Hypoxylon Cankers of Oaks
and Other Hardwoods

Hypoxylon (hi-pรณx-i-lon) cankers are prevalent and highly visible diseases affecting oaks and other hardwoods in Florida.  These cankers are caused by one or more Hypoxylon fungi.

Hypoxylon species are not considered aggressive killers.  Instead, they are usually found on trees suffering from a variety of injuries of stresses.  Hypoxylon cankers are often the "straw that broke the camel's back" in oaks suffering from water stress, root diseases, soil compaction, construction damage or other injuries.
Recognition Trees infected with Hypoxylon often show evidence of severe injuries on the branches or stem and/or advanced dieback or decline.  The bark of infected trees typically sloughs off, often near injuries or along the trunk and major branches, revealing one of two types of fungal signs.

In the spring or early summer, conspicuous, powdery, greenish-to-brown masses of spores called conidia are produced on the surface of crusty sheets of fungus tissue called stromata.

Later in the summer or fall, after the powdery conidia are gone, the fungal stromata thicken, become very firm, appearing silver-gray, brownish to black in color.  On close inspection, minute (less than 1/32"), black, slightly raised dots are visible on the surface of the stroma.  These dots are the tips of small cavities, producing spores which are discharged to the air looking for the next stressed oak.

Stromata vary from a few inches to several feet in length up and down the trunks of infected trees and are the most readily recognizable indicator of Hypoxylon infections.
Infection Biology Hypoxylon infections originate when spores come into contact with injured or stressed tissues of susceptible trees during the late summer, fall or winter.  Spores are spread by wind, splashing rain and, presumably, by certain insects, birds and rodents. The fungus develops in the bark and wood tissues.

Conidia apparently play a limited (if any) role in initiating new infections, but perhaps perform some type of sexual function which results in the production of the infective spores later in the summer or fall.
Control The presence of Hypoxylon cankers is usually an indicator of severe stress and often a warning to take precautions to reduce stresses (if they can be identified and reduced) affecting nearby trees of the same species.

(1)  The best control is a healthy tree.
(2)  The best method of control is prevention of infections by avoiding wounds, root damage, etc., and providing adequate moisture by irrigating susceptible trees during prolonged periods of dry weather.
(3)  Removal of severely infected trees to reduce local sources of inoculum (i.e., infectious spores) is recommended.  Careful pruning of branches that have localized infections should help prevent advancement of the fungus within infected trees.
This article was prepared by Dr. Edward Barnard, Forest Pathologist, Florida Division of Forestry.  Slight modifications have been made by BJ Jarvis, Pasco County Extension Director, Horticulture Agent.  For further information, please contact the Pasco County Forester at 352-523-5101.