Rijk Zwaan UK, based in York, said the new spinach downy mildew - Pf 11 race - was likely to spark huge demand for resistant varieties of spinach.
"Spinach growers are now aware that another race of spinach downy mildew has been positively identified," said leafy salads account manager Gerard van der Hut.
"You should grow as (many) resistant varieties as you can because the chemical armoury has become extremely limited.
"Chemical treatments are insufficient and growers are relying on the Horticultural Development Company to look for new types of chemicals, but this will take time."
However, chemicals were still important in the fight against early, weaker strains, which could mutate into more powerful mildew if untreated, he said.
"Harvesting of spinach takes place in the next few weeks, and later in the year we will have an idea of the quantity of seed available for next season's growing. Chemicals are still vitally important, but it is becoming very important to make sure you have resistant varieties at certain times of the year."
Rijk Zwaan produces resistant varieties such as Swan, Squirrel and Toucan, the latter being the main summer variety resistant to downy mildew.
Crop-protocol expert Ian Gillott said: "The worry is that there may not be enough seed available for resistant spinach, which is a concern to growers.
"They need to diversify their varieties and order seed well in advance without panic buying. It is absolute nonsense to overestimate your requirements."
Malcolm Kemp, who runs Kemp Herbs in Norfolk with his son David, said: "We have not been affected, but the best way of combating mildew is through hygiene."
He advised spinach growers to dispose of debris as soon as they finished harvesting and to avoid growing the crop in the same field for at least five or six weeks.
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