i have a lot this year also ... mark it off to the vagaries of mother nature ...

PM is caused by hot dank nights and high humidity ... conducive to it growing ... and they look the worst.. going into fall.. where.. w/o the heat.. the very damper nights make it thrive ...

there is nothing to do now ...

full fall cleanup reduces the spores for next year ...

perhaps a bit of prevention in spring will cut it down.. but in my world.. that is a waste of time and money.. because you can not even be sure it will be a problem next year ...

the biggest cause of PM.. is watering after dinner.. making the plants all wet.. just before nightfall ... so if you are doing that .. then dont.. and i bet 50 to 75% of your problem will go away ...

also .. lack of air movement at night doesnt help ..so if you can thin the plants.. or surrounding shrubs.. just to increase spacing.. that will also help ...

a spray of simple baking soda is about all you need.. perhaps in late spring ... if you want to be proactive ...

yes.. if the conditions are right .. it can spread everywhere ...

ken

ken gives very good advice, obviously from experiencing the same problem. We are fortunate here in the dry west where our overnight temperatures fall 40 degrees, to have very little mildew problems. Al

thx al ..

i learned the hard way ... enjoying my garden by hosing them down every evening.. lol ...

got myself a watering wand... aka breaker bar.. and so much of the problem abated.. that i could ignore what little was left ...

link below.. water your soil.. not your plants ... [and i do note.. that this may not be what the OP is doing] .. damp soil will provide whatever ambient humidity the plant needs ..

they arent dirty children who need a shower before bed ... and this is one of the reasons an automated irrigation system may become problematic ....

ken

Here is a link that might be useful: link

I too have some plants getting it bad.. but I don't water at night. Since most nights you are getting dew on everything anyway... you can't avoid it all together. My one peony just looks horrible. Might have to move it or get rid of if since it get mildew so early in the season.

I want to try the baking soda.. but i always forget to apply before it's too late.

One factor not mentioned by ken and is often overlooked by gardeners is that of a plant's predisposition to powdery mildew (PM) and other diseases. Phlox is VERY prone to this disorder, almost to the point that it is expected.

By the way, PM strains are species specific. It doesn't jump from plant to plant. However, if the conditions are right for PM in your garden, then other strains of this disease may present itself on other susceptible plants. For example, PM on your roses cannot infect your phlox.

One very unusual thing about PM is that is does not require free water in order to grow. As a matter of fact, early infections can be rinsed off with plain water. This little trick can prevent the new growth on plants from becoming infected...and allows them to harden, at which point PM spores find it difficult to germinate.

Ken's suggestion of attending to some housekeeping practices in the garden is important. That means removing old mulch, too. Grow susceptible plants in the full sun and give them plenty of elbow room. Sunlight and air circulation is important. Search out resistant cultivars, if possible. Your Phlox paniculata 'David' (not davidii) is noted to be more resistant than most but it IS phlox, after all. There might be other cultivars on the market with superior genetics.

Other than hosing down young plants before a bad bout of PM can get really started, you might want to try neem oil. I have found it to protect against AS WELL AS CURE mild infections. Purchase cold pressed neem oil and follow the directions.

in suburbia.. my phlox were in just a little bit of afternoon shade.. all kinds of mildew ... in a peat soil ... very damp ...

out here.. 100% dry sand.. never watered.. in full blistering sun... very little PM until very late in the year ... enough to ignore in my world.. more wind out here also ...

same plants.. generic plants i begged from other peeps.. the old standards ... nothing resistant ...

ken

This year, I have weeds with PM! I have never seen that before, and I had to stop in my tracks to look at it!

Dee

My David Phlox was also covered with mildew in early spring, and I cut them all to the ground, and they bloomed late without mildew. My Bright Eyes phlox had no mildew.

Betty

When I had a PM problem on my rugosa roses a couple years ago, the advice was to OVERHEAD water/hose them down.

I know, mxk3...it goes against the grain, doesn't it? There was a volunteer Crapemyrtle on this property when we bought it. Pretty color but without the genetic improvements of modern hybrids. It would develop PM as soon as the new leaves began to grow. All I had to do was hose it down every day until the foliage matured (3 or 4 weeks) and the issue was solved.

Nevermore...you need to treat this disorder early before it is able to completely infect the cells of the plant. It's too late to do much of anything at that point. Even Neem oil can only cure this disease in its earlier stages.