Black spot of elm

The fungus Stegophora ulmea (formerly known as Gnomonia ulmea) is the primary causal agent of black spot of elm. This disease specifically targets trees within the genus Ulmus, often leading to significant leaf damage in both urban forestry and nursery settings.

This pathogen affects a wide range of elm species, including American elm, Siberian elm, and English elm. While mature, healthy trees may tolerate the infection, young seedlings and saplings are particularly susceptible, often suffering from severe leaf drop if left untreated.

Symptoms are easily identified by the appearance of small, yellowish spots on the upper leaf surface in late spring. These spots progressively turn black and develop small, raised structures (pycnidia) where spores are produced. As the disease advances, leaves turn yellow and drop prematurely.

The development of the disease is heavily favored by warm, wet spring weather. High humidity is essential for the germination of fungal spores, which are spread primarily through rain splashing and wind dissemination, allowing the fungus to progress rapidly through the canopy.

The impact of black spot of elm is primarily measured by the stress it places on the host tree. Premature defoliation reduces the tree's photosynthetic capability, which depletes stored energy reserves. Over successive years, this can lead to crown dieback, increased susceptibility to other pathogens, and general decline in tree vigor.

Management and control strategies focus on sanitation and chemical intervention:

• Sanitary removal and disposal of fallen leaves during autumn to eliminate overwintering inoculum.
• Pruning for better air circulation within the tree canopy to reduce humidity levels.
• Application of protective fungicides during the bud break period if historical infection levels are high.
• Maintaining tree health through adequate irrigation and fertilization to improve natural defenses.