Powdery Mildew on Roses

By Charles Shaner- Shenandoah Rose Society

            Almost anyone who has grown roses for more than a year knows about powdery mildew.  It is a fungi that causes a powdery white or gray growth on the surface of leaves and canes.  It will affect most flowers and fruits, with young foliage and shoots being particularly susceptible.  It may cause little damage to the plants, and in the fall causes an early dormancy but usually is not fatal.

            Powdery mildew fungi infect plants with airborn spores when temperatures are 60 to 80 degrees F but will not be present during the hottest summer days.  Cool, damp, cloudy days are very favorable for powdery mildew to develop.  On roses, high humidity conditions are favored for powdery mildew to germinate and infect.  Plants that are over-crowded and shaded provide better conditions for powdery mildew to develop.

            There are some preventive measures that can be taken:

·        Purchase good quality plants from reputable nurseries, greenhouses or garden centers.

·        Prune out any diseased or dead wood.  Rake up and destroy any dead leaves.

·        Plant in a well-drained and well-prepared location where the plant can get at least 6 hours of sunlight per day.

·        Plants should be spaced for good air circulation.  Keep the centers of the plant open so air can circulate.

·        Do not work with or handle the plants when the foliage is wet.

·        Roses require the equivalent of 1 to 2 inches of rain per week.  If nature does not provide it, then we must make it up.  When watering, avoid getting the foliage wet especially late in the day.

Powdery mildew can do little to no damage on some plants and do major damage to others.  Fungicides must be used to control powdery mildew.  A regular spray program is a must for prevention and control and may need to be stepped up in cool, damp weather.  Fungicides recommended are:

·        Propiconazole (Banner Maxx):  listed for control of powdery mildew and many other diseases.

·        Myclobutanil (Systane, Eagle and Immunox):  listed for control of powdery mildew and other diseases.

·        Sulfur as a spray or dust.  Sulfur may cause some plant injury if applied when air temperature exceeds 90 F.

·        Triforine (Funginex, now Rose Defense):  only available in small quantities.

·        Fenarimol (Rubigan):  locally systemic fungicide for the prevention and control of powdery mildew.

·        Copper fungicide will control powdery mildew in some cases.

Powdery mildew can become resistant to any of the fungicides listed except sulfur.  Remember not to use the same spray all the time.

            For organic alternatives, garlic naturally contains high levels of sulfur and along with a few cloves crushed in water can be used as a homemade spray.

            Powdery mildew requires living plant tissue to grow.  It survives on buds and stem tissue and certain weeds may act as hosts through the winter.  The disease life cycle can be completed in as little as 72 hours.  It normally takes 7 to 10 days from the time of infection to develop secondary spore production.