Powdery Mildew in Gardens & Landscape
C. Rae Hozer, Cumberland County Master Gardener
best weapon against powdery mildew is disease prevention through careful site
and plant selection along with good growing practices, instead of fungicide use.
Buy/grow resistant cultivars. (Remember that seedlings from hybrid stock
typically donít have the same resistance as the mother plant.)
Select a sunny site (minimum 6 hours of sunlight). Highly susceptible plants
(phlox, roses, etc.) should not be located in damp, shady spots. Maintain enough
distance between plants for good air circulation.
Avoid overhead sprinkling that wets foliage in late afternoon or evening. Do not
let plants be stressed by lack of water during a drought.
Donít handle plants or work among them when foliage is wet.
Clean up and destroy any diseased leaves and other plant litter.
mildew shows up as a white or grayish haze on leaf surfaces, stems, flowers or
fruit of affected plants. (Plant parts coated with this fungus look like
theyíve been powdered, hence the name.) Sometimes young, infected shoots and
leaves appear twisted or curled. As the disease progresses, leaves often turn
yellow or brown and dry out. These fungi send infectious spores into the air
when temperatures are 60 to 80 degrees Fahrenheit.
plants growing in damp, shady areas are easy targets for powdery mildew. When it
is above 80 in sunlight, air temperatures in shade are cooler. High humidity
favors spread of this disease. The wet leaves many fungal diseases require
arenít necessary because powdery mildew spores germinate without a film of
water on plant surfaces. Plants growing too close shade one another and keep
humidity higher than in beds with wider plant separation.
tell us that, though symptoms look the same, powdery mildew disease on different
varieties of plants are typically caused by different fungus strains. Each
powdery mildew species has just a small number of host plants. That means
airborne powdery mildew spores from a sick dogwood tree in your yard might
infect another dogwood but wouldnít spread this disease to a nearby lilac or
rose bush, to euonymus vines climbing the enclosure for your propane tank, to
phlox and zinnias in your flower garden or to green bean and cucumber plants in
your vegetable patch. However, all of the individual plants just named are
naturally susceptible to some kind of powdery mildew. Other susceptible woody
ornamentals are deciduous azaleas, buckeye, cherry, some flowering crabapples,
honeysuckle, privet, serviceberry, tulip tree, some viburnum, walnut and willow
trees. Fleshy-stemmed ornamentals prone to powdery mildew include
chrysanthemums, dahlias, delphiniums, some begonias and snapdragons. Some
wild-growing trees, shrubs and weeds serve as host plants, too.
applications of fertilizer, lots of rain, and warm temperatures combined to give
a big boost to perennials (and weeds) in my flower gardens. Unfortunately, my
schedule kept me elsewhere instead of in those beds pulling weeds and thinning
the desirable plants. My phlox have big, bright pink and purple blooms
reminiscent of flowers in Motherís gardens when I was a child, but their lower
leaves are totally covered with powdery mildew. Newer, disease resistant phlox
cultivars are available but those arenít the varieties I have.
caption: Abundant flowers and foliage in a bed by front walk look lush from the
street but plants are overcrowded. Phlox leaves (foreground) are covered in