Coleus downy mildew
Know the disease symptoms and pathogen signs of coleus downy mildew, as well as which cultivars will hold up against this disease.
Disease symptoms and pathogen signs
Both seed and vegetatively-propagated coleus are susceptible. Disease symptoms may be obvious and include foliage drop, large brown blotches on leaves (Photo 1. A-D) and stunted seedlings or plants. The brown or blighted areas on diseased foliage have an irregular shape. Sometimes these spots look square or angular and are bordered by large leaf veins. Or, downy mildew may escape detection because only mild symptoms develop, which may mimic common growing problems. Coleus seedlings or plants may appear healthy and develop downy mildew symptoms later.
Peronospora sp., the causal agent of coleus downy mildew, reproduces via specialized asexual spores, called sporangia (Photo 1E) that may sometimes be seen 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 sporangia in the morning when the environment is cool and humid. Downy mildew development is closely related to temperature and humidity. A period of high relative humidity followed by lower relative humidity is required for sporangia to be released from stalks (sporangiophores) into the air.
Michigan State and Cornell Universities have conducted many experiments testing coleus cultivars for downy mildew susceptibility. Recently, Michigan State University compared 21 cultivars for disease. All plants were sprayed with downy mildew sporangia and kept in a humid research greenhouse. All of the cultivars tested became diseased, but some cultivars developed more downy mildew than others. Coleus cultivars that held up well in our study included ‘Fairway Red Velvet,’ ‘Fairway Salmon Rose,’ and ‘Fairway Rose.’ Although these cultivars showed downy mildew symptoms, they were relatively mild. Table 1 summarizes the results of many trials and rates 102 cultivars for susceptibility to downy mildew. Click here to view table.
Combining a less susceptible cultivar with fungicides may be necessary to manage downy mildew effectively. Fungicide studies have identified products for growers to use that are successful in managing coleus downy mildew, such as the systemic products Heritage (azoxystrobin), Subdue MAXX (mefenoxam) drench, Adorn (fluopicolide), and Fenstop (fenamidone), and protectant products such as mancozeb, copper, and Stature SC (dimethomorph). Products available for homeowners for control of downy mildew include products that contain copper, mancozeb and phosphorous acid. Products should be labeled for downy mildew and applied according to label directions.
This research was funded in part by Floriculture and Nursery Research Initiative of the Agricultural Research Service under Cooperative Agreement #59-1907-5-553 and by the American Floral Endowment.
Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.