Pathogenicity of Cytospora Fungi on Six Hardwood Species

Abstract

Cytospora canker is a serious fungal disease of many shade, fruit, and ornamental tree species in the urban forest, orchards, and nurseries. Because Cytospora species are difficult to identify and their host ranges are poorly understood, it is not known if disease occurrence on one host poses a threat to other host species. Cytospora isolates were collected from aspen (Populus tremuloides) (Cytospora chrysosperma), green ash (Fraxinus pennsylvanica) (Cytospora pruinosa), Siberian elm (Ulmus pumila) (Cytospora sacculus), alder (Alnus spp.) (Cytospora umbrina), cottonwood (Populus spp.) (Cytospora chrysosperma), and multi- and singlestemmed willow (Salix spp.) (Cytospora fugax). These isolates were inoculated into drought-stressed aspen, green ash, Siberian elm, thinleaf alder (A. tenuifolia), eastern cottonwood (P. deltoides), and single-stemmed willow. Ash, alder, and elm isolates were host specific. Aspen and cottonwood isolates were pathogenic only on aspen and cottonwood trees. Isolates from multi-stemmed willows caused cankers on aspen only and not single-stemmed willows. However, Cytospora spp. isolates collected from a single-stemmed willow were pathogenic on trees cloned from that willow. Water potential, as a covariate, did not explain variation in canker size among trees. Thus, Cytospora fungi that are host specific may not be a threat to other nearby tree species.