Is this leaf diseased and can I use it for winter mulch?


Tons of fallen leaves are looking like this. Is it some kind of fungus that’s attacking the dying leaves. If so is it safe to use these leaves to mulch the garden… or will they spread whatever is happening to them to the healthy plants in the garden?
Thanks. – A


Thank you for contacting the Toronto Master Gardeners.

The white fungus you see on the leaves of so many Norway Maples (Acer platanoides) this year, is known commonly as powdery mildew.  It develops in high humidity conditions (most of our Toronto summers), spreads with the wind and has become common on Norway Maples in the last several years.  One thing about this fungus is that different strains are host-specific.  Norway maple powdery mildew shouldn’t infect plants of other species in your garden.

The general guidance for any diseased plant material is to diligently gather any debris and dispose of it rather than leaving it on garden beds or adding it to the compost.  This will help to decrease the severity of future infections.  Depending on humidity levels and rainy conditions each summer, don’t be surprised to see this fungus again – once it’s in the neighbourhood, it’s nigh on impossible to irradicate it.

However, depending on the size of the tree, if complete leaf debris removal is too much work, it’s not the end of the world.  As there are so many Norway maples in Toronto with the fungus spreading in the right conditions, individual cultural practices can only do so much, just do your best.

Bagging the leaf debris to put out for your city garden waste collection rather than composting on site, is ideal as the city compost facilities reach high enough temperatures to kill most of the fungus.

We also recommend you have the tree pruned to open up the canopy in order to allow greater air flow and enable the leaves dry faster which will help cut down on the infection.  If the tree is large, we strongly suggest having a certified arborist tackle this pruning job.

Having this infection will not negatively impact the tree, it is just aesthetically unappealing to some gardeners, but the above suggestions should help.

We hope this information is useful.

07 November 2023