In early August 2006, a disease caused severe losses in a 1,400-ha field of 5-month-old tobacco plants in Kuandian and Fengcheng Counties, Dandong City of Liaoning in northeast China. Symptoms were observed on almost every plant. Disease symptoms were subsequently observed at nearly 100% incidence in 2,000 ha of fields from three towns in Kaiyuan County and two towns in Xifeng County, Tieling City, Liaoning Province in the second half of August 2006. Symptoms first appeared on leaves as small (2 mm) water soaked spots, and developed into expanded, dark brown lesions (2 cm) on the middle to lower leaves. Each lesion exhibited concentric rings, a necrotic center, and a tear in the center and margin that often resulted in a shot-hole appearance. Fungal isolates were obtained from the margins of lesions that were surface-sterilized by dipping each leaf section into 75% ethyl alcohol for 3 sec, then in 0.1% HgCl2 for 15 sec, rinsing in sterilized distilled water three times, and plating the leaf section onto half-strength potato dextrose agar (PDA). Six isolates were identified as Rhizoctonia solani Kühn on the basis of mycelial characteristics: multinucleate cells, septate hyphae constricted at the junction of hyphae, and hyphal branching at approximately right angles (3). The sequence of the internal transcribed spacer (ITS) 1-5.8S-ITS2 region of rDNA from each of six isolates was amplified by PCR assay using universal primers ITS1 and ITS4. The sequences (GenBank Accession Nos. JQ219152 to JQ219157) matched 100% with the ITS sequence of an isolate of R. solani AG-3 (GQ885147). Koch's postulates were conducted for each of the six isolates by wound-inoculating six tobacco leaves (cv. NC89) detached from a total of three 8-week-old plants. Each tobacco leaf was first surface-sterilized in 0.5% NaOCl for 30 sec, rinsed in sterilized distilled water, and wounded at each of four locations by inserting a needle into the leaf. Each leaf was inoculated by depositing a PDA plug (0.5 cm diameter) colonized with R. solani onto each of the four wounds; wounded control leaves (six tobacco leaves from a total of three plants) were inoculated similarly with non-colonized PDA plugs. Inoculated leaves were incubated at 28°C in natural light within a plastic container covered with a hyaline cap to maintain high relative humidity. Symptoms similar to those observed on the original plants developed on inoculated leaves within 3 days, but not on the control leaves. The pathogen was reisolated from symptomatic leaves but not from control leaves and showed morphological characteristics consistent with those of R. solani. Tobacco target spot has been recorded in South America (1), South Africa (4), Argentina, and the USA (2). However, to our knowledge, this is the first report of target spot caused by R. solani AG-3 on flue-cured tobacco in China.
References: (2) J. S. Johnk et al. Phytopathology 83:854, 1993. (4) H. D. Shew et al. Plant Dis. 69:901, 1985. (1) B. Sneh et al. Identification of Rhizoctonia Species. The American Phytopathological Society, St. Paul, MN, 1991. (3) E. Vargas. Turrialba 23:357, 1973.