Syngenta area manager for south Lincolnshire Phillip Ayres said: "High levels of watering to combat deepening soil-moisture-deficits risk washing off fungicides.
"Meanwhile, the humid conditions beneath the dense crop canopy create ideal conditions for disease."
Most growers in the Anglian region, a key potato-growing area, were operating at the limit of their irrigation capability, he said.
"Growers need to make sure they are using blight fungicide that can withstand heavy irrigation or sudden summer storm downpours," Ayres added.
He said recent trials of the product Revus suggested better blight protection following irrigation against other treatments.
Even modest irrigation of 20mm an hour after treatment with cymoxonil and mancozeb seriously reduced its performance, for example, he said.
"Revus performance was maintained at a consistently high 98 per cent-plus control right across all the irrigation programmes.
"Even when blight forecasts indicate low disease risk, beneath the leafy canopy it can remain hot and humid several days after irrigation - the ideal condition for blight."
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