Organic Control of Powdery Mildew on Garden Plants
Powdery mildew is caused by the fungus, Sphaerotheca pannosa
and that's the mouthful you can drop at any garden party from now on to
impress the heck out of your neighbours.
This parasite lives by infecting green tissue so you'll see it on your
plant's leaves, stems and flowers and buds.
Typical Fungal Problem
Like many fungal problems, this one lives on the outer surfaces of the
leaf but sends little "roots" (actually called haustoria
down into the cells to feed. (Another great term to impress the
cocktail party circuit.)
Powdery mildew is an ugly disease that disfigures the plant and leads
to serious weakening. In many cases, it occurs in late fall (Rudbeckia)
and the plant shrugs it off. In early summer infections, the plant will
struggle all summer and then die over the winter in a weakened state.
Recognizing Powdery Mildew
It appears in several ways. The most common is a gray-white powdery
dusting on the leaf surfaces. The actual color ranges from a white to
brownish-white (almost a tan color) and there are few other problems
that appear to be similar in the garden. If you see this dusting, it is
almost 100% sure you have powdery mildew.
The real tipoff to this problem is when your young leaves start to curl
and twist as they develop and do not fully unfurl. Roses twist the
entire new shoot. Other plants simply twist the leaves.
Older leaves are pretty much immune to this twisting and usually don't
show any other signs other than a dusting or small spots where the
outbreak is severe. The will also brown off once the mildew has
Lower Level First
Leaves are usually attacked on the lower surface first and then the
mildew moves around to the top of the leaf. If you're in the habit of
turning over leaves, you might notice a small, raised blister on the
leaf surface (you have to be looking pretty carefully and regularly)
and some slight purple mottling with leaf edge curling. The white
powder develops after that and will eventually cover the entire leaf.
Flower buds are often attacked and when they open they are often
distorted and do not last very long. Mature flowers are seldom attacked
although you might see some powdery mildew (white dusting) on the
If your garden tends to be a mildew trap, then your first line of
defense is to always grow disease resistant varieties. While they are
not immune to fungus problems, they'll hold off the powdery mildew
fungus as long as possible.
It is also interesting that while a plant might have resistance in one
area to that area's form of powdery mildew, it may be susceptible in
another province or state. Yes, there are "families" of powdery mildew
that vary from area to area.
Do not crowd plants any more than necessary. If in the perennial
garden, try to avoid having tall plants next to each other that will
block air circulation or worse yet, fall into each other to shade or
cover leaves (touching). This touching will increase the humidity
between the touching leaves and put a big red sign up that says,
"powdery mildew welcome here".
Do not overfeed your plants. Overfed plants are high in nitrogen and
this succulent growth is again, like hanging out a sign for both
disease and insects to feed.
As soon as you see a problem, start spraying. Spray repeatedly and
regularly to stop the problem. Remember that rain or overhead
irrigation washes off the spray protection so you'll have to reapply
after these events.
Garden cleanliness is next to powderylessness. Clean up your garden
after the season and during the season to slow down the spread of this
problem. Badly infected leaves and stems should be pruned out and
discarded (they are overwintering sites).
Use a drip irrigation system rather than an overhead system. If you
have to use overhead, then water twice a week with deep watering rather
than a little bit every day.
Read the label on any spray product you use. Remember that if a
fungicide kills fungus on the leaf, it will also kill beneficial fungus
in the soil. Do not overspray so that the soil fungi are killed.
But do cover both the top and bottom of leaf surfaces with spray to the
point of runoff. (Point of runoff means you spray but as soon as you
see the moisture on the leaf starting to accumulate into drops and run,
that is enough application.)
Organic Control Sprays
You can both spray and pour liquid seaweed onto your plant's leaves.
Research has shown that this has a powerful "booster" effect to your
plant's health and it helps fight off the powdery mildew. This is being
used in many vineyards now as an organic control because it seems to
work particularly well on crops that produce fruit. I mention it as a
Sulphur sprays are quite effective at stopping the spread of powdery
mildew. Remember that they do knock out beneficial soil fungi as well
so do only spray to runoff. You can find sulphur in almost any garden
Sodium bicarbonate (baking soda) is recommended by many gardeners and
when it is mixed at the rate of between 2 and 10 g per litre of water
(add a small dash of liquid soap as a wetting agent). (1 teaspoon to a
quart of water) I've seen research that up to 20g / litre of water has
worked well with no burning.
And to just to make your day, it has also been reported (I've never
used this myself) that urine when diluted at 1 part urine to 4 parts
water is an effective powdery mildew control. There's another reason to
take a seventh inning stretch.
Milk is another very effective spray for powdery mildew. Mix the milk
at a ratio of one part of milk to nine parts of water and spray weekly.
Do NOT go higher than 3 milk to 9 water or you'll attract other fungus
problems that want to feed on the milk. Skim milk works well as it
contains no fat to turn rancid (and attract other problems that like
the smell of rotting fats.)
There are also products on the garden center shelves featuring jojoba
oil and neem oil. I can't speak to these but some gardeners swear by
them for controlling powdery mildew.
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