terrible powdery mildew on a Magnolia

last month

last modified: last month

This is really concerning. In almost 20 years of growing deciduous magnolias, I've never seen any powdery mildew on one, much less this amount.

I often tell other members here about how garden pathogens are usually cosmopolitan, and there's little point to try to isolate problems like this. But I really wonder, could this be a totally novel strain that had never been in my garden before? It's surely too late to stop it from spreading now. Google AI says drought can make plants more prone to a flair up of powdery mildew, but in 20 years there have been plenty of droughts and this has not happened.

FWIW, this is 'Genie', a variety with a slightly different parentage than most of the Gresham-like deciduous magnolias I've grown until now. So maybe the foliage is just more susceptible because of genetic differences. It does seem somehow not as thick as clones like 'Atlas' and 'Tina Durio'.

I'm going to research whether one of the various bio-fungicides I have in my arsenal could help, but feel free to offer suggestions.

Thanks.

Featured Answer

rhizo_1 (North AL) zone 7

last month

UpperBay, you might look into the efficacy of Neem Oil as both preventative and curative.

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Comments (6)

gardengal48 (PNW Z8/9)
last month
There are many, many strains of powdery mildew but if you research the topic (not AI!!) you will find that it is mostly climate related and there is little you can do to alleviate the problem once the mildew becomes obvious (any treatment is only preventative, not curative). PM also requires living plant tissue to survive so make sure you clean up thoroughly and dispose of - not compost - all the foliage.
UpperBayGardener (zone 7) thanked gardengal48 (PNW Z8/9)
UpperBayGardener (zone 7)
Original Author
last month
Thanks, I figured using the biofungicides would be preventative, not curative. I will remove these leaves when they fall but clearly whatever spores this strain produces, are now all over the place.
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BillMN-z4a
last month
Bad year here also, with regular rains early season, high humidity and generally calm, warm weather, I think it was the perfect storm for these kinds of things.

AI is beginning to concern me. It can return information on simple questions but if things get too complicated or precise, it will spew conflicting or wrong information, sometimes in the same answer. If that continues it becomes a menace to society.

Please let us know what you find out david.

My problem here is, I'm supposed to clean up the dead leaves that Fall and dispose of them to help cut down on future instances but when you have hundreds of feet (lilacs) that feat is next to impossible, for me anyways.
UpperBayGardener (zone 7) thanked BillMN-z4a
gardengal48 (PNW Z8/9)
last month
UBG, please reread what I wrote :-) Since the problem already exists, spraying now cannot be preventative - you can't prevent what's already happened.

And since the spores need living tissue to survive, if you clean up and dispose of all the leaves - PM is a foliar disease - then the spores won't survive winter.

If you have ongoing concerns about the problem, spray as the new foliage emerges in spring and again periodically through the season, alternating fungicides.
UpperBayGardener (zone 7)
Original Author
last month
last modified: last month
"If you have ongoing concerns about the problem, spray as the new foliage emerges in spring and again periodically through the season, alternating fungicides."

Yes, this was what I meant by 'preventative'.

FWIW, though I share concerns about AI accuracy, as well I should since it's affecting my industry worse than any, if you ask it about Magnolias and powdery mildew, it hones in on M. liliflora as being particularly susceptible. Without me mentioning it ahead of time. So that means it's picking up a lot references to that species in its training data. To my knowledge, this is the only cultivar I have with substantial M. liliflora parentage. So for now I'm mainly chalking the difference with my other magnolias up to that...although certainly the weather this year didn't help. With June and July being wet, and August being relatively cool but bone dry. Maybe it set up perfect circumstances for this cultivar to have the problem.