Downy mildew is the most important disease problem in field-grown lettuce. Cool wet conditions bring a risk of the disease (caused by Bremia lactucae) so the disease is most common and spreads most rapidly in spring and autumn.
Some varieties are resistant but major gene resistance can be overcome quite rapidly so fungicides are still very important to support genetic resistance to control this pathogen. As no screening of novel fungicides for lettuce downy mildew control had been carried out in the UK for almost 10 years, the HDC commissioned project FV 357, which complemented studies on other lettuce disease to provide growers with up-to-date information.
Existing and novel fungicides were compared for effectiveness and the project considered timing, dose and persistence on crops of the Cos variety Frisco, which is highly susceptible to downy mildew. Plants were grown in pots in a polythene tunnel from June to October and were artificially inoculated with the disease. Eleven fungicides were applied once at one of four different timings: five days before inoculation; two days before; immediately prior; or two days after inoculation.
The crop was assessed three weeks after infection; Signum, Valbon and Previcur Energy performed best, followed by Revus, Invader and an experimental product coded F3. Best results were obtained from applications just before, on, or just after the day of infection. As Valbon (approved for onion downy mildew) and Revus (approved for potato blight) are not approved for lettuce, further work will be necessary to determine if Specific Off Label Approvals for use of these fungicides on outdoor lettuce could be sought.
- Horticultural Development Company
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