Selenophoma leaf spot of mahonia

The causative agent of this disease is the fungus Selenophoma mahoniae. It is a specialized pathogen that infects the foliage of woody ornamentals, particularly those in the Berberidaceae family. The fungus disrupts normal plant physiology by destroying tissue within the leaf blades.

The primary host for this pathogen is the Oregon grape or holly-leaved mahonia (Mahonia aquifolium). It is frequently encountered in landscape settings, botanical gardens, and commercial nurseries where environmental conditions, such as high density and humidity, are favorable for fungal growth.

Symptoms typically include the appearance of small, circular to irregular lesions on the leaves. These spots start as chlorotic areas, eventually turning brown or grey with a darker, distinct margin. During the later stages of infection, small, black, pinhead-sized pycnidia are visible within the centers of the dead tissue spots.

The spread of the fungus is highly dependent on environmental moisture. Rainfall, heavy dew, and overhead irrigation facilitate the dispersal of spores from the pycnidia to healthy leaves. Poor air circulation within dense hedges exacerbates the rate of infection, allowing the disease to spread rapidly through the canopy.

• Prune surrounding vegetation to improve air circulation within the plant canopy.
• Remove and destroy all fallen leaf litter to reduce the inoculum source for the next season.
• Apply preventative fungicides, especially during wet spring and autumn weather.
• Avoid overhead irrigation to keep the foliage as dry as possible during the day.

The economic and aesthetic damage caused by Selenophoma mahoniae is significant for ornamental horticulture. Severe infections result in premature defoliation, which causes physiological stress to the plant, leading to reduced vigor, stunted growth, and an increased susceptibility to secondary infections or winter damage.