Growers are being warned of a heightened risk of downy mildew infection following the recent spell of warm days and cool nights.

Alan Horgan, Certis’s technical officer, is advising outdoor and protected lettuce and spinach growers in particular to keep on top of their fungicide programme.

“Daytime temperatures of more than 16oC, coupled with cool nights as low as 7oC, generates leaf wetness, creating ideal conditions for downy mildew to sporulate and thrive,” he said. “In the right conditions, infection can develop in just a couple of hours.”

The crop environment can also contribute to disease levels, with sheltered, dense microclimates within the crop and a lack of air movement providing optimal conditions for spore germination and rapid disease infection.

“A fungicide programme using different active ingredients can reduce the risk and offers growers the best chance of control,” said Horgan.

He advises that with the issues of resistance to certain active ingredients, the use of products such as Aliette 80 WG should be included within the programme. As a systemic fungicide, Aliette provides disease protection up and down the entire plant, protecting new roots, shoots and leaves. There is also no known resistance of downy mildew to Aliette 80 WG in the UK, according to Horgan.

Incorporation into the control strategy can include applications at either propagation or following planting out in lettuce crops. It may be applied as either a spray or drench treatment for all lettuce and protected spinach applications and is subject to a 14-day harvest interval. Outdoor spinach crops require application as a spray treatment, with a seven-day harvest interval being observed. Growers are also reminded that the use of Aliette 80 WG for spinach and lettuce is off label and use is entirely at their own risk.

Horgan adds that lettuce and spinach varieties often offer limited resistance to the many strains of downy mildew. Recent Horticultural Development Company research is adding to the debate relating to strain 10 resistance being present in spinach, with race 8 now confirmed in the crop in the UK.

“Within lettuce crops, as no one variety provides resistance to all strains of the disease and as increasing chemical resistance is being experienced, growers are being met with an ever increasing and complicated task in achieving control,” added Horgan.

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