Powdery mildew or downy mildew?  Aren’t they the same thing?  Nope!
What do they have in common?  They’re
both diseases that occur on the leaves of plants.  But that’s pretty much where the similarity
ends.

Powdery mildews of plants are the ones most commonly noticed
by the casual observer.  The powdery
mildew fungi grow on both the upper and lower surfaces of leaves, making them
look like they’ve been dusted with powder.
That’s a good way to remember which is which: if it looks powdery, it’s
powdery mildew.  Plants that are commonly
infected by these fungi include lilac, peony, phlox, cucurbits, and grape.

Powdery mildew on magnolia.

 

The fungus begins to grow on leaves when a spore arrives
during a period of high humidity.  The
thread- like hyphae of the fungus grow on the surface of the leaves and extend
structures called haustoria into the cells of the leaf where nutrients are
extracted.  Some powdery mildew fungi can
also infect shoots and developing fruit.
While unsightly when heavy, powdery mildews don’t usually have a significant
impact on the health of the host plant.
By late summer when the fungus covers the leaves quite a bit, most of
the active photosynthesizing season has passed and a decreased level from the
fungal infection late in the season will have a minor effect.

Downy mildews, in contrast to the powdery mildews, do not
produce a white growth or sporulation on the upper leaf surface but on the lower or ‘down’ side.  Symptoms of downy mildew can be yellow or chlorotic spots on the upper surface of the leaf, angular leaf spots confined
by the veins or general yellowing between the veins.  On the underside of the leaf, associated with
these symptoms, sporulation will be present in periods of high humidity.  This can make the underside of the leaf look downy or in some cases dirty if the spores (sporangia) are pigmented. The sporangia are wind blown or rain splashed
to new infection sites during wet weather.  These spores need a film of water on the surface of the leaf to germinate and cause infection.
The downy mildews are not fungi; they are water molds, a group more
closely related to certain types of algae than to the fungi.
The downy mildews can cause important diseases on host plants
including grape, rose, cucurbits and basil.  On grape, downy mildew infects young fruit.

Basil downy mildew.

Prevention is the best way to control both these
diseases.  Practices that improve airflow
around the plants to reduce humidity and leaf wetness will reduce disease.  Prune plants such as lilac or grape to promote
airflow around all plant parts.  When
planting, allow enough space for air flow around the  plants as they grow.   When irrigating is necessary, water at the
base of the plant to avoid wetting the foliage.
If overhead irrigation is used, water early in the morning so the
daytime sun will dry the leaves.  Preventive fungicides can be used for both
diseases on highly susceptible hosts when a problem has occurred
previously or when needed to ensure a good harvest when growing grapes.
Some powdery mildews overwinter in fallen leaves or plant debris, so
thorough clean up of dead plant material in the fall can reduce disease the
following spring.   When using pesticides, always read and follow the label instructions carefully.

J Allen