Downy
mildew was found in a lettuce field in the Valley this week by Erika Bent (APM
Agricultural Pest Monitoring Cons. Ltd.). We don't always see lettuce downy mildew (Bremia lactucae Regel), but it can be very destructive. Spores are spread via splashing water or wind. Air currents can carry spores several kilometers. Cool temperatures (between 15' and 21' Celsius) are conducive to disease development, especially under wet conditions. This includes heavy dew set in the mornings and overhead irrigation.
Infection usually starts in the older leaves and travels inward. Pale green or yellow areas will become apparent on the upper surface of the leaves, often having an angular appearance, with sporulation occurring on the underside. This infection can spread very quickly, especially under favourable growth conditions, so be sure to check your fields often!
Fields that have been recently harvested are ideal locations for disease buildup to get out of hand, so be sure to plow down lettuce residue.
There are numerous fungicide options, which can be found on the Perennia Lettuce Management Schedule. Downy mildew can build up resistance to pesticide group modes of action very quickly, so be sure to rotate between pesticide groups at every application. While there are varieties that have been bred to be resistant to downy mildew, due to the quickly evolving nature of this disease, varieties may not be resistant to all strains of downy mildew.
