First Report of Leaf Spot Disease Caused by Glomerella magna on Lobelia chinensis in China.

Abstract

Lobelia chinensis is a perennial herbaceous plant in the family Campanulaceae that is native to China, where it grows well in moist to wet soils. It is commonly used as a Chinese herbal medicine. In May 2012, symptoms of leaf spot were observed on leaves of L. chinensis in Nanning, Guangxi Zhuang Autonomous Region, China. The leaf lesions began as small, water-soaked, pale greenish to grayish spots, which enlarged to gray to pale yellowish spots, 4 to 6 mm in diameter. At later stages, numerous acervuli appeared on the lesions. Acervuli were mostly epiphyllous, and 40 to 196 μm in diameter. On potato dextrose agar (PDA), a fungus was consistently recovered from symptomatic leaf samples, with a 93% isolation rate from 60 leaf pieces that were surface sterilized in 75% ethanol for 30 s and then in 0.1% mercuric chloride for 45 s. Three single-spore isolates were used to evaluate cultural and morphological characteristics of the pathogen. Setae were two to three septate, dark brown at the base, acicular, and up to 90 μm long. Conidia were long oblong-elliptical, guttulate, hyaline, and 11 to 20 × 4.1 to 6.3 μm (mean 15.2 × 5.1 μm). These morphological characteristics of the fungus were consistent with the description of Colletotrichum magna (teleomorph Glomerella magna Jenkins & Winstead) (1). The rDNA internal transcribed spacer (ITS) region of one isolate, LC-1, was sequenced (GenBank Accession No. KC815123), and it showed 100% identity to G. magna, GenBank HM163187.1, an isolate from Brazil cultured from papaya (2). Although KC815123 was identified as G. magna, it shows 99% identity to GenBank sequences from isolates of C. magna, and more research is needed to elucidate the relationships between these taxa, especially with consideration to host specificity. Pathogenicity tests were performed with each of the three isolates by spraying conidial suspensions (1 × 106 conidia/ml) containing 0.1% Tween 20 onto the surfaces of leaves of 30-day-old and 6- to 8-cm-high plants. For each isolate, 30 leaves from five replicate plants were treated. Control plants were treated with sterilized water containing 0.1% Tween 20. All plants were incubated for 36 h at 25°C and 90% relative humidity in an artificial climate chamber, and then moved into a greenhouse. Seven days after inoculation, gray spots typical of field symptoms were observed on all inoculated leaves, but no symptoms were seen on water-treated control plants. Koch's postulates were fulfilled by reisolation of G. magna from diseased leaves. To our knowledge, this is the first report of G. magna infecting L. chinensis worldwide.