First Report of Leaf Spot on Tibouchina semidecandra Caused by Beltrania rhombica in China
Authors: Shi Z. R. , Xiang M. M. , Zhang Y. X. , Huang J. H. 
Published: June 2012

Tibouchina semidecandra Cogn. is a popular ornamental plant in tropical and subtropical areas (1). In August 2011, a leaf spot was observed on approximately 70% of 5,000 potted plants of T. semidecandra in a nursery in Zhongshan, Guangdong Province, China. Each leaf spot was round with a brown center surrounded by a reddish brown border, and ranged from 8 to 10 mm in diameter. A fungus was isolated consistently from the lesions by surface-sterilizing symptomatic leaf sections (each 3 cm2) with 75% alcohol for 8 s, washing the sections with sterile water, soaking the sections in 3% NaOCl for 15 s, rinsing the sections with sterile water three times, and then placing the sections on potato dextrose agar (PDA) at 28°C. Each of three single-spore isolates on PDA produced gray, floccose colonies that reached 70 mm in diameter after 5 days at 28°C. Setae were dark brown, straight, erect, distantly and inconspicuously septate, and 125 to 193 × 3.0 to 4.5 μm. Conidiophores were light brown, cylindrical, simple or sometimes branched at the base, and 105 to 202 × 3 to 5 μm. Separating cells were hyaline, oval, and 12 to 13 × 4 to 5 μm. Conidia were unequally biconic, unicellular, dark brown with a pale brown or subhyaline band just above the widest part, and 26 to 31 × 8.5 to 12 μm (mean 27.3 × 10.6 μm) with a conspicuous appendage at the apex that was 6 to 14 × 1 to 1.8 μm. These characteristics were consistent with the description of Beltrania rhombica Penz. (3). The internal transcribed spacer (ITS) region of the ribosomal DNA (rDNA) of one isolate (GenBank Accession No. JN853777) was amplified using primers ITS4 and ITS5 (4) and sequenced. A BLAST search in GenBank revealed 97% similarity to the ITS sequence of an isolate of B. rhombica (GU797390.1). To confirm pathogenicity of the isolate, ten detached leaves from 3-month-old plants of T. semidecandra ‘Purple Glorybush’ were inoculated in vitro with 5-mm diameter, colonized mycelial plugs from the periphery of 5-day-old cultures of the isolated fungus. The agar plugs were put on the leaf surface and secured with sterile, moist cotton. Sterile PDA plugs were similarly used as the control treatment on ten detached leaves. Leaves were placed in petri dishes and incubated in a growth chamber with 12 h of light/day at 28°C. Necrotic lesions appeared on leaves after 2 to 3 days of incubation, whereas control leaves inoculated with sterile PDA plugs remained asymptomatic. B. rhombica was consistently reisolated from the lesions using the same method described above, but was not reisolated from the control leaves. Although there are approximately 77 reported hosts of B. rhombica (2), to our knowledge, this is the first report of B. rhombica causing a leaf spot on T. semidecandra. Because the disease caused foliar damage and reduced the ornamental value of the nursery plants, control measures may need to be implemented for this species in nurseries.

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