Cause The fungus Peronospora lamii. The disease has been found on Lamium in Oregon. The cultivars Beacon Silver and White Nancy are susceptible. The disease is favored by cool, wet conditions. Spores are produced under high humidity and are released in large numbers when relative humidity drops (particularly in the morning hours). Spores are carried through the air, spreading the disease under favorable environmental conditions.
Symptoms Angular leaf spots, particularly on the lower leaves. Leaf spots are often delineated by the leaf veination. May be reddish or purple depending on cultivar grown. Undersides of the leaves often have a gray fuzzy fungal growth.
Cultural control
- Inspect new plugs and plants for signs of this disease.
- Grow in dry conditions were leaves are not wet for extended periods of time.
Chemical control No fungicides are specifically registered for this crop; however, the following may be effective. Rotate fungicides from different groups that have different modes of action for resistance management.
- Segway SC at 2.1 to 3.5 fl oz/100 gal water. Drench rate is based on pot diameter. Group 21 fungicide. 12-hr reentry.
- Stature SC at 6.12 to 12.25 fl oz/100 gal water. Did very well in one trial in NC. Group 40 fungicide. 12-hr reentry.
Reference Thines, M., Telle, S., Ploch, S., and Runge, F. 2009. Identity of the downy mildew pathogens of basil, coleus, and sage with implications for quarantine measures. Mycological Research 113:532-540.