Wasabi (Wasabia japonica (Miq.) Matsum.), a member of the Brassicaceae family, is a commercially important crop in East Asian countries such as China, Japan, Korea, and Taiwan. In Korea, wasabi is under commercial development since it has become popular as a condiment due to its strong pungent constituents. In May 2013, wasabi plants showing typical symptoms of white blister rust disease were first observed in plastic greenhouses in Taebaek City, Korea. Leaves of infected plants had whitish sori on the lower surfaces and chlorotic blotches on the corresponding upper leaf surfaces. Later, sori changed to creamy to light tan with necrosis of leaf lesion. New infections might occur anytime during the growing season. A representative sample was deposited in the Korea University Herbarium (KUS-F27596). Microscopic examination of fresh materials was performed under a light microscope. The grouped sporangiophores were hyaline, clavate or cylindric, and measured 20 to 35 × 10 to 14 μm. The sporangia were arranged in basipetal chains, hyaline, globose to subglobose, with uniform wall thickness and measured 16 to 21 × 13 to 18 μm. The primary sporangia were morphologically similar to the secondary sporangia, although the former exhibited a slightly thicker wall than the latter. No resting organs were observed. Previously, the white blister rust pathogen on wasabi has been considered either Albugo candida or A. wasabiae, although the latter name is often considered a synonym of A. candida. Based on the morphological characteristics and the specific host plant, the causal agent of this disease was identified as A. candida (2). To confirm this morphological identification, genomic DNA was extracted from infected plant tissue, and the amplification and sequencing of the internal transcribed spacer (ITS) region of rDNA of the Korean specimen were performed using procedures outlined by Choi et al. (1), with oomycete-specific primer set, DC6 and LR0. The resulting 835-bp sequence of the region was deposited in GenBank (Accession No. KF887494). Since this was the first ITS sequence submitted for A. candida on wasabi, comparable data were not available. A comparison with the ITS sequences available in the GenBank database revealed that it is identical to A. candida found on Capsella bursa-pastoris (AF271231), and shows a high similarity of 99% with many A. candida sequences originating from other brassicaceous plants. Therefore, the pathogen found in Korea was confirmed to be A. candida. In Korea, it has been reported that A. candida attacks Brassica juncea, B. campestris subsp. penikensis, and B. napus (3), but to our knowledge this is the first record of A. candida on wasabi (4). The white blister rust caused by A. candida is one of the most devastating diseases of wasabi in Japan and Taiwan where the crop is widely cultivated. On the other hand, in the United States, Canada, and New Zealand, where wasabi is a new crop on a commercial scale, there is no record of this disease. These facts taken together suggest that wasabi white blister rust be not only currently spreading in East Asia, but it also poses a new and serious threat to production of this crop in countries in which it is currently absent.
References: (1) Y. J. Choi et al. Mol. Phylogenet. Evol. 40:400, 2006. (2) Y. J. Choi et al. Fungal Divers. 27:11, 2007. (3) Y. J. Choi et al. Plant Pathol. J. 27: 192, 2011. (4) D. F. Farr and A. Y. Rossman. Fungal Databases. Syst. Mycol. Microbiol. Lab., Online publication, ARS, USDA, Retrieved November 15, 2013.