Dutch elm disease

Ophiostoma spp.

1. Identification

Ophiostoma spp. is a group of pathogenic fungi associated with serious vascular diseases in trees, including the well-known Graphiosis or Elm Wilt Disease (in English, Dutch elm disease). These fungi colonise the conducting vessels, blocking the flow of sap and causing wilting and progressive death.

2. Causal agent

Ascomycete fungi of the genus Ophiostoma, described by H. & P. Sydow, characterised by rapid growth in living wood and the production of conidia in simple structures (synnemata or conidiophores).
The most relevant species include:

• Ophiostoma ulmi
• Ophiostoma novo-ulmi (the most aggressive form)
• Ophiostoma himal-ulmi

3. Main hosts

• Elms (Ulmus spp.) — primary and most affected hosts
• Other broadleaf trees may harbour the fungus or serve as support for the insect vector
• Cut or stored wood may act as a reservoir

4. Symptoms

• Sudden wilting of branches, generally starting in the canopy
• Yellowing and drying of the leaves
• Brown streaks in the xylem vessels when a cross-section is made
• Progressive decline of the tree
• Total death in severe cases or when the infection is systemic

5. Disease cycle

The cycle involves the interaction between the fungus and insect vectors, mainly bark beetles (Scolytinae), such as Scolytus spp.

• The insects transport the fungal spores between trees.
• When they bore into the bark to feed or lay eggs, they introduce the fungus into the vessels.
• The fungus colonises the xylem, blocking the conduction of water.
• The tree reacts by forming tyloses and gums, aggravating the vascular blockage.
• Infested cut wood is an important source of dissemination.

6. Favourable conditions

• High presence of vector beetles
• Trees weakened by water stress, inadequate pruning or mechanical damage
• Stored elm wood without treatment
• Moderate to warm temperatures that favour vector activity

7. Disease management

• Removal and destruction of infected wood to eliminate sources of inoculum
• Control of vectors through traps or regulatory measures
• Sanitary pruning in the early stages of infection
• Maintenance of tree vigour to reduce susceptibility
• Use of tolerant elm varieties, when available
• Regular monitoring in areas with a history of the disease
• Compliance with phytosanitary regulations concerning the transport of wood

Bibliographic references

• Agrios, G. N. (2005). Plant Pathology. Elsevier.
• EPPO. Diagnostic Protocols for Ophiostoma spp.
• Brasier, C. M. (1991). Ophiostoma novo-ulmi: pathogen evolution and spread.
• FAO. Forest Health Management Guidelines.