Lesions tend to be circular and spread across leaf veins. Fungus mycelium and spores are external to the host. Host tissues usually yellow after fungus growth is evident. Bremia lactucae 2. Fungus spores are produced singly on a branched conidiophore.
Lesions tend to be angular and confined within leaf veins.
Fungus mycelium is internal to the host, and only the conidiophore and spores are external. Host tissues usually yellow before fungus growth is evident. Control of powdery mildew is problematic. Fortunately, most powdery mildew infections in lettuce occur near maturity, so harvest may be possible before treatment is necessary. If treatment is called for, wettable sulfur and several copper compounds are the only materials available which will provide some suppression of powdery mildew in lettuce.