Elm yellows

Elm yellows is a devastating plant disease caused by a specialized cell-wall-less bacterium known as a phytoplasma, specifically Candidatus Phytoplasma ulmi. This pathogen colonizes the phloem tissue of the host plant, effectively clogging the transport system and preventing the flow of essential nutrients and sugars, leading to systemic decline and death.

The primary hosts for this pathogen are tree species within the genus Ulmus (Elms). Different species show varying degrees of susceptibility, with some being highly vulnerable to rapid decline once infection is established. The disease is recognized globally as a significant threat to the ecological stability of forests and urban landscapes where elms are present.

Symptoms of Elm yellows typically manifest during the mid-summer months. The most characteristic signs include yellowing (chlorosis) of the foliage, followed by sudden wilting and premature leaf drop. Infected branches may show signs of tissue necrosis under the bark, which often emits a distinct wintergreen-like odor. Severe cases result in thinning of the canopy and the eventual death of the entire tree.

Transmission occurs through insect vectors, primarily leafhoppers, which acquire the phytoplasma while feeding on the phloem of infected trees and subsequently inoculate healthy trees during feeding. Additionally, the disease can spread through natural root grafts between adjacent elm trees, facilitating the spread of the pathogen through a grove or hedge of elms.

Management of Elm yellows is challenging because there is no known cure for infected trees. Effective control relies on strict sanitary protocols, including the identification and prompt removal of symptomatic trees to eliminate the source of infection. Controlling leafhopper populations in high-value ornamental areas can also serve as a secondary measure to reduce the risk of secondary transmission.