Cause Bremia lactucae, has been reported from Oregon and found by the OSU Plant Clinic but not Plasmopara halstedii, which can also cause downy mildew. The name Bremia centaureae has been reported from California. These fungi overwinter on wild host species in the Asteraceae family or in plant debris left in the soil. The disease is worse in damp, foggy weather and at cool temperatures (43°F to 53°F). Although not tested directly, it is expected that downy mildews from lettuce and Bachelor's Button are different and will not infect each other.
Symptoms On the lower surface of mature leaves, light gray to white downy patches are sometimes preceded by slight chlorosis. Downy patches are matched on the upper leaf surface by severe chlorosis. Lesions frequently appear angular. Affected portions turn brown, and leaves eventually die. Younger leaves (near flowers) may be distorted and twisted.
Cultural control
- Control weeds in and near the planting, because many weeds are hosts.
- Space plants wide apart to provide full light and air circulation.
Chemical control Focus on cultural controls. If using chemicals, alternate them to prevent building up resistant fungal populations.
- Aliette at 1 to 2 lb/100 gal water. Try on a few plants before using on a large scale. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- According to IR-4 data, Heritage is safe on some species of this genus. Use at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker on a few plants first before wide spread use. Group 11 fungicide. 4-hr reentry.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Orvego at 11 to 14 fl oz/100 gal water. Group 40 + 45 fungicide. 12-hr reentry.
- Segovis at 0.6 to 3.2 oz/100 gal water plus another fungicide. Group 49 fungicide. 4-hr reentry.
Reference Morgan, W.M. 1981. Bremia lactucae. CMI Descriptions of Pathogenic Fungi and Bacteria, No. 682. Surrey, England: Commonwealth Mycological Institute.
Thines, M., Runge, F., Telle, S., and Voglmayr, H. 2010. Phylogenetic investigations in the downy mildew genus Bremia reveal several distinct lineages and a species with a presumably exceptional wide host range. European Journal of Plant Pathology 128:81-89.