Platychora leaf spot

The disease is caused by the ascomycete fungus Platychora ulmi, an obligate parasite of elm trees (genus Ulmus). This pathogen specifically targets the foliage, leading to localized necrosis and significant physiological stress for the host tree. It is recognized as a significant foliar pathogen in various ecosystems across the temperate regions where elms grow.

This fungus primarily affects different species of elm, including Ulmus glabra and Ulmus laevis. It is commonly found in forest stands, urban parks, and windbreaks. The disease density often depends on local humidity levels and the genetic susceptibility of the specific elm populations present in the area.

Visible symptoms include the development of dark, often circular or irregular lesions on the leaves. Within these lesions, tiny black stromata develop, which are fruiting bodies of the fungus. As the infection progresses, affected leaves turn yellow prematurely, become brittle, and undergo early abscission, which leaves the tree canopy sparse well before the autumn season.

Development and spread are heavily dependent on moisture. During wet, humid springs, the fungus releases spores that are dispersed by wind and rain splashes to new, healthy leaves. The pathogen overwinters successfully in fallen leaf litter, which acts as the primary reservoir for the infection. Without proper removal of this debris, the disease cycle continues consistently year after year.

The harmful impact of Platychora ulmi manifests as a reduction in the tree's photosynthetic capacity. This energy deficit inhibits growth and reduces the tree's ability to store carbohydrates, making it more susceptible to secondary stressors like drought or insect infestation. In chronic cases, the cumulative effect can lead to severe weakening or decline of the tree health.

• Sanitation: Raking and destroying fallen infected leaves.
• Pruning: Removing dead or diseased branches to improve air circulation.
• Chemical control: Applying appropriate fungicides during the early leaf expansion phase.
• Maintenance: Ensuring trees are well-watered and fertilized to maintain vigor.