Thousand Cankers Disease of Walnut (Geosmithia morbida)

Integrated Pest Management Program and Utah Plant Pest Diagnostic Laboratory

PLP-015

January 2025

Claudia Nischwitz, Extension Plant Pathologist • Marion Murray, Extension IPM Specialist

What You Should Know

• Thousand cankers disease is caused by the fungus Geosmithia morbida. • It is transmitted by the walnut twig beetle (Pityophthorus juglandis). • Once symptoms are visible, trees can die within two to three years.

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INTRODUCTION

housand cankers is a disease of walnuts, caused by a fungus (Geosmithia morbida) that is spread by the walnut twig beetle (Pityophthorus juglandis, Fig. 1). The beetle is endemic to the native range of Arizona walnut (Arizona, New Mexico, and Chihuahua, Mexico), and was first identified in Utah in 1988. Widespread black walnut mortality in the early 2000s in Colorado and Utah led to the discovery of the pathogen-vector complex. The name of the disease comes from the numerous necrotic lesions (cankers, Fig. 4) found on the cambium of infected trees (Tisserat et al., 2009). The fungus kills black walnut trees (Fig. 2), often within three years of developing the first symptoms (Cranshaw & Tisserat, 2008).

Fig. 1. Walnut twig beetle (Pityophthorus juglandis).

Fig. 2. 400-year old black walnut killed in northern Utah.

HOSTS Black walnut (Juglans nigra) and black walnut hybrids are very susceptible to Geosmithia. California walnut (J. hindsii, J. californica) and Persian walnuts (J. regia) are slightly susceptible. Cankers do not seem to form on Arizona walnuts (J. major) (Cranshaw & Tisserat, 2008). Thousand Cankers Disease of Walnut

Fig. 3. Exit holes caused by the walnut twig beetle. Page 1

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Thousand Cankers Disease of Walnut (Geosmithia morbida)

Published on Jun 13, 2025

Utah State University Extension

"Thousand cankers is a disease of walnuts, caused by a fungus (Geosmithia morbida) that is spread by the walnut twig beetle (Pityophthorus juglandis, Fig. 1). The beetle is endemic to the native range of Arizona walnut (Arizona, New Mexico, and Chihuahua, Mexico), and was first identified in Utah in 1988. Widespread black walnut mortality in the early 2000s in Colorado and Utah led to the discovery of the pathogen-vector complex. The name of the disease comes from the numerous necrotic lesions (cankers, Fig. 4) found on the cambium of infected trees (Tisserat et al., 2009). The fungus kills black walnut trees (Fig. 2), often within three years of developing the first symptoms (Cranshaw & Tisserat, 2008)."