Downy Mildew on Coleus
Over the last 20 years, the floriculture pathology research program at Michigan State University has had two over-arching goals. First, we seek to understand pathogens and how they thrive, move and survive. Second, we work out integrated management solutions that include environmental control, cultivar resistance and fungicide product selection and application. Diseases change in their occurrence and importance over time, and we’ve had the opportunity to study many pathogens, including foliage blights, and crown and root rots. We work with graduate students and technicians, each focusing on a particular pathogen and the management strategies needed.
One of our recent efforts has included control strategies for a new downy mildew (Peronospora sp.) on coleus. Certain crops have a long history of downy mildew, such as roses and snapdragons. Others, such as coleus and impatiens, have only recently had problems with this disease. Downy mildew is a nasty pest. It’s tough to control and can cause a lot of damage in a short period of time. Downy mildew distorts plants and blights the foliage. Severely infected plants often drop their leaves. The undersides of blighted leaves may develop downy mildew seeds (spores), which can be carried from diseased plants to healthy ones by air currents. Because the spores are microscopic, they easily escape detection, but these spores can be hitchhikers on plant surfaces. If conditions are moist, spores can travel long distances on plant surfaces. If a thin film of water forms on the plant’s surface, spores will germinate and penetrate the plant. Once the downy mildew is inside of the plant, the pathogen forms a network of fungal threads that withdraw nutrients.
Our studies show that if conditions are moist and cool (59-68° F), the downy mildew readily grows and develops within the plant, causing spotting or blighting of the foliage. Once the downy mildew is well established, the fungus will begin to reproduce and make more spores.
A different scenario can play out when the downy mildew has successfully penetrated the plant but the environment becomes hot and dry (77-86° F), unfavorable conditions for the disease. The downy mildew may become “quiet,” neither growing further within the plant nor dying out. Plants with “quiet” downy mildew infections may appear perfectly healthy. This downy mildew “quiet” phase causes confusion for growers because a plant that appears healthy may develop disease almost overnight. Downy mildew can be carried on the seed in some crops. Since downy mildew can ramp up quickly, preventive measures may be needed.
Downy mildew can look different in different cultivars. While all coleus cultivars that we tested developed downy mildew, some became more diseased than others. For instance, ‘Freckles’, ‘Beauty’, ‘Russet’ (germplasm courtesy of the North Central Regional Plant Introduction Station), ‘Fairway Salmon Rose’ and ‘Fairway Rose’ all held up well in our test with only mild downy mildew symptoms. Other cultivars, including ‘Dragon Black’ and ‘Volcano’, showed more symptoms and included a lot of leaf dropping. Without disease-resistant coleus cultivars, fungicides are an important part of controlling downy mildew. We’ve conducted comparative trials with currently registered products as well as some that are not yet registered. An interesting aspect of this research has been seeing good efficacy against downy mildew when Subdue MAXX EC is applied as a drench. Because Subdue MAXX EC is applied as a drench to control Pythium, the fact that it works against downy mildew when used in this way is a great bonus. The new product Adorn 4SC applied as a drench was also tested but didn’t control downy mildew quite as well as Subdue MAXX EC. All other fungicides were applied as a spray; some of the stand-out materials included Stature SC; FenStop SC; Heritage; a new active ingredient from Syngenta called mandipropamid; and a new active ingredient from BASF.
Because fungicide programs for downy mildew protection must include a plan to delay the pathogen’s resistance to a particular fungicide or active ingredient type, it’s good that there are several effective products that can be used in rotation.