weeks of crop growth, and soil moisture deficits crept up to 65-80mm in September on unirrigated crops.
“At that stage, you can do little to undo the potential for bruising – those growers who continued with their scheduling and irrigated or received rainfall were the ones who had crops that didn’t bruise.”
It is not an easy message to sell – some growers point out that delayed skin set can become a problem in wet soils, he acknowledges. “There might have to be a trade-off if it rains after irrigation is resumed, but to me bruising will always be the more critical of the two and the Potato Council work is trying to understand this better.”
Others fear harvesting difficulties, but any irrigation applied will make little difference if rain arrives in sufficient quantity to cause such problems, Mark maintains. “If crops are still actively growing to the end of August, don’t give up scheduling two weeks earlier.”
He hopes to use some of the 600+ commercial fields that come under CUF’s commercial irrigation scheduling programme in 2012 and monitor produce as it comes out of store and goes through QA.
“This will add to the programme and should give us more data to work with, to help quantify the benefits of late-season watering in a dry August,” says Mark.