University of Vermont Extension System
Department of Plant and Soil Science

Commercial Ornamental Horticulture (COH) Leaflet 39

Powdery Mildew on Phlox and Monarda

Leonard P. Perry, Extension Professor

The Problem

• one of main diseases of important genera such as these
• white powdery growth covering leaves, severe causes leaf drop, affects vigor and resistance to stress over time, less attractive for sales

 Cause

• Erysiphe cichoracearum fungus generally; strains vary with location, climate
• white fungal threads produce spores early in season, which germinate, penetrate leaves, and absorb nutrients causing leaves to turn yellow and eventually die
• overwintering "pepper appearing" structures form late in season, overwinter in buds and on debris, release spores following spring to continue disease

 Conditions affecting mildew

• wind triggers spore release and aids distribution, but also keeps leaves dry so spores don't germinate as easily
• leaf wetness for short periods promotes the diseases, forceful water (70psi) or continual wetness prevents
• high relative humidity promotes spore germination, some also germinate in dry
• temperature above 86ºF kills spores except under high humidity, moderate temperatures promote the disease
• light: plants in shade usually have more disease from lower temperatures and more dampness
• nutrition: plants with luxuriant growth from excess nitrogen have more disease

 Management Options

1. Host resistance: select resistant cultivars (see below)
2. Eradication: keep debris, dead leaves removed
3. In greenhouses: decrease humidity through air flow, air exchange, proper watering
4. In outdoor production: use proper spacing, fungicides (see below and references)
5. In landscapes: thin stems and increase spacing for more air movement, water early in when leaves will dry quickly
6. Begin scouting early in season (Memorial Day), begin application of sprays at onset of disease, continue through season if weather is conducive for disease

 Phlox Cultivar Resistance

• ratings in 1986 (White Flower Farm, CT), 1996 (Chicago Botanic Garden, Richard Hawke), replicated in 1991-92 (Vermont and Long Island--Margery Daughtrey)
• Cultivars with least or no mildew in at least 2 of the 3 ratings:

Monarda Cultivar Resistance

• ratings in 1996 (Chicago Botanic Garden--all 19 cultivars with no mildew, as in Vermont in 1995), replicated 1994-97 (Vermont)
• Cultivars with least or no disease in Vermont ratings:

• Cultivars with high disease in Vermont ratings:

Organic Controls Research (Vermont)

• 1991-93 in Vermont, 'White Admiral' phlox, replicated
• Best control (no differences among treatments):

• Least control (worst treatments, not effective):

• Horticultural oil is only one registered for use as a fungicide
• Worst treatments in this study often provide some control in less stressful situations, in landscapes, or for individual plants
• Antidessicants form a barrier, preventing diseases from entering the leaf; so as leaves grow they must be reapplied to remain effective (not done in above studies)

 Chemical Controls Research

• Long Island (Margery Daughtrey)
• 1990, 4 phlox cultivars, horticultural oil at both 2 and 4 week intervals provided as good or better control than Bayleton
• 1991-92, replicated for phlox and monarda cultivars:

Some Useful References

• Pest Management Recommendations for Production of Herbaceous Perennials, Cornell University (607-255-2080)
• Ball Pest and Disease Manual, Powell and Lindquist, Ball Publishing (888-888-0013)
• Diseases and Pests of Ornamental Plants, Pirone, Wiley Publishing
• Herbaceous Perennials Production: A guide from propagation to marketing, Perry, NRAES, Cornell University (607-255-7654), expected publication May 1998

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Return to Perry's Perennial Commercial Page

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture. Lawrence Forcier, Director, UVM Extension System, Burlington, Vermont. University of Vermont Extension System and U.S. Department of Agriculture cooperating, offer education and employment to everyone, without regard to race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status.

• 2/12/98