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A lilac bush was destined for removal and I separated as many cohesive root and shoot bunches as I could. I planted some immediately and potted others, almost all of which are doing well. I notice one in particular has a white powdery substance on the surface of most of its leafs, which appears to have spread over the course of a few days but then more or less stopped changing its extent. Wiping it off with my fingers, it comes off pretty easily. I separated it from the other trees and took some photos:

White powdery substance on lilac trees

White powdery substance on lilac trees

White powdery substance on lilac trees, some rubbed off by thumb

In the last image to the left of my thumb you can see some of the powder I rubbed off with my thumb. It didn't have a noticeable smell.

Any idea what the substance is and how I should respond to it for the plant's health?

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This is powdery mildew. Bamboo gave me a 1:9/milk:water to spray on the plants and it was amazingly effective. Otherwise, properly mixing neem/water to spray at night would work well. This is the only fungal disease that can be controlled after the fact with infection.

Are these plants outdoors in the sun or are they on a covered patio? What have you added for fertilizer? How do you water? Is it really humid there?

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  • Thanks for the info. These are outdoors but under a low hanging maple canopy, which keeps it pretty humid. I haven't fertilized these yet, as I potted them just a few months ago in nutrient-balanced potting soil (probably is time to fertilize and even repot though). I let rain do most watering but if it's very dry I spray these and other nearby plants with a hose. If anything I let them get too dry but it does get quite humid.
    – cr0
    Commented Jul 13, 2017 at 3:04
  • Whole milk, any milk works? I tend to drink nut milk but if regular milk works id grab some of that instead of using neem to avoid unnecessary harm to other creatures not infecting the plant
    – cr0
    Commented Jul 13, 2017 at 3:05
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    She said skim milk. No fat might wash off too easily. Like a sticker spreader a little fat? I feel it is too easy a solution...but it still worked! Neem really smells toxic or harmful. It is as far as I will go. But milk and water? I got a pump sprayer a gallon or two size to spray everything in the morning, not at night. Cow's milk, not soy, not nut...plain old cow's milk, grins. Wish I knew how this works, how the milk plays a part as a fungicide.
    – stormy
    Commented Jul 13, 2017 at 6:36
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    Don't forget to get the undersides of the leaves as wet as the tops.
    – stormy
    Commented Jul 13, 2017 at 6:38
  • I tried the 1:9/milk:water spray on both sides of all the leafs of the infected plant. It helped, but did not totally get rid of the mildew (still some really bad spots). Did you wipe or rinse off the solution after applying it? I just left it on but am wondering if rinsing it down afterward would help strip the mildew, as it seemed like the milk loosened it and further spraying helped remove some of it.
    – cr0
    Commented Aug 1, 2017 at 16:50

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