Fungicide applications to disinfest the walnut twig beetle (Pityophthorus juglandis) of Geosmithia morbida

The walnut twig beetle, Pityophthorus juglandis (Blackman) causes minor damage to Arizona walnut (Juglans major) in its native range of New Mexico and Arizona.  Recently, the beetle has emerged as a lethal pest of black walnut (Juglans nigra), a species native to the eastern USA and highly valued for its timber and nuts.  The beetle creates galleries in the phloem and carries Geosmithia morbida, a canker-inducing fungus.  This research aims to explore the association between the beetle and the fungus by attempting to create Geosmithia-free populations of the beetle.  Preliminary studies indicated that G. morbida growth was inhibited by greater than 97 percent in potato dextrose agar amended with 10 µg/g or higher concentrations of the fungicide azoxystrobin.  However, regrowth of the fungus was noted immediately following the transfer of the agar plug containing the fungus from the fungicide-amended to non-amended agar.  In contrast the fungus was completely inhibited and killed in agar amended with 0.6 µg/g azoxystrobin and 1.0 µg/g propiconazole (active ingredients in the fungicide Headway).  Current research is determining whether this fungicide combination will be effective in disinfesting live walnut twig beetle adults and larvae of G. morbida.