Rusty crust fungus

Rusty crust fungus (Hypoxylon rubiginosum) is a species of sac fungus within the Hypoxylaceae family. It is recognized in arboriculture and plant pathology as a saprotrophic fungus that often turns opportunistic, colonizing the wood of deciduous trees that are already suffering from physiological stress or environmental damage.

This pathogen hosts on a variety of hardwood species, including oak, beech, maple, and several fruit-bearing trees. By targeting the cambium and xylem, the fungus exploits structural vulnerabilities, often colonizing tissues that have been previously exposed through pruning wounds, frost cracks, or mechanical abrasions.

The primary symptom of an infection is the emergence of flat, crust-like stromata on the bark surface. Initially, these structures are a distinct rusty-red color, gradually turning dark brown or black as the fungus matures. This process leads to localized necrosis of the bark and significant structural breakdown of the underlying wood, often manifesting as white rot.

Environmental favorability for Hypoxylon rubiginosum is high in humid, temperate conditions. The fungus propagates through airborne spores, which can travel significant distances, though transmission via contaminated pruning tools and human intervention in the orchard remains a primary factor in localized disease outbreaks.

Mitigation and control strategies are essential to prevent the spread of the fungus and to maintain the longevity of the trees:

• Eliminate infected wood immediately by pruning well below the visible affected area.
• Sterilize all cutting tools with alcohol or a strong fungicide to prevent cross-contamination.
• Encourage tree vigor through proper fertilization, irrigation, and avoiding site compaction.
• Protect tree trunks from sunscald and other environmental stressors that could facilitate fungal entry.