Powdery Mildew on a Dahlia
Powdery mildew is favored by high humidity at night because it
creates a favorable environment for spore formation, and low
humidity during the day, which allows those spores to be blown from
host to host. Temperatures between 70 and 80 degrees are ideal for
Cultural controls include making
sure lilacs are exposed to full sun and good air circulation. If the
shrubs are planted too close together, you might consider removing
every other one to allow good air circulation. Pruning the largest,
oldest stems out at ground level reduces the overall size of lilacs
and pushes a flush of young stems to sucker up from roots that will
flower better and flower at a more appropriate height for cutting.
Thin the young stems to the sturdiest, most vigorous ones and make
sure they are spaced to allow air circulation in the center of the
shrub. Rake up and destroy fallen leaves in fall to reduce the
amount of spores overwintering near susceptible plants.
Watering Can Help!
There is evidence that powdery
mildew is more common on drought-stressed lilacs, so providing
supplemental water when the weather is hot and dry can be helpful.
Fertilize very moderately, only based on soil test results.
Fertilization pushes succulent growth that is more susceptible to
Fungicide applications are
rarely warranted since the disease is not life threatening.
Varieties of Lilac
Should you choose to plant
lilacs in the future, consider planting resistant species such as
dwarf Korean lilac (Syringa meyeri), Manchurian lilac (Syringa
pubescens subsp. patula ‘Miss Kim’), and littleleaf lilac (Syringa
pubescens subsp. microphylla).
Azaleas and Rhododendrons