Leaves dropping off plants, brown blotches on leaves and stunted seedling: all telltale symptoms of downy mildew on coleus. 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. It is best to look for these sporangia when the greenhouse environment is humid and damp.
The fungus that causes downy mildew on coleus poses a challenge to growers. 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 carry over 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 is 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 also may 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.
A recent Michigan State University study compared 21 coleus cultivars for leaf blighting and sporangia production.
All plants were sprayed with the downy mildew pathogen and kept together in a humid research greenhouse. All of the 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 ‘Fairway Mosaic’, ‘Fairway Red Velvet’, ‘Wizard Velvet Red’, ‘Fairway Salmon Rose’ and ‘Fairway Rose’. Although these cultivars showed downy mildew symptoms, they were relatively mild. One of these, ‘Fairway Red Velvet’, appeared to have very sparse sporulation. Another cultivar, ‘Fairway Lemon’, also showed very sparse sporulation, although the blighting was at the moderate level.
Since so many cultivars are susceptible to downy mildew, fungicides are needed for protection. Fungicide studies have been conducted at Michigan State University with products that are currently registered and others that are not yet registered. One study included a new active ingredient called mandipropamid and compared it with the registered products of Heritage (azoxystrobin), Stature (dimethomorph), and Subdue MAXX (mefenoxam) alone and in combination. In this trial, all treatments were effective and limited downy mildew. In another trial, fungicides tested included Stature DM 50WP, Pentathlon LF (mancozeb), Subdue MAXX EC, Insignia (pyraclostrobin), and Terrazole 35WP (etridiazole). All treatments effectively protected the plants except for Terrazole 35WP. A third study compared the new active ingredient fluopicolide and other registered products including FenStop SC (fenamidone), Stature DM 50WP, Pentathlon LF and Segway 400SC (cyazofamid). All of these treatments were very effective in preventing downy mildew.
Applying fungicides for downy mildew protection must include a plan that delays the ability of the pathogen to become accustomed to a particular fungicide or active ingredient. This can be accomplished by mixing two fungicides together, but each one must have a different mode of action. Another common approach is to mix up the fungicide program by alternating products and active ingredients.