Aleurodiscus oakesii, the oak parchment, cause of "smooth patch disease"

Tom Volk's Fungus of the Month for April 2006

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Aleurodiscus oakesii fruiting bodiesAs I write this in early April, it's just getting to be Spring here in Wisconsin, and mushroom pickin's are slim. I know some of you are already getting morels, false morels, scarlet cups, eyelash cups, and black tulip fungi, but until the end of the month we have to settle for last year's leftovers. One of these leftovers is Aleurodiscus oakesii, a small but very common and overlooked fungus. Unlike mushrooms with which you may be more familiar, Aleurodiscus species produce a flat crusty fruiting body that is rarely more than a centimeter (half inch) in any direction and usually about a millimeter or two thick. However its effects can be seen from quite a distance. Although A.oakesii is not a parasite, it lives on the bark of trees, primarily oaks and other members of the Fagaceae, such as chestnut, although it is found commonly on many other hardwoods. In most areas the most common host are members of the white oak group, the oaks with non-pointed lobes on their leaves. The fungus is described from eastern North America, but it can also be found in Great Britain, China, Japan, and probably elsewhere. The food source for the fungus is actually the dead bark of the tree. You may remember from your botany or plant anatomy class that the outer bark of woody trees is no longer functioning in the transport of sugars. This task is taken by the inner bark with its functional secondary phloem. (See, you can learn some botany here today, too!)

smooth patch disease on oakHowever, A.oakesii does digest the outer bark eventually, and this results in smooth patches on the normally rough and furrowed bark. This effect is cleverly called "smooth patch disease" by plant and forest pathologists. It is not very harmful to the tree, although some may cringe at the cosmetic effects this may have on trees in their yards.

In the picture to the right, notice the large lighter patch on the bark, extending from the base of the tree up to about 2 meters (6 feet). If you went up close to the lighter patch, you would notice that the lighter color is due to the bark's being more smooth than the surrounding bark. In addition, you would notice abundant fruiting bodies of Aleurodiscus oakesii. closer view of smooth patch disease on oak. diseased part on leftTo the left you can see an infested area (on the left) next to an uninfected area (on the right). This disease seems to be very common. I did not have a picture of the smooth patch, so I drove around the park near my house and found this example within five minutes. If you live east of the Rocky Mountains in North America, I guarantee you have seen it dozens of times, mostly without knowing it. Look for the smooth patches on the bark, then the little cup shaped to flattened fruiting bodies. If you look around closely enough you'll find it!

Aleurodiscus oakesii microscopic characteristicsLike most other Corticioid fungi (crust fungi), such as Phanerochaete chrysosporium, Phlebia coccineofulva, Hyphoderma puberum, and Pulcherricium caeruleum, Aleurodiscus oakesii is far more interesting microscopically than macroscopically. It's loaded with interesting microscopic characters. Most prominent are the spiny ornamented basidiospores (C and D in the picture to the right) that turn blue in Melzer's Reagent (iodine plus chloral hydrate). Another important feature are the acathohyphidia (A and B), which are a fancy kind of cystidia, sterile structures that grow between the basidia (E). "Acanth-" means thorn or prickle, and "hyphidia" are little hyphae, so acathohyphidia are "prickly little hyphae." It is unclear why they are there, but they may function as deterrents to insect predators. There are a few dozen other species of Aleurodiscus around the world, but almost all of those are larger and more closely appressed to the wood, i.e. not curling up on the edges. The prettiest one is Aleurodiscus aurantiacus, an orange crust fungus common on alder. Many of the others are gray/blue or white to tan.

original description of Corticium oakesii Berk. & Curt So what's with the unusual name, Aleurodiscus oakesii? "Aleuro-" means flour, referring to the white powdery nature of the surface of the fruiting body, and "discus" refers to its disc-shaped structure. Thus Aleurodiscus means "flour disc." It is widely assumed by both professional and amateur mycologists that the epithet oakesii refers to the host, the oak tree. However, it is actually a pun (intended!) made up by the mycologists who first described the fungus as Corticium oakesii Berk. & Curt. in 1873. [Greveilea 1(11):66]. To the left you can read the original description of the species. However, since all mycological species desriptions must be written in Latin, you may have to brush up on your dead language translation! However, you'll be surprised how much of it you will understand if you know some mycological terminology. The Reverend M.J. Berkeley and M.A. Curtis named the fungus in honor of William Oakes (1799-1848), a famous Botanist from New England who did much of his work in the White Mountains of New Hampshire. Apparently Oakes collected the original sepcimen.

According to The Gray Herbarium at Harvard University William Oakes, coutesy of Lisa DeCesare of the Harvard Botanical Libraries ,

"William Oakes was born on July 1, 1799. He entered Harvard in 1816 and developed a strong interest in natural history under the tutelage of William Peck. After graduating from Harvard in 1820, Oakes studied law for three years. In 1824 he moved to Ipswich, Mass., and began to practice law. A few years later he gave up law and devoted himself to the study of natural history, particularly that of New England. In 1842 he was asked to write a brief description of the flora of the White Mountains for a geological survey report on New Hampshire, and he expended much energy on this project for the rest of his life. He died on July 31, 1848, when he fell off a ferry between Boston and East Boston and drowned."

However, according to Scott Redhead & Lorelei Norvell. [1999. American epitaphs and epithets. Mushroom, The Journal 17(4): 27-29] a later dedication of a book by his friend John Robinson indicated that his death may have been far more sinister. I guess we'll never know.

I hope you enjoyed learning about the cute little crust fungus, Aleurodiscus oakesii, the "flour disc named for William Oakes." See if you can find it-- it's more common than you might think!

If you have anything to add, or if you have corrections, comments, or recommendations for future FotM's (or maybe you'd like to be co-author of a FotM?), please write to me at my email address

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