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You Bet Your Garden
Question of the Week © 2017 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.


Can You—SHOULD You—Compost Diseased Tree Leaves? Special treat: Last year around this time, Mike celebrated the 17th Anniversary of "You Bet Your Garden" on Public Radio. For the occasion, he was joined for the entire hour by master fruit grower and pruning expert Dr. Lee Reich and Texas organic advocate Howard Garrett, "The Dirt Doctor". At the end of the show, instead of answering the weekly 'Question' himself, Mike read it to his two guests and asked what THEY would say. Their answers were a real shocker!

It was an episode that can't be beat, so to celebrate the show's 18th year on the air (October 3rd!), Mike is repeating that fabulous broadcast. (And taking the week off to start collecting THIS year's falling leaves for his own mulch and compost makings!)

And so, back by popular demand (and because Mike needed a week off), here 'tis:

Mike: Simon in Windsor, Ontario, Canada ("on the border with Detroit") writes: "A large maple tree in my backyard developed black spots on all of the leaves this year. A maple belonging to my neighbors across the street has the same issue. I believe these may be 'tar spots'. The trees otherwise seem to be doing fine. I suspect this happened last year as well but I wasn't paying much attention then.

My concerns:

1) I save my fall leaves to add to my compost throughout the year. My compost never gets very hot and I'm concerned that if I use these leaves I will just be re-inoculating the tree next year. My compost is made in a wooden 4-bin system filled with kitchen vegetable scraps, yard debris (mostly old vegetable plants), previous years' leaves and straw (bought for Hallowe'en decoration). I always have lots of worms and bugs crawling around in it.

2) I was planning on also stealing my neighbors' leaves (as I know you have also been known to do). Now their leaves may be off limits too!

My tree is not fed; it is not near a treated lawn (we don't do that much in Canada). There is wood mulch near the tree; but it's a natural non-dyed cedar mulch pulled away from the plants.

My questions are:
1) Can I just use the leaves in my compost?
2) Can I use them if I get my compost hot enough? And if so, how do I get it hot enough?"

Mike: I'll begin by saying that tar spot is caused by a disease organism, that it's mostly a 'cosmetic' problem that doesn't actually harm the tree and that leaving infected leaves under the tree continues the cycle of disease the following year. That said, is it wise for him to want to try and put infected leaves into his compost?

Lee: I put everything in my compost, even my old jeans. They take a couple of years to finish, and the same may be true of these spotty leaves. Composting is a factor of time and/or temperature—so if your compost doesn't get hot, you give the materials more time. But once the material is composted I don't think you'll have any risk of that compost re-inoculating the tree with a disease like tar spot. So I would not hesitate to include these leaves and I would use the neighbors' leaves as well.

Howard: I agree with Dr. Reich. I think the best way to get rid of disease pathogens IS to compost them. Let the beneficial microbes in a good healthy compost pile neutralize the pathogens.

Lee: Again, I put everything in the compost. I've composted for decades and there's nothing I wouldn't put in the compost. People say not to compost anything that's diseased or pest-ridden; but I contend that if you look at any part of any plant closely enough, you'll find something wrong with it—and that would mean that you wouldn't compost anything!

Mike: But Lee—you grow a lot of disease-prone fruits like apples and peaches. You wouldn't throw something like a peach with brown rot into the compost pile, would you?

Lee: I would, I repeat, throw ANYTHING into my compost pile.

Howard: I would too. All disease pathogens are is organisms that are out of balance; and the way to bring things back into balance is to compost them with lots of other organisms. You can control pathogens that are fungal, bacterial—you can even control viral pathogens; it's just a little harder.

Mike: But don't these kinds of diseases have to go through hot composting? Or can Nature handle a slow curve ball?

Lee: If you can't make the compost hot, let it compost longer. It's just like Howard says—these things are out of balance and the way to bring them back into balance is through composting, not by throwing them away.

Mike: And would you then spread this compost back under the affected tree?

Howard: Absolutely. This disease is not damaging to the tree; it's just a cosmetic problem. And trees are tough; they can handle these kinds of problems as long as they have the proper growing conditions. And first on that list is that they're planted high enough in the ground so you can see the root flare. If you can't see the root flare, the soil or mulch or whatever that's got it buried has to be pulled back or blown away until all of the trunk is exposed to the air.

Mike: Wow. When I read that this disease was carried over from year to year by infected leaves overwintering under the tree, I was ready to tell this guy to throw them away or burn them…

Lee: Oh no.

Howard: Never!

Mike: Well, there you have it; I am outvoted two to one! Not only do my distinguished guests say that you CAN compost diseased leaves, they're going a step further and saying that you should.


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