Microdiplodia dieback

Microdiplodia dieback is a plant disease caused by the fungus Microdiplodia heteroclita. This pathogen primarily affects the bark and vascular tissues of woody plants, leading to the necrosis of stems and branches, which can significantly affect the overall health and vigor of ornamental and forest trees.

Host plants commonly include species from the Oleaceae family, such as ash (Fraxinus) and lilac (Syringa). In some instances, it has been reported on maples and elms. The pathogen is widespread in both wild environments and managed landscapes, often acting as a facultative parasite that targets weakened hosts.

The primary symptom is the appearance of sunken or discolored patches on the bark. As the fungus progresses, pycnidia, which appear as tiny black pustules, emerge through the epidermis. If left untreated, the dieback can lead to the death of affected branches, causing severe canopy thinning and potentially the decline of the entire tree.

The development of the fungus is favored by high humidity and temperature fluctuations that stress the plants. Spores are primarily dispersed through rain splash, wind, and movement of contaminated tools during pruning. Once the spores land on a bark wound or lenticel, the fungus colonizes the inner bark and cambium layer.

Management strategies focus on a combination of cultural and chemical controls. Pruning out infected branches back to healthy wood is the most critical step to prevent spread. It is essential to sanitize tools with alcohol or bleach after each cut. Additionally, maintaining tree vigor through proper irrigation and fertilization helps the host resist colonization by the fungus.