Disease Note.

Identification of Races of Melampsora larici-populina, the Eurasian Poplar Leaf Rust Fungus, on Populus Species in California and Washington. J. Pinon, Laboratoire de Pathologie forestiere, I.N.R.A.-C.R.F., F-54280 Champenoux, France. G. Newcombe and G. A Chastagner, Washington State University, Puyallup 98371-4998. Plant Dis. 78:101. Accepted for publication 22 September 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0101C.

The poplar leaf rust fungus, Melampsora larici-populina Kleb., is native to Eurasia but was first confirmed in North America in 1991 on hybrid poplars along the lower Columbia River in Washington and Oregon (I). Since then, M. larici-populina has been found in 18 counties in California(D. Dilly,personal communication). In order to determine races of M. larici-populina in the two regions, rust-infected leaves were collected from Populus spp. near Moorpark and Riverside, California, and from P. trichocarpa Torr. & A. Gray and P. trichocarpa X P. nigra L. near Woodland, Washington. Then, 106 uredinia from Moorpark, 20 uredinia from Riverside, and 92 uredinia from Woodland were isolated for monouredinial inoculations of leaf disks of a set of poplar differentials used in race determinations in western Europe (2). All uredinia isolated were of M. larici-populina, as judged by urediniospore morphology (1). None of the isolates was virulent on P. X euramericana (Dode) Guinier 'Ogy' and 'Ghoy', P. candicans Ait., and P. trichocarpa X P. deltoides J. Bartram ex Marsh. 'Beaupre', but 93, 90, and 95% of the isolates from Moorpark, Riverside, and Woodland, respectively, were virulent on both P. X euramericana 'Brabantica' and 'Unal'. The remaining isolates were virulent on either Brabantica or Unal or were avirulent. These data indicate that race El predominates in locations sampled in both California and Washington, while three other, unnamed races occur at low frequency. Neither race E2 (virulent on Ogy) nor race E3 (virulent on P. candicans) was found. European races are avirulent on Beaupre and Ghoy, as were the isolates in this study.