University of Vermont Extension System
Department of Plant and Soil Science

Powdery Mildew on Phlox and Monarda:                                 COH 39
Overview, Cultivar Resistance

Dr. 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

• 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

• 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

• Host resistance: select resistant cultivars (see below)
• Eradication: keep debris, dead leaves removed
• In greenhouses: decrease humidity through air flow, air exchange, proper watering
• In outdoor production: use proper spacing, fungicides (see below and references)
• In landscapes: thin stems and increase spacing for more air movement, water early in when leaves will dry quickly
• 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:

• Promising newer cultivars: Shortwood, Robert Poore

NC State Univ. cultivar ratings (Dick Bir, Fletcher NC MHCREC), replicated 1998
 

Least disease—early season (early June)	Least disease—late season (late July)
Claire Grace Marshall’s Delight Stone’s Throw Pink	Beauty of Cobham Blue Stocking Cambridge Scarlet Elsie’s Lavender Mahogany Marshall’s Delight Vintage Wine

Severe disease pressures at this location, with much rain and fog. At least 45% defoliation on all early in season, and at least 65% defoliation on all late in season.

Chicago Botanic Garden cultivar ratings (Richard Hawke), replicated 1993-1996 (more common shown)
 

Least or no disease	High disease
Colrain Red Gardenview Scarlet Marshall’s Delight Ohio Glow Petite Delight Raspberry Wine Squaw Violet Queen	Adam Beauty of Cobham Cambridge Scarlet Croftway Pink Mahogany Mrs. Perry Panorama Prairie Fire Snow Queen fistulosa fistulosa ‘Claire Grace’

Vermont cultivar ratings, replicated 1994-1997 (Leonard Perry et al)
 

Least or no disease	High disease
Blue Stocking (purple) Marshall's Delight (pink) Violet Queen (purple)	Adam (red) Cambridge Scarlet (red) Croftway Pink (pink) Souris (pink)

Vermont cultivar ratings, replicated 1999- present

Promising for least disease: Jacob Cline, Petite Delight, Petite Wonder COH39 11/15/01

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.

Last reviewed 11/15/01