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Fall 2010

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

November 30, 2010

Powdery mildew: A leaf parasite hunkers down for the winter

People raking their yards may notice the occasional leaf blanketed with a white frosting of powdery mildew

Powdery mildew on a Tulip Poplar leaf

Not the same mold found growing on basement walls and shower stalls, these fungi grow only on living plants. The cool nights and warm days of spring and fall encourage the fungus to spread across the surfaces of stems,fruits and leaves, into which the mildew sends tiny root-like pegs to
pilfer nutrients.

In addition to its spreading mat of fungal filaments, the mildew sprouts vertical threads tipped with vegetative spores, which are carried off by
breezes and deposited onto distant leaves -- or dropped farther out onto the same leaf.

Another strategy of infection involves the release
of sexual spores. Powdery mildew mats can grow directly from these reproductive cells, but before they can make sexual spores of their own, two mildew mats of the same kind must first connect
and fuse their cells and cell nuclei.

Cross section of a leaf infected with powdery mildew

Some mildew mats can even join together their own nuclei -- effectively fertilizing themselves. Once that happens, autumn's cool temperatures trigger the formation of cleistothecia, tiny black spherical structures that bundle up sexual spores. These structures provide a winter refuge for the fungus.

If it isn't eaten by a tiny invertebrate, a cleistothecium will break open during warm spring weather. Sexual spores will emerge to drift away on air currents and will be lucky to land on a leaf they are capable of infecting.

Each of the many species of powdery mildew is very particular about which plants it will parasitize. The fungus on the leaf, left, didn't threaten the life of the tree, but it did obstruct the leaf's ability to photosynthesize.

Powdery mildew cleistothecium

Sources: American Phytopathological Society; Tree of Life Web Project; Missouri Botanical Garden; Cornell University