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
developed.
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 sepals.
Treatment
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.
No Crowding
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”.
Don’t Overfeed
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.
Cleanliness
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 good alternative.
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 shop.
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.