Kalanchoe (Kalanchoe blossfeldiana Poelln.) is widely cultivated in Korea as an ornamental houseplant and succulent garden plant because of its ease of propagation, low water requirements, and wide variety of flower colors. In August 2010, suspected nursery-stage kalanchoe leaf scorch was found at a grower's greenhouses located in Gimhae, Korea. In some greenhouses, 20 to 30%, and occasionally as much as 50%, of the plants were affected. Symptoms on kalanchoe include browning of the leaf margins and yellowing or darkening of tissues between the main leaf veins. As the disease progresses, affected leaves dried up, turned brown, and became brittle. A velvety, blackish olive mold formed on the surface of the dead tissue, followed by plant defoliation. Fresh leaf specimens were collected from infected plants and the causal pathogen was purified with a single-spore isolation technique and transferred onto potato dextrose agar (PDA). Colonies on PDA developed a gray or grayish brown, hairy, velvety mycelium that was mostly immersed and also formed conidia. Conidia were pale to mid brown, oblong, smooth or verruculose, with three to five transverse and one to two longitudinal septa in two to three transverse divisions, and 32 to 55 × 11 to 18 μm. Conidiophores were pale to mid brown, solitary or in fascicles, unbranched or occasionally branched, straight or flexuous, more or less cylindrical but enlarged slightly at one to three apical percurrent proliferations, septate, and 80 to 300 × 2 to 5 μm. A representative isolate of the pathogen was inoculated on kalanchoe leaves for pathogenicity testing. Cultures grown on PDA were flooded with sterile distilled water and after rubbing with an artist's paintbrush with hair bristles, the resulting suspensions were filtered through sterile cheesecloth. Conidial suspensions were adjusted to 2.5 × 104 conidia/ml with sterile distilled water. The leaves of five 1-month-old potted plants were wounded by applying pressure with forceps having serrated teeth, bruising the tissue. Wounded plants were sprayed with a conidial suspension until runoff. Five plants sprayed with sterile distilled water served as controls. The plants were maintained for 48 h at 25°C in a humidity chamber with 100% relative humidity and were then moved to a greenhouse. Symptoms similar to those observed in the farmer's greenhouse developed on wounded leaves within 9 days. The causal pathogen was reisolated from the lesions to prove Koch's postulates. To confirm the identity of the fungus, the complete internal transcribed spacer (ITS) rDNA and glyceraldehyde 3-phosphate dehydrogenase (gpd) gene were amplified and sequenced (1). Amplification of the ITS region generated a 579-bp sequence (GenBank Accession No. HQ840713) and gpd was 558 bp (GenBank Accession No. JF776462). The ITS and gpd sequences were 100% similar to the sequences of Stemphylium xanthosomatis (GenBank Accession Nos. AF442804 and AF443903, respectively). On the basis of symptoms, mycological characteristics, pathogenicity, and molecular data, this fungus was identified as S. xanthosomatis. The type culture of the fungus is stored at the Korean Agricultural Culture Collection (KACC 45812), National Academy of Agricultural Science, Korea. To our knowledge, this is the first report of leaf scorch caused by S. xanthosomatis on kalanchoe in Korea.
Reference: (1) M. P. S. Câmara et al. Mycologia 94:660, 2002.