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First Report of Gray Mold Caused by Botrytis cinerea on Marigold (Tagetes erecta) in Korea

Aktaruzzaman, M., Afroz, T., Kim, B. S., Shin, H. D.
Plant disease 2018 v.102 no.8 pp. 1656
Botrytis cinerea, DNA-directed RNA polymerase, Tagetes erecta, aesthetic value, conidia, conidiophores, culture media, flowers, fungi, genes, glyceraldehyde-3-phosphate dehydrogenase, gray mold, growth chambers, heat shock proteins, internal transcribed spacers, leaves, mycelium, pathogenicity, pathogens, ribosomal DNA, runoff, sclerotia, sodium hypochlorite, stems, teleomorphs, temperature, Korean Peninsula
Marigold (Tagetes erecta, Asteraceae) is an herbaceous flowering plant used for medicinal and ornamental purposes. In September 2017, marigolds had approximately 10 to 15% of the flower heads infected by the gray mold fungus. The disease was widely distributed in Gangneung-Wonju National University, Gangneung, Korea. Water-soaked, brown or gray spots followed by abundant mycelium with conidia appeared on affected flower heads that reduced the aesthetic value of the flowers. Leaves and stems were not affected. Diseased tissue was sampled, and surface sterilized by immersing in 1% sodium hypochlorite (NaOCl) for 1 min, rinsed three times with sterile distilled water, placed on potato dextrose agar (PDA, Difco), and incubated at 20 ± 2°C for 7 days. Conidia from naturally infected flowers were ellipsoidal or ovoid, 6.1 to 8.5 × 5.1 to 9.8 μm (n = 50) and from PDA cultures were 5.1 to 10.5 × 4.3 to 7.2 μm (n = 50). Conidiophores from PDA cultures were straight or flexuous, septate, with an inflated basal cell, brown to light brown, and measured 105 to 425 × 9 to 28 μm (n = 20). After 21 days, the fungus formed black sclerotia ranging from 0.7 to 4.8 × 1.0 to 3.7 mm (n = 20) near the edge of the Petri dish. A representative isolate (MGGM002) was deposited in the Department of Plant Science, Gangneung-Wonju National University. Morphological characteristics were consistent with those of Botrytis cinerea Pers.: Fr. (Ellis 1971). To confirm the identity of the causal fungus, the internal transcribed spacer (ITS) region of rDNA of the isolate was amplified with universal primers ITS1/ITS4 and sequenced. BLAST analysis of the resulting 514-bp nucleotide ITS segment (GenBank accession no. MG744435) showed 100% identity with the sequence of Botryotinia fuckeliana, teleomorph of Botrytis cinerea (KY364366). Additionally, three nuclear protein-coding genes were sequenced: glyceraldehyde-3-phosphate dehydrogenase (G3PDH), heat-shock protein 60 (HSP60), and the DNA-dependent RNA polymerase subunit II (RPB2; Staats et al. 2005). The G3PDH, HSP60, and RPB2 sequences (MG744436, MG744437 and MG744438 respectively) were 99 to 100% identical to those of B. cinerea (KY201464, KX266738, and KY275258). To determine pathogenicity, a conidial suspension (1 × 10⁵ conidia/ml) was collected from 2-week-old cultures on PDA and sprayed onto three potted marigold plants with flowers until run-off. Another three plants, serving as controls, were sprayed with sterile water. Inoculated plants and controls were incubated for 48 h in a growth chamber (20°C, 90 ± 10% RH). After 5 days, water-soaked lesions with conidia developed on inoculated flower heads and leaves whereas control plants remained symptomless. The pathogenicity test was repeated twice with similar results. The pathogen was successfully reisolated from inoculated flowers and confirmed to be B. cinerea by morphology, thus fulfilling Koch’s postulates. Gray mold of T. erecta caused by B. cinerea has been recorded in China, Australia, and the U.S.A. (Farr and Rossman 2018). To our knowledge, this is the first report of gray mold on T. erecta in Korea. According to our field observations, gray mold was active at low temperatures (16 to 24°C) and lesions expanded rapidly in humid conditions. Gray mold developed on the foliage following artificial inoculation and it is likely that it could do so in nature.