Too late in the year to treat powdery mildew?

[DrTolhur]

I've got a whole bench of Japanese maples, and several of them have contracted a powdery mildew infection. This is my first time dealing with it, and treatment seems simple enough (I'd probably just go the neem oil route), but I'm wondering if it's worth doing at this point in the year. Trees around here are starting to turn, so it's probably just a couple weeks before the infected trees drop their leaves anyway. I'm not sure if I'd be able to get rid of the mildew soon enough to make any difference to the trees at this point. Thoughts? Should I start spraying with neem oil, or just let it run its course for the rest of the season?

If it makes any difference, all of the infected trees are young and in very early development. A couple have what I'd deem a pretty heavy infection.

 

[Deep Sea Diver]

Well, there is that nagging thought of treating it now …so your leaves can continue photo synthesizing at a higher level to help thicken your trunk, however incrementally.…

cheers
DSD sends

 

[ShadyStump]

Powdery mildew is a constant issue here. I would recommend treating it immediately. It has the potential to damage any soft growth, not just the foliage, and if nothing else you'll reduce the amount of it that might survive winter on the trunk, branches and pot, reducing the chances and severity of infection next year.

In the future, it's much easier to prevent powdery mildew than to treat it after you've spotted an infection. Avoid watering when it's about to get dark and cool, and keep your trees in the sun as much as possible - the cool, dark, moist conditions are what it spreads in easiest - and spray with neem oil at least monthly starting as soon as you see buds breaking in spring. Every other week is recommended, but may or may not be necessary in your climate. Preventing it from setting in next year is going to be important. Once they've been infected it's almost impossible to get rid of it permanently.
If you do see any signs of infection, start spraying weekly.

 

[KiwiPlantGuy]

Hi,
Is there something I am missing but I thought Neem oil was an insecticide??
So treating powdery mildew with an oil?
My take would be a spray of Mancozeb and Clearys? (Topsin down here) too protect the remaining leaves.
Other option is just to remove infected leaves, or ignore as it won’t do much harm and just look bad.
Charles

 

[Wires_Guy_wires]

Hydrogen peroxide at 1.5% will treat light mildew but it will leave patches of damaged foliage where the milldew has been growing.
I'm also not aware of neem oil having any effects. If it happens too early in the year, I spray with copper sulphate.

 

[penumbra]

Peroxide ^^^^, use now and in the future use it before you see the mildew.

 

[ShadyStump]

Is there something I am missing but I thought Neem oil was an insecticide??
So treating powdery mildew with an oil?

Neem is not a very effective insecticide for anything larger than aphids, but it is a decent pest deterrent. It makes plants hard for them to eat.
To be fair, my experience using it is mostly in the vegetable garden where a nontoxic oil is going to be preferred to most other options, and this is what OP said they have.
In that context gourds and other squash are particularly susceptible, an early infection destroying entire gardens, or on the farming scale decimating entire crops if not caught in time. I'm my climate it can eat clean through all the foliage of a pumpkin patch inside of a month, so on a small tree it can definitely be a death sentence.
My recommendation has been successful in the past under these conditions.

For strictly bonsai applications, @Wires_Guy_wires advice may well be the better option if available.

 

[AJL]

It might also help to pick off and bin all infected leaves, including any fallen leaves, and move the worst infected plants well away from the rest of your Acers to reduce risk of further cross- infection.

 

[Vali]

I've treated it successfully with lime sulphur at 2% concentration, both on a small oak in a pot and in the apple trees in my garden.

 

[Wires_Guy_wires]

One of the key causes of mildew seems to be susceptibility of the host plant.
I have a couple garden plants in full sunshine and in a windy spot and they still get it. Plants right next to it never develop issues, same species of plant. Picking off leaves only removes the leaves, not the issue.
I've heard that certain trichoderma fungi might act as a protective measure. Try googling trichoderma viride, usually the other one pops up as well. I forgot its name.

 

[Bonsai Nut]

Is there something I am missing but I thought Neem oil was an insecticide??

Neem is an organic pesticide and also a topical fungicide. It is a good option if you are treating fungus on food crops where you wouldn't want to use non-organic fungicides or systemics.

 

[ShadyStump]

move the worst infected plants well away from the rest of your Acers to reduce risk of further cross- infection

This is a good point.

 

[DrTolhur]

To clarify, I currently possess no treatment options, so I'll need to buy whatever it is I choose to use. Sounds like there's a general consensus on hydrogen peroxide, though, rather than neem oil. I just saw neem oil mentioned online in multiple sources, which is why I said that. Wires_guy mentioned using copper sulphate "if it's too early." What constitutes too early? And why not use copper sulphate all the time instead of hydrogen peroxide?

 

[penumbra]

And why not use copper sulphate all the time instead of hydrogen peroxide?

Conversely, why not reach for peroxide first. A dollar and nineteen cents and you are good to go.
While you have been debating for the last 24 hours on what to do, I would have accomplished this in the first hour ....... if I had the problem at all.

 

[DrTolhur]

Conversely, why not reach for peroxide first. A dollar and nineteen cents and you are good to go.

Also a good question. I assumed the comment about "too early in the year" for hydrogen peroxide to mean it would be harmful for some reason.

 

[WNC Bonsai]

To clarify, I currently possess no treatment options, so I'll need to buy whatever it is I choose to use. Sounds like there's a general consensus on hydrogen peroxide, though, rather than neem oil. I just saw neem oil mentioned online in multiple sources, which is why I said that. Wires_guy mentioned using copper sulphate "if it's too early." What constitutes too early? And why not use copper sulphate all the time instead of hydrogen peroxide?

Copper tends to accumulate in soils potentially causing adverse effects to the microbial function and fertility of the soil.

 

[penumbra]

Peroxide is gone within minutes or hours and has no residual I am aware of, though I expect someone here will argue this point.
Even when peroxide is used as a soil treatment, the beneficial microbes repopulate within 24 to 72 hours.

 

[ShadyStump]

Also a good question. I assumed the comment about "too early in the year" for hydrogen peroxide to mean it would be harmful for some reason.

I think you misinterpreted something there.
The statement was more like if the powdery mildew shows up too early in the year.

I've never tried the peroxide but myself, but it does seem like it should be effective.
I will say that neem oil is a better preventative than treatment in my experience.

 

[Deep Sea Diver]

Hydrogen peroxide is used on powdery mildew routinely. Vine Maples are the worst for it around here.

Keep spraying frequently, including the trunk and top of the soil to help prevent quick reinfection.

cheers
DSD sends

 

[ShadyStump]

Hydrogen peroxide is used on powdery mildew routinely. Vine Maples are the worst for it around here.

Keep spraying frequently, including the trunk and top of the soil to help prevent quick reinfection.

cheers
DSD sends

This is where I'll argue the superiority of neem oil as a preventative. It's persistent for up to two weeks in dry weather. If you miss a day, no big deal.


Edit: Just acknowledging that I have experienced severe damage from powdery mildew in the past, so I may be taking it much more seriously than people in other climates do.
No issues with any of my trees yet but I've lost entire gardening seasons to the stuff.