The following is a good article reminding to keep scouting for downy mildew in greenhouse production areas.
Continue Watching Out for Downy Mildew
Cool, wet weather of the past few weeks has been favorable for the development of downy mildew on coleus. Symptoms on leaves range from small brown blotches to angular brown lesions to distortion and defoliation, depending on cultivar. Symptoms of downy mildew can mimic symptoms of Botrytis blight, drought stress or nutritional problems, so it’s important to get a confirmed diagnosis if you suspect downy mildew. The photo shows symptoms on Helichrysum. Growers can expect to find this disease in seed and vegetatively propagated types of coleus. John Speaker, IPM Scout, noted that he usually find it in vegetatively grown coleus first. If it shows up there it will spread to seed grown coleus. The fungus reproduces via specialized spores called sporangia. If you examine the undersides of the foliage you may sometimes see the gray colored sporangia on the underside of the coleus leaves. In some instances, these sporangia may be few in number and very difficult to see without the help of a microscope. Other times, the sporangia are produced in high numbers and form a fine carpet of grayish fuzz on the underside of the leaf that is obvious to the naked eye. It is best to look for these sporangia when the greenhouse environment is humid and damp.
Downy Mildew Control: Check all incoming coleus plants and cuttings for symptoms of the disease. Cultural practices that reduce humidity in the crop (increased air circulation, increased plant spacing) can help reduce spread of the disease. Discard infected plants by placing in plastic bags before carrying them out of the greenhouse to avoid spreading sporangia during plant removal. An effective protectant fungicide program includes rotating between Stature (dimethomorph), Fenstop (fenamidone), and mancozeb products (such as Protect DF). After the crop is finished, remove plant debris and wipe down surfaces with a greenhouse disinfectant. If you’ve had a problem with coleus downy mildew, make sure to discard any coleus stock plants to keep the disease from surviving in your operation until your next crop.
Information from the June 12, 2009 edition of the Greenhouse TPM/IPM Bi-Weekly Report from the University of Maryland Cooperative Extension Central Maryland Research and Education Center