Powdery mildew has been a pervasive problem in my garden for years. Last year it attacked my dogwood tree, peonies, phlox, and all of my squash plants. It is the most common fungus issue gardeners face.
Identifying powdery mildew is easy. It's white-ish or grey-ish, almost moldly looking stuff that slowly starts to creep over the green leaves of your plants turning what was once pretty green into an unhealthy looking mess. It's not deadly to you or your plants, but if left to rage out of control it can weaken perennial plants and by autumn it can completely shrivel infected plants.
Powdery mildews are worst in warm, dry climates but the relative humidity needs to be high for spore germination. These conditions occur in crowded plantings where air circulation is poor and in damp, shaded areas. Powdery mildew infections rise with relative humidity but it does not occur when leaf surfaces are wet.
The trouble with it is that once it's there it's nearly impossible to get rid of — the best you can do is prevent it in the first place and once you spot it, you can really only aim to stop what has started from spreading — and now is the time to do it!
The first line of attack is to increase airflow in the plant and remove infected leaves. Know that dense, shady areas favor disease development so do what you can to remedy these environmental issues. There are hundreds of different powdery mildew fungi and they are generally host specific – meaning that the mildew on your lilacs will not spread to your grapes or your roses. But if you start to see a problem on more than one type of plant, this is a sign that you have an environmental problem to address.
You should resist the urge to fertilize your sickly plant because that will generally cause the plant to throw out new growth — just what the fungus loves. Young, succulent growth usually is more susceptible than older plant tissues so don't feed the fungus.
Once you have done what you can environmentally, there are a few organically safe products to use on powdery mildew (ask at your local garden center). I tried one last year and I can't say it was remarkably effective. Generally this is a high maintenance problem as all the solutions need re-application every few days.
This year I am going to go with the what works for so many other plants troubles. I'm breaking out the Neem oil. Neem oil coats the leaves and protects them while also acting as a suffocating agent for the fungus, so this should help stop the spread, at least. It's not a one shot fix — it too has to be re-applied every few days. Also, Potassium bicarbonates (common product names are Kaligreen and First Step) are recommended but if I can't put my hands on either of these two commercial products I plan to use Sodium bicarbonate (as in Baking Soda from my kitchen cabinet).
Researchers at Cornell University have found success in a spray that a combined Neem oil (or Sunspray) and baking soda (1 tablespoon baking soda plus 2.5 tablespoons of Sunspray oil in 1 gallon of water). It is still experimental, but the more experienced I become in the gardening world, the more I learn that the simplest and more natural remedies are often the best.
Have you had success with stopping powdery mildew? What did you do? There are lots of suggestions out there like using milk, hydrogen peroxide and other remedies, but I've never had notable success with them myself.