Tobacco (Nicotiana tabacum L.) is a leafy, annual, solanaceous plant grown commercially for its leaves. Guizhou Province produces more than 30% of the total Chinese tobacco crop. In July 2010, a disease was observed in a commercial field of 5-month-old N. tabacum plants in Bijie, Guizhou in southwestern China. Symptoms first appeared on the leaves as small spots that later increased in size and developed into expanded, dark brown lesions covered with green-gray spore masses. Lesions expanded rapidly under cool, humid conditions. Isolates of Botrytis cinerea were collected from diseased leaves with typical symptoms. Diseased leaf samples were washed with distilled water three times, placed in a moist chamber, and incubated at 25°C in darkness for 48 h to encourage sporulation. Spores produced on leaves were transferred to individual agar discs (5 mm in diameter) with an inoculating needle and then the agar discs were transferred to potato dextrose agar (PDA) and incubated at 25°C for 7 days. Fungal colonies were at first colorless and later became gray to brown when the conidia differentiated. The size of conidia ranged from 5.0 to 9.5 × 6.5 to 12.5 μm (average 7.3 × 8.7 μm) based on 50 spore measurements. Microsclerotia produced in the culture were round or irregular and ranged from 1.2 to 3.0 × 1.0 to 2.5 mm (average 2.1 × 2.0 mm). The pathogen was identified as B. cinerea Pers.:Fr on the basis of morphology and sequence of ITS1-5.8s-ITS2 region of rDNA amplified by PCR using universal primers ITS-1 (5′-TCCGTAGGTGAACCTGCGG-3′) and ITS-4 (5′-TCCTCCGCTTATTGATATGC-3′). The sequence (GenBank Accession No. HQ902163) exactly matched the sequences of two Botryotinia fuckeliana (anamorph B. cinerea) accessions, (e.g., GenBank Accession Nos. HM849615.1 and HM849047.1). Koch's postulates were conducted by wound inoculating five tobacco leaves (cv. K326) after surface disinfesting them with 5% NaOCl. Plugs of the fungus (5 mm in diameter) obtained from the colony margins were transferred onto 3 × 3 mm wounds made with a needle on the surface of five sterilized leaves. Inoculated leaves were incubated at 25°C, 100 to 120 μE·m–2·s–1, relative humidity >80%, and 16 h light per day for disease development. Typical symptoms developed on leaves within 7 days after inoculation. The pathogen was reisolated from affected leaves but not from the noninoculated control leaves. Botrytis gray mold blight has been recorded on N. tabacum in New Zealand, the United Kingdom, and northern China (1–3). However, to our knowledge, this is the first report of Botrytis blight on N. tabacum in Guizhou Province of China and the disease must be considered in existing disease management practices.
References: (1) W. Brian et al. Mol. Plant Pathol. 8:561, 2007. (2) A. G. Mcleod et al. N. Z. J. Crop Hortic. Sci/Exp. Agric. 12:866, 1958. (3) Z. Y. Zhang. Page 37 in: Flora Fungorum Sinicorum. Vol. 26. Science Press, Beijing, 2006.