In June 2000, during a routine examination of native willow (Salix sp.) growing in northwestern Minnesota, several plants were observed with multiple stem cankers. The cankers, some reaching 6 cm in length, were found on 1- to 3-year-old wood. Within the canker, the bark was blistered in several areas, exposing urediniospores averaging 15 × 24 μm. Although all North American willow leaf rusts are now included in the collective species Melampsora epitea, the spores fit the description of M. paradoxa (syn. M. bigelowii) (1). While evidence of previous sporulating cankers was present on older wood, no symptoms of leaf rust were present on the foliage in the area. Similarly, evidence of overwintering of M. bigelowii on young willow stems has been reported in Michigan (2). Willow species and cultivars used in biomass plantings need to be screened for rust resistance to local rust populations as the presence of a leaf rust infecting stems could become damaging to cultivated willow. For this reason, revision of the willow rust species and determination of their distribution and host range should be undertaken.
References: (1) J. C. Arthur. 1962. Manual of the Rusts in United States and Canada, Hafner Publishing, New York. (2) J. R. Weir and E. E. Hubert. Phytopathology 8:55, 1918.