Plants in the genus Malus Mill. are grown in temperate regions for fruit crops such as apple and ornamental landscape plants such as flowering crab apple. Gymnosporangium yamadae Miyabe ex G. Yamada, cause of Japanese apple rust, is known to attack economically important species of Malus in Asia. In August 2004 and July 2008, the aecial stage of a rust fungus was observed in Wilmington, DE and nearby in Media, PA on leaves of toringo crab apple (Malus toringo (Siebold) Siebold ex de Vriese), a cultivated plant native to Asia. On the basis of morphological and molecular characteristics, the fungus was identified as G. yamadae. This pathogen has not been previously reported in North America. The identification was confirmed by morphological comparisons with specimens of G. yamadae from Asia and descriptions (1--3) as well as using the D1/D2 domain of 28S rDNA sequence data G. clavariiforme (GenBank Accession No. AR426211), G. clavipes (GenBank Accession No. DQ354545), G. cornutum (GenBank Accession No. AF426210), G. juniperi-virginianae (GenBank Accession Nos. AF522167, AY629316, and DQ354547), G. libocedri (GenBank Accession No. AF522168), G. sabinae (GenBank Accession Nos. AF426209 and AY512845) and G. yamadae (GenBank Accession Nos. FJ559373 and FJ559375). The specimens from North America included aecia of G. yamadae that are foliicolous, hypophyllous, roestelioid, and 4 to 7 mm high. The peridia are yellow-brown to brown and cornute to tubular with a closed brown tip at the apex and lacerate sides; the peridial remains often form a reticulate pattern. The peridial cells are long-narrow rhomboid, 83 to 120 μm long with smooth outer walls and verrucose to echinulate inner and side walls. The aeciospores are globose, 18 to 28 × 19 to 29 μm, with a slightly coronate surface and brown-yellow walls 2 to 3 μm thick. The telia known on Juniperus spp. were not observed. The specimens were deposited in the U.S. National Fungus Collections (BPI 878846, BPI 878847, BPI 878848, and BPI 878849). The 28S rDNA sequence of G. yamadae from BPI 878849 was deposited in GenBank as Accession No. FJ455091. Aecia of G. juniperi-virginianae, cause of cedar apple rust, differ from G. yamadae in having recurved peridial walls at maturity.
References: (1) F. D. Kern. A Revised Taxonomic Account of Gymnosporangium. Pennsylvania State University Press, University Park, PA, 1973. (2) S. K. Lee and M. Kakishima. Mycoscience 40:109, 1999. (3) S. K. Lee and M. Kakishima. Mycoscience 40:121. 1999.