Leaf rust symptoms have been noticed sporadically on northern highbush blueberry plants (Vaccinium corymbosum L.) in Michigan for the past 8 years. In 2009, leaf rust was seen in several cultivated blueberry fields and on greenhouse-grown blueberry plants in southwest Michigan. In 2010, leaf rust was widespread throughout western Michigan and particularly evident in the fall, sometimes resulting in premature defoliation. Cultivars Rubel, Jersey, Elliott, Liberty, and Brigitta were most commonly affected. Both the 2009 and 2010 growing seasons were characterized by above-average precipitation in early to mid-summer. Early symptoms on the adaxial leaf surface consisted of roughly circular yellow spots that later developed brown, necrotic centers. Older lesions were more angular and sometimes surrounded by a purplish border. In the fall, a “green island” effect was sometimes apparent around the lesions. On the abaxial side, numerous yellow-to-orange rust pustules (uredinia) were visible. Uredinia were dome shaped, erumpent, 100 to 400 μm in diameter, clustered, and sometimes coalescing. Urediniospores were broadly obovate with dark yellowish content and measured 19 to 25 × 16 to 20 μm (average 22 × 18 μm, n = 30). Spore walls were hyaline, echinulate, and 1.0 to 1.5 μm thick with obscure germ pores. Uredinia were examined with light and scanning electron microscopy for the presence of conspicuous ostiolar cells characteristic of Naohidemyces vaccinii (Wint.) Sato, Katsuya et Y. Hiratsuka, but none were observed. No telia or teliospores were observed. On the basis of morphology, the pathogen was identified as Thekopsora minima P. Syd. & Syd. (3,4) and a sample was deposited in the U.S. National Fungus Collection (BPI 881107). Genomic DNA was extracted from urediniospores of rust isolates from six different locations, and a 267-bp fragment of the ITS2 region was amplified and sequenced using the primers ITS3 and ITS4 (GenBank Accession No. HQ661383). All sequences were identical to each other and shared 99% identity (232 of 234 bp) with a T. minima isolate from South Africa (GenBank Accession No. GU355675). The alternate host, hemlock (mostly Tsuga canadensis L.) is a common and valuable conifer in the Michigan landscape. Hemlock trees were not examined for the presence of aecia but are assumed to play a role in the epidemiology of the disease in Michigan because leaf rust tends to be more severe near hemlock trees. Pucciniastrum vaccinii (G. Wint.) Jorst. was considered the causal agent of blueberry leaf rust until Sato et al. (1,4) identified three unique species. While T. minima has been reported on black huckleberry (Gaylussacia baccata [Wangenh.] K. Koch) in Michigan (4), to our knowledge, this is the first report of T. minima on highbush blueberry in the state. T. minima has been reported on highbush blueberry in Delaware and New York (4), Japan (2), and South Africa (3). The severity of the outbreak in 2010 warrants further research into economic losses, epidemiology, and management of the disease.
References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Botany and Mycology Laboratory, ARS, UDSA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, 2010. (2) T. Kobayashi. Page 1227 in: Index of Fungi Inhabiting Woody Plants in Japan. Host, Distribution and Literature. Zenkoku-Noson-Kyoiku Kyokai Publishing Co., Tokyo, 2007. (3) L. Mostert et al. Plant Dis. 94:478, 2010. (4) S. Sato et al. Trans. Mycol. Soc. Jpn. 34:47, 1993.
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