Coleus Downy Mildew Strikes Again
Tom Creswell, Plant Diagnostician, P&PDL
Director, Purdue University
First identified on coleus in the US in 2005, downy mildew has
become an annual threat to production of this popular annual. The
disease is caused by the fungus Peronospora lamii, and
is spread primarily by air-borne spores. The fungus can also infect
basil and agastache. The symptoms may range from barely noticeable
spots to leaf distortion to defoliation of the entire plant. Cultivars
show extensive variation in how they react to the fungus and symptoms
may mimic impatiens necrotic spot virus, Botrytis blight,
drought stress or spray burn. High humidity favors spore production
and spread of the disease. Spores are produced on the undersides
of leaves and can resemble Botrytis sporulation.
Recommendations for greenhouse growers:
Check incoming coleus and agastache carefully for any signs
of disease before placing them in the greenhouse.
Keep greenhouse humidity low to reduce spore production.
Confirm suspected downy mildew by sending
samples for diagnosis to your state’s plant disease
Cover diseased plants in a plastic bag before removing them
from the greenhouse to reduce spread of spores.
Spray with an appropriate fungicide to protect uninfected plants
before the problem appears.
The disease is less likely to be a problem in the landscape
than the greenhouse.
Look for healthy transplants without leaf spots or distortion
of the leaves.
Avoid sprinkler irrigation. Water by trickle irrigation on
the soil instead.
Bag and remove badly infected plants from the garden.
Most fungicides used against this fungus are available only
to professional applicators. For more information on fungicides
labeled for use, see "Downy
Mildew of Landscape Plants" (pdf file)
Click image to enlarge
Spores are produced on the undersides of leaves
Microscopic spores of the downy mildew fungus