Plant Diseases

Q: I noticed that my bee balm and phlox looked bad this year. The leaves were covered with a white powdery substance. What is this stuff and is there anything I can do to control or prevent it next year?

A: A fungus (from the genus Erysiphe) attacked your plants! Many Erysiphe species produce powdery mildew—what looks like white dust on plant leaves, stems, and even flowers.

Distinguishing powdery mildew from other plant problems
There are other plant mildews that cause a white, powdery appearance—most notably downy mildew. For more information on the difference, see the blog Mildew on Plants? What to Watch for!

Also, serious infestations of tiny insect pests, such as spider mites, white flies, woolly aphids, or mealy bugs, may make your plants look white! If you look closely, you can see spider mite webbing, white flies flying, or aphid or mealy bug individuals to determine if you have an insect problem rather than a disease! 

What conditions favor powdery mildew?

• Low light
• Wet leaves and high humidity
• Overcrowded gardens with poor air circulation
• Excessive nitrogen fertilization

Powdery mildew’s preferred climate—hot dry days, cool nights, and morning fog—means that for gardeners in Western North Carolina, powdery mildew is a fact of life. 

Plant damage
Powdery mildew requires living plant tissue for its vegetative part (the mycelium) to grow. Mats of branching mycelium threads absorb nutrients. As these mats spread, they decrease photosynthesis, causing affected leaves to look yellow (chlorotic). 

Susceptible plants
There isn’t much that powdery mildew doesn’t go after. It affects more than 1,300 plants! The good news is that it is host specific—different fungal strains affect different plants. In addition to the perennials you mentioned, different fungal species cause problems for some ornamental trees, such as our beloved flowering dogwood (Cornus florida), vegetables—cucumbers in particular—and many fruits, including apples, strawberries, peaches, and grapes.

Minimizing powdery mildew infections
Given the growing conditions in Western North Carolina, eliminating powdery mildew is a bit like Don Quixote tilting at windmills! 

Selecting disease resistant varieties is a better approach. Ask for disease resistant cultivars when shopping—or go with a list of desirable varieties. One good source is our own NCSU Mountain Horticultural and Crops Research Center in Fletcher, N.C. Testing done by Extension Specialist Dr. Richard Bir gives the following perennial cultivars high marks.

Among bee balm (Monarda didyma) varieties:

• Early Blooming – ‘Claire Grace,’ ‘Marshall’s Delight,’ and ‘Stone’s Throw Pink.’
• Late Blooming – ‘Beauty of Cobham,’ ‘Blue Stocking,’ ‘Cambridge Scarlet,’ ‘Elsie’s Lavender,’ ‘Mahogany,’ ‘Marshall’s Delight,’ and ‘Vintage Wine.’

Among garden phlox (Phlox paniculate) varieties:

• ‘David,’ ‘Robert Poore,’ ‘Bright Eyes,’ ‘Eva Cullum,’ ‘Fairest One,’ ‘Franz Shubert,’ ‘Natascha,’ ‘Orange Perfection,’ ‘Rosalinde,’ and ‘Starfire.’

Because powdery mildew is such a threat to flowering dogwoods, major agricultural universities, including NCSU, have undertaken decades of research to create disease-resistant varieties. When purchasing look for crosses between Cornus florida and Cornus kousa, such as ‘Stardust,’ ‘Stella,’ and ‘Celestial.’

NOTE: Resistance doesn’t mean immunity, but it does mean that there is a reduction of disease growth in the plant.

Improve your growing conditions. Once you have selected the right variety, give your plants the best chance for success.

• Grow them in full sun—if your plants can tolerate it. Powdery mildew prefers shade!
• Give them breathing room. Encourage good air circulation by thinning and pruning.
• Reduce or eliminate overhead watering.
• Remove and dispose of infected leaves during the growing season. In the fall, remove and destroy infected leaves and branches.
• Apply fungicides that target powdery mildew, remembering that most need to be applied before the onset of infection.
• Use horticultural oil—but only if conditions are right. Some oils may cause damage if applied during warmer summer weather.

Yes, powdery mildew is a formidable opponent. But by selecting the right plants and creating an environment that impedes fungal growth, you and your garden can enjoy success next season!

Article written by Janet Moore, Extension Master Gardener^SM Volunteer.

Learn more
Let’s Talk About Powdery Mildew
by Dr. Bill Hanlin, Horticulture Assistant
NC Cooperative Extension, Wilkes County, NC

Monarda and Powdery Mildew Resistance
and
An Evaluation Report of Selected Phlox Species and Hybrids
by Richard G. Hawke, Coordinator Plant Evaluation Programs
Chicago Botanic Garden