Several mushroom species that are known as the Jack O’ Lantern fungus are very common on and around oaks and other species in the late summer and fall (Photos 1 & 2). Knowing this fungus (Photo 3) is important to arborists, as it looks very similar to the well-known, root-killing and decay pathogen Armillaria (Photo 3A). The fungus also can produce an abundant number of mushrooms on and around urban trees, creating concern from tree owners about the tree stability and identity of the fungus. Fruiting of the Jack O’ Lantern fungus is probably more common than Armillaria in urban landscapes. But as far as is known, the Jack O’ Lantern is a saprophyte, living on dead roots, stumps, buried wood or deadwood in trees that have been killed by other agents (Photo 4).

Jack O’ Lantern gets its name from the orange color of the cluster of mushrooms that develop starting in the late summer. The mushrooms are about the color of a ripe pumpkin when fresh (Photo 5) but fade to dull orange as they age. There are several species of Jack O’ Lantern fungi, but they are all in the genus Omphalotus (see species descriptions later in this article). The fungus appears as clusters or groups of mushrooms on the lower trunk or buttress roots, and also on the ground where it is attached to dead roots (Photo 6). Single mushrooms also may occur, but they are usually near larger clusters. The fungus lacks a ring around the stem, is orange colored when fresh and sometimes has a lateral stem (attached off-center of the cap or sometimes at the edge of the mushroom). The gills of the Jack O’ Lantern are decurrent, meaning they run a short way down the stem (Photos 5 & 6).

Jack O’ Lantern looks very similar to Armillaria, and for arborists the distinction is very important. Armillaria mellea and related species all have a ring around the stem and are honey colored or light brown (Photos 7 and 7A). Armillaria tabescens (ringless Armillaria) is probably the closest to Omphalotus in appearance and was classified in the same genus (Clitocybe) in the past (Photo 6A). However, A. tabescens – which is a known root killer of oaks and a wide range of other species and can contribute to the decline and death of landscape trees – is also honey colored and has a central stem. Both Armillaria and Omphalotus have white spore prints.

The Jack O’ Lantern mushroom has been known in eastern North America under several names since it was described as Agaricus illudens in the early 1800s. However, continuing mycological research resulted in different opinions regarding the appropriate name for the species. Thus, in mushroom literature and even tree-pathology publications, it can be found in several genera, most commonly Clitocybe and Omphalotus.

In the mid-1900s, the species was thought to be the same as Omphalotus olearius of Europe, and reference to this fungus in North America was frequently by that name. Current research shows the eastern North American species is distinct and should be called Omphalotus illudens. Two other species of Jack O’ Lanterns occur in North America, Omphalotus olivascens, in the coastal Southwest, and O. subilludens, reported to be found in Florida and Texas. However, the most common Omphalotus species by far is O. illudens, which can be found east of the Mississippi River, north to south.

It is common for Jack O’ Lantern to fruit on trees that also have other decay pathogens present. The presence of Jack O’ Lantern on or around a tree can be reason for some concern, because it at least indicates that the tree has an adequate number of dead roots or wood present in the butt that it can support fruiting of this fungus. However, there is no evidence that Jack O’ Lantern has the ability to kill or decay living roots or attack healthy, undamaged wood in the base of trees. Arborists should follow through on making sure they have the right identity of the fungus present and do basic common decay inspection of trees with Jack O’ Lantern, such as sounding with a mallet and probing.

The Jack O’ Lantern has a number of other features arborists should take note of. The fungus is reported to be a common cause of gastrointestinal upset if eaten. So do not attempt to eat Jack O’ Lantern. It also has the potential to be confused with the common and well-known edible chanterelle mushroom that has a similar color and appearance. Knowing the difference between these two fungi is obviously important if you are considering a meal. To add to the Halloween aura of the Jack O’ Lantern, the fungus is reported to have bioluminescent gills that glow at night (Photo 8). However, this seems to be a rarely seen phenomenon despite being commonly cited as a feature of the fungus.

Note: The author gratefully acknowledges Harold H. Burdsall, Jr., Ph.D., retired USDA mycologist and now with Fungal & Decay Diagnostics, LLC
(fungaldecay.com), for photographs and assistance with this article.

Christopher J. Luley, Ph.D., is president and tree pathologist with Urban Forest Diagnostics, LLC, located in Naples, New York.