Watch for downy mildew on coleus

Downy mildew on coleus was a big problem last year and it’s causing trouble for growers in 2008. Both seed- and vegetatively propagated coleus are susceptible. Disease symptoms may be obvious and include leaves dropping off of plants, large brown blotches on leaves or stunting. Downy mildew may escape detection because only mild symptoms develop that can mimic other common growing problems.

{sidebar id=57}

When the greenhouse is humid and damp, downy mildew is produced via specialized spores called sporangia that may sometimes be seen on the underside of coleus leaves. These sporangia may be few in number and very difficult to see without a microscope.

Other times, sporangia are produced in high numbers and form a fine carpet of grayish fuzz on the underside of leaves that is obvious to the naked eye. Coleus seedlings or cuttings may appear healthy but develop downy mildew later.

Varying symptoms

Downy mildew can look different depending on the cultivar. The amount of blight and leaf drop caused by downy mildew may vary. At Michigan State University, we compared 21 coleus cultivars for leaf blighting and disease. All plants were sprayed with downy mildew sporangia and kept in a humid research greenhouse. All of the cultivars tested became infected, but some cultivars developed more downy mildew than others.

Cultivars that held up well in the study included ‘Fairway Mosaic,’ ‘Fairway Red Velvet,’ ‘Wizard Velvet Red,’ ‘Fairway Salmon Rose’ and ‘Fairway Rose.’ Although they showed downy mildew symptoms, they were relatively mild.

‘Fairway Red Velvet’ appeared to have very sparse sporulation. ‘Fairway Lemon’ also showed very sparse sporulation, although the blighting was at the moderate level.

Controlling downy mildew

Since so many cultivars are susceptible to downy mildew, fungicides are needed for protection. Fungicide studies have been conducted at Michigan State with products that are registered and others that are not yet registered.

One study included a new active ingredient called mandipropamid, which was compared to Heritage, Stature and Subdue MAXX EC (applied as a drench) 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, Subdue MAXX EC (applied as a drench), Insignia and Terrazole 35WP. 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, Stature DM 50WP, Pentathlon LF and Segway 400SC. 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 control approach is to vary the fungicide program by using a different product or active ingredient for each spray treatment.

The author thanks Blair Harlan for his technical assistance. This research was funded in part by the USDA/ARS through a Cooperative Agreement as part of the Floriculture and Nursery Research Initiative and by the American Floral Endowment.

{sidebar id=1}

- Mary Hausbeck

August 2008