Watch out for downy mildew on coleus

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

There has been a report of downy mildew on cutting propagated coleus. If you’re growing these very popular plants, Dr. Mary Hausbeck, MSU greenhouse plant pathologist, has summarized what you need to know about how to identify the disease and prevent or control an outbreak.

Downy mildew checklist

  • Yellowish or pale green foliage
  • Downward curling of the leaves
  • Distortion of the leaves
  • White to light gray fuzz on the undersides of the leaves
  • Emerging leaves that are small and/or discolored (yellow or pale green)
  • Plants may be stunted

The most obvious sign of downy mildew is the white, grayish fuzz that develops on the underside of the leaves. This can go unnoticed because growers are not accustomed to turning over the leaves to look for plant problems. Perhaps other than an especially severe case of Botrytis, no other disease causes such obvious gray fuzz, especially on the underside of the leaf. This disease should not be confused with powdery mildew, which affects some floriculture crops and has white fuzzy growth.

When scouting coleus, examine fully expanded leaves on one out of every 30 plants, paying special attention to the undersides of leaves. Plants may be infected with downy mildew but not show white or grayish fuzz immediately, so scout at least weekly. Diseased plants should be discarded immediately but not placed in a cull or compost pile. Healthy-appearing plants adjacent to the diseased plants should also be discarded. All remaining coleus anywhere on the premises should be treated with fungicides.

Fungicides should be applied preventively before disease develops. Downy mildew has the bad habit of changing and becoming resistant to the systemic fungicides (example: Subdue MAXX). Since Subdue MAXX is especially at risk it should only be used once in a production program and must only be applied as a drench.

An example of a fungicide program for downy mildew

Subdue MAXX drench at planting only.
Weekly foliar sprays could include the following:
Spray 1 - Mancozeb
Spray 2 - Stature DM 50WP+ Mancozeb
Spray 3 - Mancozeb
Spray 4 - Aliette + Mancozeb
Spray 5 - Mancozeb
Spray 6 - Strobilurin (examples: Compass 50WDG or Heritage 50WG) + Mancozeb
Spray 7 - Repeat the program beginning with Spray 1.

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