Thousand cankers disease

Geosmithia morbida

1. Identification

Geosmithia morbida Kolařík, Freeland, Utley & Tisserat is the fungus responsible for the disease known internationally as “Thousand Cankers Disease”, or “Doença dos Mil Cancros” in Portuguese. The disease results from the interaction between the fungus and the insect vector Pityophthorus juglandis.

2. Causal agent

An ascomycete fungus of the genus Geosmithia, characterised by rapid growth in culture and the production of cylindrical conidia. It develops in the subcortical tissues, giving rise to multiple cankers that compromise sap circulation.

3. Main hosts

• Walnut trees (Juglans spp.), notably Juglans nigra, the most susceptible species.
• Other species of the genus may be affected, although with less severity.

4. Symptoms

• Small subcortical cankers that multiply along the trunk and branches.
• Yellowing, wilting and loss of vigour of the canopy.
• Drying of branches and progressive decline.
• Presence of galleries of the insect vector under the bark.
• Death of the tree after the accumulation of numerous cankers.

5. Disease cycle

The fungus is introduced into the walnut tissues by the beetle Pityophthorus juglandis during feeding and oviposition. Each attack gives rise to a new canker. The insect disperses the fungus between trees and within the same tree. The progression is cumulative and depends on the population density of the vector.

6. Favourable conditions

• Warm, dry climates favour the activity of the insect vector.
• Trees subject to water or nutritional stress show greater susceptibility.
• Movement of infested wood accelerates dissemination.

7. Disease management

• Prevention: avoid transporting walnut wood originating from infested areas.
• Forest sanitation: removal and destruction of severely affected trees.
• Monitoring: regular inspection for symptoms and the presence of the insect vector.
• Integrated management: reduction of stress factors, maintenance of tree vigour and rigorous phytosanitary surveillance.
• Vector control: dependent on local regulations and authorised methods.

Bibliographic references

• Tisserat, N. et al. (2011). Thousand Cankers Disease of Walnut. Plant Health Progress.
• Kolařík, M. et al. (2011). “Geosmithia morbida sp. nov.” Mycologia.
• Seybold, S. J. et al. (2013). Thousand Cankers Disease: A Diagnostic Guide. USDA Forest Service.
• EPPO. Pest Risk Analysis for Geosmithia morbida and Pityophthorus juglandis.