First report of botrytis blight caused by Botrytis cinerea on Cineraria in Korea

In April 2014, hundreds of ready-to-market cineraria [Senecio cruentus (Masson ex L’Her.) DC., Asteraceae] plants, grown in pots for 4 months in plastic greenhouses in Gangneung City, were severely damaged (∼30% incidence) by a disease previously undocumented in Korea. Diseased plants had small to large, brown or dark brown lesions on leaves and flowers. Botrytis sp. was consistently isolated from affected tissues. The pathogen appeared to infect leaves along the margins and occasionally on other parts of the leaf blade and flower petals. The pathogen produced profuse conidia on the surface of dead and dying leaves and flowers, resulting in a moldy gray appearance and greatly reducing the aesthetic and commercial value of the plants. Conidia (n = 30) were one-celled, ellipsoid or ovoid, 5.9 to 10.8 × 5.2 to 8.6 μm on naturally infected leaves, and 6.1 to 10.6 × 5.1 to 7.8 μm on potato dextrose agar (PDA). Monoconidial isolates were obtained by plating conidial suspensions on PDA and selecting single colonies. Incubation of isolates at 20 ± 2°C for 21 days produced blackish, spherical to irregular microsclerotia of 1.2 to 4.0 × 1.2 to 3.0 mm (n = 20). A representative isolate (GM007) was deposited in Gangneung-Wonju National University and used for further studies. Morphological characters were consistent with those of Botrytis cinerea Pers.: Fr. (Ellis 1971). To confirm identification, the internal transcribed spacer (ITS) region of rDNA of the isolate was amplified with primers ITS1/ITS4 and sequenced. BLAST analysis of the resulting 518-bp nucleotide segment (GenBank Accession No. KM840848) showed 100% identity with the sequence of Botryotinia fuckeliana (teleomorph of B. cinerea). For further confirmation, nuclear protein-coding genes (G3PDH and HSP60) were sequenced (Staats et al. 2005). The resulting sequences (DDBJ Accession Nos. LC009697 and LC009698, respectively) showed 100% identity with those of B. fuckeliana (KP120866, KJ937067). To conduct the pathogenicity test, inoculum was prepared by harvesting conidia from 2-week-old cultures on PDA. A conidial suspension (2 × 10⁴ conidia/ml) was sprayed onto healthy leaves of three 3-month-old potted cineraria plants. Three plants were sprayed with sterilized water, serving as controls. Plants were covered with plastic bags for 3 days after inoculation to maintain high relative humidity and were placed in a growth chamber at 20 ± 2°C. The first foliar lesions developed on leaves 4 days after inoculation, whereas control plants remained symptomless. The pathogenicity test was carried out twice with similar results. The pathogen was successfully reisolated from inoculated leaves, fulfilling Koch’s postulates. Association of cineraria with B. cinerea has been reported in the United States and the United Kingdom (Farr & Rossman 2015). To our knowledge, this is the first report of B. cinerea causing Botrytis blight on cineraria in Korea. According to our field observations, leaf lesions expanded rapidly under cool, humid conditions of nonheated greenhouses. Under dry conditions, however, the disease developed slowly or even became quiescent. We suppose that ventilation of the greenhouses may effectively reduce severity of Botrytis blight on cineraria.