Downy Mildew on Coleus: Update
Coleus downy mildew (Peronospora sp.) has been detected in a greenhouse in North Carolina. Although this disease is not new to North Carolina, this serves as a reminder to scout regularly, particularly in the spring when temperatures are cool and humidity is high, as it can spread rapidly through a greenhouse.
Symptoms include irregular blotches on the leaves that may be angular in shape (but not always), foliage drop, and stunting and can occur at any stage of growth. Spores may be evident on the underside of the leaves as a fuzzy mat that is gray-purple to gray-black in color. Sporulation is more likely to be encountered under favorable conditions. A hand lens under adequate light may be necessary to see the spores. However, plants that do not show symptoms also may be infected.
To manage the disease, increase ventilation in greenhouses to reduce humidity and allow adequate space between plants to promote air circulation. Cultivars vary in susceptibility, as tested by Cornell University and Michigan State University, and a summary can be found here.
Preventative fungicide applications combined with the use of less susceptible varieties can provide adequate management. Some examples of commercial fungicides that are effective include: dimethomorph (Stature), azoxystrobin (Heritage), mefanoxam (Subdue Maxx), mancozeb (Protect), and copper-based fungicides. As always, read and follow label instructions on all pesticides including alternating fungicides in the same FRAC group.
To prevent disease from getting started, ensure that plants coming into the greenhouse are healthy (although this can be difficult if plants have latent infections). Keep new plants quarantined in a separate room, if possible, to be sure disease does not appear. Remove and destroy or bury all coleus at the end of the season as these plants can provide a harbor for the disease into the next year. Avoid landscape plantings of coleus near the greenhouse.
For more information, there are multiple articles available on this subject below.
This article was prepared with assistance from Mike Munster, Plant Disease and Insect Clinic, and Dr. Brian Whipker, Department of Horticultural Science.