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First Report of Fusarium Wilt of Tobacco Caused by Fusarium kyushuense in China

Tobacco (Nicotiana tabacum L.) is a leafy, annual, solanaceous plant grown commercially for its leaves. China accounts for more than 39.6% of total global tobacco production (3). In May 2012, seedlings of tobacco cv. Honghuadajinyuan in a Guiyang tobacco commercial field (Guizhou, China, 26.35° N, 106.42° E) developed symptoms of severe wilting, chlorosis, and stunting. The main stem and taproot exhibited reddish to light brown vascular discoloration; further progression of these symptoms eventually caused mortality of infected seedlings. To isolate the causal agent, necrotic tissues from the symptomatic root were placed on potato dextrose agar (PDA) and incubated at 25°C in darkness. Colonies with white to rose mycelia and red-brown colony colors developed on PDA after 5 days of incubation. Microconidia were abundant, straight or slightly curved, clavate, 0- to 3-septate, and 7.5 to 20.0 × 2.5 to 5.0 μm. Macroconidia were straight or slightly curved, slender, 3- to 5-septate, and 25.0 to 45.0 × 3.3 to 5.0 μm. Based on the observed colony attributes, growth patterns, absence of chlamydospores, micro- and macro-spore attributes (1), and PCR amplification (using primers ITS1/4) combined with translation elongation factor primers (EF1/2) (2), the fungus was identified as F. kyushuense O'Donnell & T. Aoki. Sequence of ITS1-5.8s-ITS2 region of rDNA (GenBank Accession No. JX235957) exactly matched the sequences of F. kyushuense accession AB587020.1 (100% similarity). Analysis of the elongation factor (EF-1alpha) gene of the fungus (JX658565) resulted in a 99% match for F. kyushuense accession AB674297.1. Pathogenicity of the fungus was confirmed by performing Koch's postulate as follows. Pure cultures of the fungus F. kyushuense obtained from symptomatic tissues of tobacco seedlings were grown on PDA for 6 days. Tobacco plants to be used in pathogenicity tests were germinated and grown on potting soils in a plastic container. Additional fertilization was supplied by adding 0.2 g/L of 20-20-20 (N-P-K) in the float water. When seedlings got 6-leaf stage, they were ready for pathogenicity tests. Spores harvested from these culture plates were suspended in sterile distilled water, adjusted to a concentration of 1 × 10⁴ conidia/ml, and inoculated by irrigating 10 ml of the conidia suspension onto roots of each of the 12 tobacco seedlings with 6-leaf stage. A group of 12 seedlings of the same age treated with sterile water served as control. Inoculated seedlings were maintained at 25°C, 100 μE m^–2.s^–1, relative humidity >70%, and 16 h light per day, and monitored for 9 days for symptom development. Seedlings inoculated with conidia developed disease symptoms with roots with vascular discoloration of roots whereas control seedlings remained symptomless. F. kyushuense was reisolated from the symptomatic seedlings 9 days after inoculation. F. kyushuense has also been isolated from rice seeds in China (4), and from diseased wheat in Japan (1). The common tobacco Fusarium disease reported in China was caused by F. oxysporium f. sp. nicotianae. However, to the best of our knowledge, this is the first report of F. kyushuense causing wilt on tobacco in China and the disease must be considered in existing disease management practices.

References: (1) T. Aoki and K. O'Donnell. Mycoscience. 39:1, 1998. (2) D. M. Geiser et al. Eur. J. Plant Pathol. 110:473, 2004. (3) US Census Bureau. Foreign Trade Statistics. Washington DC, 2005. (4) Z. H. Zhao and G. Z. Lu. Mycotaxon. 102:119, 2007.