Elm yellows

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Elm yellows
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Elm Yellows, also known as Elm Phloem Necrosis, is a systemic and often fatal disease affecting elm trees. It is caused by a type of bacteria-like organism known as a phytoplasma, which infects the phloem tissue of the tree, disrupting the transport of nutrients and leading to the characteristic symptoms of the disease.

Pathogen

The causative agent of Elm Yellows is a phytoplasma, a type of mollicute that lacks a cell wall and is obligate to the phloem of plants. Phytoplasmas are transmitted by sap-feeding insects, primarily leafhoppers and planthoppers. The specific phytoplasma associated with Elm Yellows is classified within the 16SrV group.

Symptoms

The symptoms of Elm Yellows can vary depending on the species of elm and environmental conditions, but they generally include:

• Yellowing and wilting of leaves, often starting at the top of the tree and progressing downward.
• Premature leaf drop, leading to a sparse canopy.
• Necrosis of the phloem tissue, which can be observed by peeling back the bark to reveal a butterscotch-colored discoloration.
• In some cases, the production of witches' brooms, which are dense clusters of shoots.

Transmission

Elm Yellows is primarily spread by insect vectors, particularly leafhoppers, which acquire the phytoplasma by feeding on infected trees and subsequently transmit it to healthy trees. The disease can also spread through root grafts between adjacent trees.

Diagnosis

Diagnosis of Elm Yellows can be challenging due to the similarity of its symptoms to other diseases such as Dutch Elm Disease. Laboratory testing is often required to confirm the presence of the phytoplasma, typically through molecular techniques such as polymerase chain reaction (PCR).

Management

Management of Elm Yellows involves a combination of cultural practices and vector control:

• Removal of Infected Trees: Infected trees should be removed and destroyed to prevent the spread of the disease.
• Vector Control: Reducing the population of leafhoppers through the use of insecticides can help limit the spread of the phytoplasma.
• Resistant Varieties: Planting resistant or tolerant varieties of elm can reduce the impact of the disease.

Prevention

Preventive measures include monitoring for symptoms, controlling vector populations, and avoiding the planting of susceptible elm species in areas where the disease is prevalent.

Also see

• Dutch Elm Disease
• Phytoplasma
• Leafhopper
• Plant pathology

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