Powdery Mildew

July 15, 2010 at 10:06 am Leave a comment

Not all garden pests pose an immediate threat to your flowers and trees, some just make your gardens look unsightly.  Powdery mildew is an example of one of those pests that doesn’t necessarily kill the plant but does make plants less attractive to gardeners.  What is powdery mildew?  Powdery mildew is a number of different species of fungus that are spread as spores via the wind.   Some species of fungus attack certain host plants (i.e calendula, zinnia, catalpa, lilac, and cucumber) while others are less selective.  Powdery mildew can also be spread when plants are transplanted to an area of the garden where there was no powdery mildew present.

What is the lifecycle of the fungus?   The fungus spends the winter months in infected plant parts and on the surface of fallen leaves.  They produce structures called cleisothecia that able to withstand the harsh winter conditions (appear as black dots within the white patches).  The following spring the fungi release spores from the cleisothecia that travel on air currents to infect new plants (University of Guelph, 2010).

What damage does this pest cause?  The first sign of a powdery mildew infection is a dusty grey to whitish coating on the leaf and surfaces of other plant parts.  Some of the powder can be rubbed off the plant by using your fingers.  Powdery mildew causes stunting and distortion of leaves, buds, and fruit.  Some plants are more sensitive to powdery mildew infection such as tuberous begonias and may be more susceptible to death.

How do you control powdery mildew?  The ideal growing conditions for powdery mildew are high humidity and when plants are crowded together providing little air circulation.

You can create less favourable conditions for the fungus by cutting the lower branches of trees to provide more air circulation.  Depending on the garden conditions it may be possible to completely remove trees or shrubs.  Experienced gardeners may be able to steer you towards more powdery mildew resistant species of plants.  At the end of the growing season cutting off the tops of annuals and perennials and burying them in a compost pile will reduce the amount of powdery mildew that survives the winter.  The fungal spores remaining on the plant will be killed by the heat of the compost pile (Cornell University, 2010).  Please check out the references below for further information on powdery mildew.

References

Cornell University’s Plant Diagnostic Clinic “Factsheet on Powdery Mildew” http://plantclinic.cornell.edu/FactSheets/powdery/powdery.htm

Accessed on July 13, 2010

University of California’s IPM Online “Powdery Mildew on Ornamentals”

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7493.html

Accessed on July 13, 2010

University of Guelph’s Pest Diagnostic Clinic “Control of Powdery Mildew in the Home Garden”  http://www.uoguelph.ca/pdc/Factsheets/Diseases/PowderyMildewinGarden.htm

Accessed on July 13, 2010

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Entry filed under: Bugs, diseases, Pests, Trees. Tags: , , , , , .

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