Well, you might just consider doing nothing. Mildew on lilacs is unsightly, and some years can be pretty intense, and then other years it's not much of a problem - but it won't kill the shrub.

In other words, imo, this is a garden aesthetic problem, not a horticultural problem.

I suppose another option you might try is to use the "vibrant media" product that is embedded in your post - Arm and Hammer baking soda. There is apparently some indication from rose growers that it may be effective in controlling mildew. Certainly would be safe to use! Altho the inclusion of that very product as vibrant media in my post is by no means meant as an endorsement! LOL!

brian

Susceptible type of plant + suitable environment for mildew = infestation. Remove the plant or change the site conditions and the infestation goes away.

For a PDF on results of testing baking soda for control of plant diseases...

Here is a link that might be useful: Will fungi fail and roses rejoice?

sounds like a monster ....

any chance at a picture????

my gut tells me to tell you to just get rid of it.. perhaps replace it with a mildew resistant lilac .. if there is such ....

sometimes it is just time to stop banging your head against the wall ....

bayer can be used on any sized plant [if labeled for said plant].. JUST READ THE DIRECTIONS .. or easier.. call the 800 number for specific info .... but it will not be cheap on such a large tree

Here is a link that might be useful: 800 number is right there

I'm with Brian on this - PM on lilacs is pretty much just a cosmetic issue. On an established shrub, it really doesn't do any serious harm. If you feel you must treat, you can try a dilute milk spray (or the baking soda approach) before the problem becomes noticeable. Both the lactic acid in the milk and the baking soda alter the surface pH of the foliage, making it inhospitable to the fungal spores. Repeat applications may be necessary, especially after heavy rainfalls.

From PDF I linked to above:

There are a handful of articles testing the value of SBC
on ornamental plants other than roses in field situations.
Unfortunately, the results are not encouraging. SBC did not
reduce powdery mildew on lilacs, marigolds, or rosemary and
in the latter case caused severe foliar damage. Neither was it
effective in treating white rust in field-grown chrysanthemums.
Only when combined with horticultural oil was SBC useful
in treating powdery mildew in Euonymus. Baking soda itself is
not likely to control fungal disease in your garden or landscape,
but very easily could cause leaf damage if used at a higher
concentration. Low concentrations of SBC, combined with a
horticultural oil, may have some effectiveness on mild cases of
powdery mildew.
The most promising strategy IÂve seen for controlling fungal
diseasesÂthe use of microbial antagonists in combination with
SBCÂhas only been developed for post harvest disease control.
So far, there are no similar, published efforts for addressing foliar
diseases

For additional research on the use of bicarbonates to control powdery mildew......

On lilacs specifically: Journal of Aboriculture research

Note that a combination of sodium bicarbonate together with all-season horticultural oil effected almost 100% control.

Here is a link that might be useful: additional bicarbonate research

I can testify to the success of the baking soda + horticultural oil on roses and crapemyrtles. No lilacs where I'm from. I've also used plain horticultural oil.

You can also simply spray the lilac with plain water every day until the threat passes. I did that successfully for three years with a very susceptible crapemyrtle that came with the house I bought. As I recall (I've gotten rid of that tree by now), I only had to spray it for a couple of weeks when the leaves first came out.

Neem oil is also effective against PM.

A little late here, but PM on Syringa may decrease vigor sliiightly, but that is it. If the spouse doesn't like the white, then I would use Neem as rhizo sez, in a tank sprayer - works well for me when I see it here. Of course no surprise that I can't view the ISA doc on their server, and our sub doesn't go back that far, so can't lend my valuable 2¢ there... ;o)

Dan

I'm also with Brian on this one. Plenty of lilacs in the D.C. area with hot, humid summers. PM is pretty much ubiquitous as a result, but doesn't faze/harm lilacs. I believe Father Fiala (breeder/authority on lilacs who authored the monotype "Lilacs" on the genus) also said to disregard powdery mildew. If it really is unsightly to you, my own experience is that tree lilacs (S. reticulata) and some of the S. patula varieties do not seem to get PM although they don't smell like a S. vulgaris. Of the S. vulgaris varieties, President Poincare, Nadezhda, Krassavitsa Moskvy, Congo and Agincourt Beauty have done very well here (we have about 30 varieties in the yard) and seem more resistant to PM than common lilac and some of our other varieties.

Again, when attempting reduction with hosing down be sure to do it so that it washes the plants instead of merely dampening them.

And do it in the morning, during a dry day. Causing them to remain damp for an extended period (all day or over night) could even encourage mildew growth.

The optimum with mildew-susceptible garden plants is probably consistently moist root zones combined with tops that are completely dry at all times.

Although some mildew spores travel with their own moisture droplet attached, so they can germinate on dry leaf surfaces.