Forest Health > Insects and Diseases publication
Diseases of... Hardwood Stem
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HYPOXYLON CANKERS
Silver-gray crust-like stromata of Hypoxylon atropunctatum on the stem of a dying oak. |
| Pathogens: |
Fungi Hypoxylon spp.,
especially Hypoxylon atropunctatum
and Hypoxylon truncatum |
| Common Host: |
Oaks Quercus spp.,
Other Hardwoods |
Significance: |
Hypoxylon cankers are prevalent and highly visible disease problems affecting oaks and other hardwoods in Florida. Hypoxylon spp. are not considered aggressive killers. Instead, they are usually secondary in that they take advantage of trees suffering from any number of injuries of stresses. Hypoxylon cankers are often the finishing blow to oaks suffering from water stress, root disease, soil compaction, construction damage or other, related injuries. |
Recognition: |
Trees infected with Hypoxylon spp. often show evidence of either 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 (sing., conidium) are produced on the surface of crusty sheets of fungus tissue called stromata (sing., stroma). Later in the summer or fall, after the powdery conidia are gone, the fungal stromata thicken, become very hard and assume a silver-gray, brownish, or black color, depending upon the particular fungus involved. In many cases, minute (less than 1 mm), slightly-raised dots or bumps may be observed on the surfaces of these late-season stromata. These dots are the tops of small cavities or locules called perithecia (sing., perithecium) embedded in the fungal stromata in which the pathogens produce another type of spores (ascospores). Stromata vary from a few centimeters across to several meters in length up and down the trunks of infected trees and are the most readily recognizable indicator of Hypoxylon infections.
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Infection Biology: |
Hypoxylon infections originate when ascopores of the various pathogens come into contact with injured or severely stressed tissues of susceptible hosts during the late summer, fall or winter. Spores are spread via wind and splashing rain and presumably by certain insects, birds, and rodents. The fungus develops in the bark and wood tissues and the following spring or summer the bark is sloughed, revealing the typical powdery masses of conidia. 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 ascospores later in the summer or fall. |
Control: |
Trees with extensive Hypoxylon infections are usually beyond repair. 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 the infected trees. Prevention of infections through avoiding wounds, root damage, etc., and providing adequate moisture via irrigation to susceptible trees during prolonged periods of dry weather is the best method of 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.
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Bulletin No. 196-A | Printed October, 1983 | Contact the Forest Health Section |
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