Not all the perfumes of Arabia can wash this little leaf clean

At a meeting of the Wayland Garden Club last month, a gardener reported tar spot fungus on her Norway maple leaves and concluded that this meant she had to remove all the leaves from her property. She reasoned that they couldn’t be used for compost or mulch for fear of spreading the fungus to her lawn and garden. 

Are leaves infected with fungus harmful to other plants?

    You too may have noticed that Norway maple leaves have developed black spots, turned brown and started to fall much earlier than usual. 

Black tar spots on Norway maple leaves

You could imagine that the trees are dying. Fortunately or unfortunately, the fungal infection is just cosmetic. This year’s very wet spring gave a boost to various fungi, including those that cause both tar spot and anthracnose, another fungal condition that causes leaves to brown and drop off, but the trees will leaf out again next spring. 

This Norway maple's leaves have prematurely turned brown and started to drop off

You don’t have to protect other plants from contact with the infected leaves.

    I say fortunately or unfortunately because I’m ambivalent about harm to the Norway maples that line our streets. Like many cities, mine used to favor tall, graceful elms as street trees. 

American elms in Central park

Dutch elm disease, another much more lethal fungal disorder originally from Asia, arrived in New England in 1928 and burst out of control during World War II, when attention to containing its spread was diverted to war efforts. To replace the dying elms, the cities planted Norway maples. They were tough enough to thrive in urban conditions. What could go wrong?     

     Eighty years later, we’re dealing with the consequences of this decision. Free of the competitors they faced in their native range in Europe and western Asia, these trees have become invasive in North America. 

Norway maple in flower, its prettiest time-photo Daniel Case

Since 2009 it’s been illegal to sell or plant them in Massachusetts.

    When we moved into our house in 1985, we had Norway maples as street trees (still present), large specimens stood in neighbors’ yards, and a volunteer thicket of them grew behind the back fence. Now there are fewer Norway maples around us, but I still pull hundreds of their seedlings from my planting beds every spring.

Norway maple seedlings: there are always more coming

    Norway maple has long been my most hated tree. I’ve often wondered how I’d feel if a pest finally came along that could kill them all. The Asian long-horned beetle, which enjoys munching maples, looked like a good candidate. 

Asian long-horned beetles on a maple. This menace hasn't reached my yard yet-photo USDA.

I admit it’s a thrill to see the tar spots inflicting Norway maples and bypassing red maples, a near relative that is native, non-invasive, and less susceptible to the fungus.

Red maple leaves free of fungus

    Call me a wimp, but I can’t wish all those Norway maples dead. They’re a menace, but they’re still providing us with “environmental services” such as sequestering carbon and cooling and cleaning the air. Their dense shade is a bane for gardeners but very welcome to drivers seeking parking places on hot days. Let’s say they have the defects of their qualities. They’re tough, hardy, and good at reproduction. Can we, who planted them, hold them responsible?

Winged maple seeds are called samaras