Pipturus albidus (Hook. & Arn.) A. Gray or māmaki is a flowering plant species in the Urticaceae (nettles) endemic to the Hawaiian Islands. Māmaki is a forest and agricultural commodity, as well as a traditional medicinal and fiber crop. In August 2013, leaf rust was observed in Kuristown, Hawaii, on 15 māmaki plants. Infected leaves had vein-delimited chlorotic spots on the adaxial surface and yellow to orange uredinia on the abaxial surface. Uredinia were scattered, minute, pulverulent, subepidermal, and dome-shaped with a central pore, consistent with Pucciniastrum. Urediniospores were 16 to 23 × 10 to 14 μm, echinulate, ellipsoid to pyriform, walls hyaline, 0.5 μm thick, contents pale yellow to bright yellow. No teliospores were observed. A voucher specimen was deposited in the U.S. National Fungus Collections (BPI 892695). The only species of Pucciniastrum previously known on Pipturus, Pucciniastrum pipturi Syd. [syn. Uredo pipturi (Syd.) Hirats. f.], has larger urediniospores, 26.5 to 40.0 × 19.5 to 27.5 μm, and is currently reported from Japan and the Philippines (3). The pathogen was identified as Pucciniastrum boehmeriae (Dietel) Syd. & P. Syd., which infects Boehmeria Jacq., also in the Urticaceae, and has urediniospores that are 18 to 27 × 13 to 18 μm and similar in shape (2). DNA was extracted from uredinial lesions and the nuclear ribosomal internal transcribed spacer (ITS2) region and the 5′ end of large subunit (28S) rDNA were amplified and sequenced following the protocol of Aime (1). The resulting fragment (GenBank Accession No. KF711854) was 100% identical to authenticated and vouchered P. boehmeriae ITS2/28S rDNA sequences (AB221449 to AB221451 and AB221391 to AB221393) (4). Sequences from P. pipturi are not available for comparison, but host family, molecular, and morphological data support the identification of the rust as P. boehmeriae, which is found throughout eastern Asia. To our knowledge, this is the first report of P. boehmeriae on māmaki and the first report in Hawaii on any host. Plant health professionals and regulatory officials can utilize this information to establish survey methods and implement appropriate management practices for this rust disease.
References: (1) M. C. Aime. Mycoscience 47:112, 2006. (2) N. Hiratsuka. Revision of Taxonomy of the Pucciniastreae. Kasai Publishing and Printing, Tokyo, 1958. (3) M. Kakishima and T. Kobayashi. Mycoscience 35:125, 1994. (4) Y.-M. Liang et al. Mycoscience 47:137, 2006.