Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Phlox -- Downy Mildew
 
Cause: Downy mildew on phlox is caused by the fungus-like organism Peronospora phlogina. Although not formally reported form the PNW it has been found by both the OSU and WSU Plant Disease Clinics. This fungus is only known to infect plants in the phlox family Polemoniaceae, such as the genera Collomia sp., Gilia sp., Microsteris sp., Navarretia sp., and Phlox sp. Some of these plants are native to western U.S. and may be hosts for the disease. Cool, wet conditions are favorable for infection and development of sporangia. The sporangia are wind-borne spores that are easily disseminated throughout a localized area such as a greenhouse.
Symptoms: Irregular yellow areas on the leaf and slow growing, slightly distorted stems are the most common symptoms. The presence of white to grayish mildew growth on either the lower or upper surfaces of leaves and on newer portions of stem are an indication of downy mildew. It sometimes be confused with powdery mildew.

Sporulation of the fungus can be seen on the leaves.

 
Cultural control: It will be easier to control if spotted before the disease is wide spread.

  1. 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.
  2. Immediately remove and discard infected plants.
  3. Heat and vent greenhouses late in the day to lower the relative humidity at night.
  4. Maintain adequate fertility.
Chemical control: Protective applications are important, particularly when the disease has been a problem in previous years. Once the disease is detected, apply 2 to 3 repeat applications at the closest intervals allowed. Close intervals may be necessary when environmental parameters can not be manipulated to avoid conditions that favor disease. Rotate between chemicals to reduce the possibility of the fungus developing resistance to fungicides.
  1. Alude at 1.25 to 2.5 quarts/100 gal water applied as a foliar spray at 14- to 21-day intervals. 4-hr reentry.
  2. Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter-sticker. 4-hr reentry.
  3. Insignia at 4 to 8 oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventatively only. 12-hr reentry.
  4. Protect DF at 1 to 2 lb/100 gal water plus a spreader-sticker. 24-hr reentry.
  5. Phyton 27 at 1.5 to 3 fl oz/10 gal water. 24-hr reentry.
References:
Daughtrey, M. L., Wick, R. L., and Peterson, J. L.. 1995. Compendium of flowering potted plant diseases. APS Press, St. Paul, MN. 90 pp.
Content edited by: Warren Copes on January 1, 2009
 
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