P&PDL Picture of the Week for
February 23, 2009

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 diagnostic lab.

  • 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.

For gardeners:

  • 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

Downy mildew on coleus

Downy mildew on coleus

Downy mildew on coleus

Downy mildew sporulation on coleus

Spores are produced on the undersides of leaves

Downy mildew spores

Microscopic spores of the downy mildew fungus

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service