Hypoxylon fungus

Very often when we are called to look at dead or dying oaks we find fungi in the genus Hypoxylon. These fungi are very ubiquitous and form latent colonies on healthy oaks and other hardwoods. Once the trees are weakened by stresses, such as drought, herbicide damage, insects or other diseases, Hypoxylon mobilizes to take advantage of the resources provided by a sick tree. In some cases, it may even slow the spread of more devastating diseases; trees succumbing to oak wilt are often rapidly colonized by Hypoxylon, which can use up the sugars and starch in the sapwood, preventing the oak wilt fungus from spreading.

Stromatal mat and fruiting bodies of a decaying fungus on an oak killed by flooding.

Stromatal mat on an oak killed by fungus.

Close up of stromatal mat.

Stromatal mats on willow oak that was flooded by beavers.

Stromatal mat of Hypoxylon on willow oak that has been killed by oak wilt.

 

Because Hypoxylon is so commonly found on dead and dying trees it is understandable that decline is often attributed to it. It is unlikely that Hypoxylon is ever the primary cause of mortality, but it can certainly play an important part in the decline of stressed trees. If other diseases or insects can be ruled out, conditions in the year or years prior to the onset of symptoms may be key.

Hypoxylon atropunctatum was associated with dieback and mortality of numerous oaks in the southeastern U.S. in 1981 after conditions of extreme heat and drought in 1980.

Fungi in the genus Hypoxylon are relatively easy to recognize. It decays the inner bark and sapwood so that the outer bark sloughs off, revealing the distinctive tan or gray stromatal mats. As the stromata mature they become black. Under some conditions black dome-like perithecial stromata may be formed.