Elm witches' broom

Elm witches' broom is a systemic plant disease caused by the phytoplasma Candidatus Phytoplasma ulmi. It primarily impacts various species of the Ulmus genus, triggering distinct physiological changes that lead to the proliferation of abnormal shoots and the decline of the tree's overall vigor.

The most characteristic symptom is the development of dense, clustered masses of stunted twigs, commonly referred to as "witches' brooms." Affected branches exhibit shortened internodes and undersized, often chlorotic foliage, which disrupts the photosynthetic capacity and aesthetic value of the tree significantly.

The pathogen resides within the phloem tissue of the host plant and is disseminated by insect vectors, specifically leafhoppers and planthoppers. Once an insect feeds on an infected tree, it acquires the phytoplasma and transmits it to healthy elms, allowing the disease to spread throughout a landscape or nursery environment.

The progression of the disease is favored by warm, humid conditions that support vector population explosions. Over time, the tree experiences progressive decline, characterized by sparse canopy development, stunted growth, and increased susceptibility to secondary stressors, eventually leading to tree mortality.

Management of elm witches' broom relies heavily on integrated preventive strategies because there is no direct chemical cure for phytoplasma infections in forest settings:

• Regular inspection and removal of infected branches or entire trees to reduce inoculum sources.
• Effective management of insect vectors using appropriate insecticides during peak activity periods.
• Sourcing and planting of certified disease-free elm stock to prevent introduction.
• Ensuring healthy growth conditions to enhance the tree's natural resilience.