Macrophoma leaf spot of ash

The disease known as Macrophoma leaf spot, or ash bark necrosis, is caused by the fungus Macrophoma fraxini. It is a fungal pathogen that specifically targets the bark of ash trees (Fraxinus species). By colonizing the bark, the fungus interferes with the nutrient transport system of the plant, potentially leading to the death of branches and overall tree decline.

The primary host for this pathogen is the European ash. The fungus affects both young twigs and branches of mature trees. It is frequently found in managed forests, nurseries, and urban landscaping environments, where environmental stress can weaken the natural defense mechanisms of the host trees.

The diagnostic signs of the infection are characterized by the development of small necrotic spots on the bark surface. As the fungus matures, it produces fruiting bodies known as pycnidia. These appear as minute, black, pimple-like structures embedded in the bark. Eventually, the affected bark may crack and detach, leading to the girdling of the branch and subsequent wilting of the foliage above the infection site.

Favorable conditions for the development and spread of Macrophoma fraxini include high humidity, prolonged rainy seasons, and poor air circulation within the tree canopy. The fungal spores are primarily dispersed via splashing rain and wind currents, facilitating rapid colonization of neighboring healthy tissues or nearby susceptible trees during favorable weather windows.

Management and prevention strategies focus on reducing the fungal load within the environment. Key practices include:

• Sanitary pruning and removal of infected branches, ensuring cuts are made into healthy, living wood.
• Proper disposal of infected material, typically through burning or deep burial, to prevent spore re-infection.
• Application of copper-based fungicides as a preventative measure in the early growing season.
• Improving tree vigor through proper fertilization, irrigation, and minimizing physical wounds to the bark.