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First Report of White Blister Rust Caused by Albugo lepidii on Broad Leaf Pepperwort (Lepidium affine) in China

Zhang, S. H., Guo, Q. Y., Yang, H., Cao, Z., Song, J. G., Xu, B.
Plant disease 2018 v.102 no.7 pp. 1463
Albugo, Bayesian theory, DNA, Helianthus annuus, Lepidium latifolium, agricultural land, blister rust, cotton, evolution, greenhouses, herbaria, host specificity, internal transcribed spacers, leaves, oospores, pathogenicity, pathogens, probability, seedlings, sporangia, statistical analysis, stems, weeds, wheat, zoospores, China
Lepidium affine Ledeb. (Brassicaceae), known as broad leaf pepperwort, is a perennial herb as a weed in farmlands distributed in central and northwestern Asia. From May to August of 2016, white blister rust was observed on L. affine in many farming areas cultivated with cotton, sunflower, and wheat in Altay, Xinjiang, China. Approximately 35 to 60% of 300 plants surveyed were affected in this area. The chlorotic spots with white to slightly yellow blister-like sori 2 to 5 mm in diameter were observed mostly on the abaxial surfaces of leaves and also occurred on the adaxial surfaces of leaves, the stems, and the seed heads. Two representative specimens were deposited in the Mycological Herbarium of Tarim University (HMUT 2901 and HMUT 4175). Sporangiophores were hyaline, clavate or cylindrical, straight to slightly curved, and 17.8 to 35.7 × 8.4 to 17.3 µm (n = 30). Sporangia were hyaline, globose or subglobose, 11.9 to 17.2 µm in diameter, and wall 1.5 to 2.5 µm in thickness (n = 35). Oospores were pale yellow to dark brown, globose or subglobose, (32.4 to) 44.6 to 55.7 (to 61.7) µm in diameter, wall 3 to 8 µm in thickness, with rough tubercles 2.4 to 5.2 µm in height (n = 30). The sizes and characteristics of sporangia were consistent with those of Albugo lepidii (Choi et al. 2007; Rector et al. 2016). To confirm the identification, genomic DNA was extracted directly from sori on diseased leaves from isolates HMUT 2901 and HMUT 4175. The fragments of internal transcribed spacer (ITS) and cox2 were amplified and sequenced as described by Xu et al. (2016). BLAST search indicated that the ITS sequences (GenBank accession nos. MG010814 and MG010815) were 99% similar to A. lepidii on L. altifolium (KM068789), L. apetalum (AY929835), and L. sativum (GU292145). It also indicates that the cox2 sequences (accession nos. MG010812 and MG010813) were 99 to 100% similar to A. lepidii on L. altifolium (KM085008), L. apetalum (AY927054), and L. sativum (GU292105). Based on the concatenated ITS and cox2 sequences, the minimum evolution, maximum likelihood, and Bayesian analysis clustered HMUT 2901 and HMUT 4175 with the reference sequences of A. lepidii with high bootstrap support values (99 and 93, respectively) and maximum posterior probability. For pathogenicity tests, sporangia collected from field diseased leaves were suspended in sterile water at 5°C for 8 h to improve zoospore release (Koike 1996). The suspension was sprayed on six healthy seedlings of L. affine at 1.0 × 10⁵ sporangia/ml. Sterile water was sprayed on three healthy seedlings as controls, all plants were incubated in a greenhouse for 15 days at 24°C, and the pathogenicity test was repeated once again. Typical white blister rust symptoms, including sori on the leaf surfaces, appeared on inoculated plants but not on the controls. Based on morphological and molecular evidence, as well as the host specificity, the pathogen on L. affine was identified as A. lepidii. To our knowledge, this is the first report of white blister rust caused by A. lepidii on L. affine in China. The pathogen may be a potential bioherbicide to regulate L. affine.