First Report of Garlic Rust Caused by Puccinia allii on Allium sativum in Minnesota

High-quality garlic is an emerging crop grown in Minnesota for local markets, community supported agriculture, and select restaurants. In July 2010, Allium sativum cv. German Extra Hardy Porcelain plants showing foliar symptoms typical of rust infection were brought to the Plant Disease Clinic at the University of Minnesota by a commercial grower from Fillmore County, Minnesota. Infected leaves showed circular to oblong lesions (1 to 3 mm long), which ranged in color from yellow-orange (uredinia) to black (telia). Urediniospores collected from uredinia were globoid to ellipsoid, yellowish in color, and measured 18 ± 1 × 30 ± 2 μm with a wall thickness of 2.4 ± 0.5 μm. Teliospores were two celled, 18 ± 3 × 47 ± 10 μm, with a projected cross-sectional area (1) of 826 ± 87 μm²; cell walls were smooth, brown, 1.6 ± 0.3 μm (proximal cell) to 2.1 ± 0.5 μm (distal cell) thick, and 4.2 ± 0.8 μm at the apex. The pathogen was identified as Puccinia allii (2) and a sample was deposited in the U.S. National Fungus Collection (BPI 884132). DNA was extracted from infected leaf tissue and the nuclear ribosomal internal transcribed spacer region (ITS) and 5' end of the large subunit (LS) was amplified and sequenced as described by Anikster et al. (1). The 1,257-bp sequence from the sample collected in Minnesota (GenBank Accession No. JX402206) was identical to ITS/LS sequence of a sample of P. allii collected from garlic in California (GenBank Accession No. AF511077), with the exception that MN sequence contained nine "A"s rather than 10 in the hyper-variable area at the 3' end of the ITS region. P. allii has been shown to be a species complex comprising at least two different types, "leek type" and "garlic type" (1). Based on the ITS sequence and the projected cross-sectional area of the teliospores, the sample of P. allii from MN is consistent with the garlic type. Garlic rust occurred in localized foci late in the growing season and therefore did not cause significant loss to the 2010 crop. Reoccurrence of garlic rust was not reported in either 2011 or 2012 growing seasons in Minnesota. P. allii all but eliminated commercial garlic production in California in the late 1990s (1) and has the potential to cause significant negative impact to the emerging garlic crop in Minnesota. However, the epidemiology of garlic rust in the northern U.S. is not well understood and therefore predicting the risk of the Minnesota garlic crop to rust is difficult. References: (1) Y. Anikster et al. Phytopathology 94:569, 2004. (2) L. J. Szabo et al, Rust. Pages 41-44 in: Compendium of Onion and Garlic Diseases and Pests, Second Edition. H. F. Schwartz and S. K. Mohan, eds. APS Press, St. Paul, 2008.