Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Rose -- Downy Mildew
 
Cause: Peronospora sparsa, a fungus-like organism that can be systemic in roses. It overwinters as mycelium in stems, root crowns, and roots or as oospores in leaf debris and stems. Humidity above 85% and cool temperatures favor the disease. Infection usually occurs on young plant parts. Cuttings taken from infected stock plants will carry the disease. Infected stock may or may not show symptoms.
Symptoms: Purplish red to dark brown, irregular spots on leaves. Leaflets may yellow but contain "green island" areas. Major leaf veins often restrict fungal growth such that lesions become angular as they enlarge. A grayish spore mass may be on the leaf underside if humidity persists. However, the diagnostic sporulation of this fungus is not abundant. Defoliation also may occur in extreme cases. Small spots or long purplish areas may form on canes and may kill twigs. Some people may confuse this with black spot or even rust.

Note the reddened areas on the sepals and stem.

Irregular-shaped, purple spots form on the upper leaf surface, while light, gray-brown spore masses form on the underside of the leaf (middle leaflet on right side of this picture).

 
Cultural control:

  1. Lower humidity in the greenhouse by raising temperatures and venting at key times of day.
  2. Rake leaves and prune out old flowers and stems.Burn or bury them or send them to a landfill.
  3. Take cuttings from plants with no history of the disease.
  4. Scout plants carefully for symptoms of the disease especially the undersides of leaves and any stock plants or plants held over from the previous year.
  5. Although hot water soaks have been effective they have also damaged cuttings. Hot water soaks are not recommended until time and temperature durations have been investigated.
  6. Maintain adequate fertility.
Chemical control: Focus on cultural control methods first. Do not use metalaxyl or mefenoxam-based products (group 4 fungicides) alone; they are not registered, may not offer much control, and encourage resistance to develop. Chemical control tactics help prevent infection but may not be helpful once the disease starts.
  1. Aliette WDG at 2.5 lb/100 gal water. Do not use within 1 wk of copper spray. 12-hr reentry.
  2. Alude at 1.25 to 2.5 pint/100 gal water applied as a foliar spray at 14- to 21-day intervals. 4-hr reentry.
  3. Compass O 50 WDG at 1 to 2 oz/100 gal water. Do not use organosilicate additives. Rotate to a different fungicide with a different mode of action after each application. 12-hr reentry.
  4. Cygnus 50 WG at 3.2 to 6.4 oz/100 gal water plus a non-organosilicone spreader-sticker. Best used before symptoms develop. 12-hr reentry.
  5. Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. 4-hr reentry.
  6. Heritage at 2 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Heritage can also be used at 1.6 to 8 oz/A for commercial rose production. 4-hr reentry.
  7. Insignia at 4 to 8 oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventatively only. 12-hr reentry.
  8. Nu-Cop 50 DF at 1 lb/100 gal water. 24-hr reentry.
  9. Segway at 1.5 to 3 fl oz/100 gal water. 12-hr reentry.
  10. Stature SC at 6.12 to 12.25 fl oz/100 gal water. Do not make more than 2 sequential applications. 12-hr reentry.
References:
Aegerter, B.J., Nunez, J.J. and Davis, R.M. 2002. Detection and management of downy mildew in rose rootstock. Plant Disease 86:1363-1368.
Horst, R. K. and Cloyd, R. A. 2007. Compendium of Rose Diseases and Pests. Second Edition. APS Press. St. Paul, MN.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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