Downy mildew was first observed on coleus in New York and Louisiana in 2005 and, by 2006, it was found throughout most of the United States. Symptoms include leaves dropping off of plants, brown blotches on leaves (see Figure 1A), and stunted seedlings. Both seed and vegetatively propagated types are susceptible. The brown or blighted areas on diseased foliage have an irregular shape and can cause the leaf to twist and drop. Sometimes these spots look square or angular and are bordered by large leaf veins. The fungus reproduces via specialized spores called sporangia 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 (Figure 1B). These lemon-shaped sporangia are produced on a spore stalk (Figure 1C). It is best to look for these sporangia when the environment is humid and damp.
The fungus that causes downy mildew on coleus is tricky and elusive. Sometimes the disease is obvious and other times it may lie quietly in the plant tissue until the conditions are just right. For this reason, it is important that all coleus that may be left between production seasons be destroyed. Resist the urge to carryover favorite cultivars. Also, it is not advisable to use coleus in outdoor plantings that border production greenhouses. It is possible that coleus seedlings and/or cuttings may arrive at your greenhouse and appear healthy, only to develop downy mildew later. Since this disease is relatively new to the United States, research has been needed to determine whether there is a difference among cultivars and which fungicides work best. Knowing these fundamentals will go a long way in producing a crop that looks healthy and stays healthy in the landscape.
Although it seems that all coleus cultivars may be affected by downy mildew, the amount of blighting and leaf dropping that results may vary among the cultivars. Coleus cultivars may also differ in how many sporangia are produced on coleus leaves. This is an important characteristic because the sporangia are responsible for spread of the downy mildew. Wind currents or splashing water dislodge sporangia and make them available to infect nearby healthy plants. In a recent Michigan State University study, 15 coleus cultivars were compared for leaf blighting and sporangia production. All of the varieties, with the exception of ‘Dragon Black,’ were supplied from the USDA-ARS North Central Regional Plant Introduction Station for testing. Plants were sprayed with the downy mildew pathogen and kept together in a humid research greenhouse. All cultivars tested developed disease symptoms although some cultivars became more diseased than others. Examples of the coleus cultivars that held up well in our study included: ‘Freckles,’ ‘Beauty,’ ‘Russet,’ and ‘Harlequin,’ and ‘Pegasus’. Although these cultivars showed downy mildew symptoms, the disease was relatively mild. High susceptibility to downy mildew was observed on cultivars ‘Duke Yellow,’ ‘White Gem,’ and ‘Cristata’ (see table). The popular variety ‘Dragon Black’ was again observed to be very susceptible and if you are determined to grow this variety, preventive fungicide applications might be necessary. In other studies, varieties ‘Volcano,’ ‘Wizard Rose,’ ‘Wizard Golden,’ ‘Wizard Coral Sunrise,’ and ‘Versa Watermelon,’ were all determined to be highly susceptible as well. Most varieties that had the highest percentage of sporulating foliage also had the highest concentration of sporulation on each leaf. Unfortunately at this time we cannot for sure say that we know of a variety that is completely resistant to this new pathogen, however, some varieties appear to be less affected. Below is a table showing what the general susceptibility levels of some varieties we have tested.
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