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First Report of Castor Leaf Spot Caused by Cladosporium tenuissimum in Zhanjiang, China
- Liu, Y. L., Yin, X. G., Lu, J. N., Li, Y., Zhou, Y. H.
- Plant disease 2019 v.103 no.2 pp. 375
- Cladosporium, DNA primers, Ricinus communis, bananas, common names, conidia, conidiophores, cotton, culture media, disease control, greenhouses, hyphae, internal transcribed spacers, leaf blight, leaf spot, leaves, loci, oil crops, pathogenicity, pathogens, relative humidity, ribosomal DNA, temperature, watermelons, China
- Ricinus communis L., common name castor, is an important oil crop (Liu et al. 2016). Leaf spots on castor with disease incidence of about 20.5% have been observed in about 1,800 plants in the field (0.66 ha) located in Zhanjiang (21.17 N, 110.18 E), China, since 2016. Early symptoms appeared on affected leaves as light brown or gray necrotic spots. Successively, a mold appeared on both sides of the spots. The spots were irregular in shape and increased in size and coalesced. Seven samples of symptomatic leaves were collected in the field, and they were washed and surface disinfested before isolation. Potato dextrose agar (PDA) was used to isolate the pathogen. Successively, pure cultures were obtained by transferring hyphal tips on PDA. Thirteen isolates were obtained from affected leaves. Two monospore isolates (RiK-1 and RiK-2) were obtained from the isolates, with each isolate being identical based on preliminary molecular analyses by BLASTn. Therefore, the representative isolate (RiK-1) with typical morphology and molecular characteristics was selected for further study. The isolate RiK-1 was grown on PDA and formed colonies of approximately 3 cm (diameter) in 7 days, at 25 ± 1°C. Colonies were olive green and appeared velvety. Conidiophores were up to 500 or 700 µm long, thick walled, septate, and unbranched or rarely branched. Ramoconidia were cylindrical to clavate, slightly inflated at the apex, and bearing two to three flat, thickened scars. Conidia were single celled, lemon shaped, and smooth walled, 2 to 5 µm (diameter), up to 9 µm long. They formed fragile chains of up to six conidia. Morphological characteristics of colonies of isolate RiK-1 were consistent with the description provided for Cladosporium tenuissimum (Bensch et al. 2010). Molecular identification was performed by sequencing of the nuclear ITS-rDNA region using primers ITS1 and ITS4. The sequence of isolate RiK-1 was deposited in GenBank (accession no. MH383051). A BLASTn search of the sequence showed 99 to 100% identity with C. tenuissimum (MF473305.1) and C. oxysporum (KX258798.1). Isolate RiK-1 was closer to C. tenuissimum, which is characterized by nodulose conidiophores with a head-like swollen apex, different from C. oxysporum, which is characterized by nodulose to nodose conidiophores with conidiogenous loci restricted to swellings. Therefore, isolate RiK-1 was identified as C. tenuissimum. A pathogenicity test was performed in vivo, in the greenhouse with a relative humidity of 80%, at temperatures ranging from 24 to 30°C. Two methods (wound inoculation and no wound inoculation) were used on leaflets. Sterile cotton ball was dipped in a spore suspension (1 × 10⁶ conidia/ml) or in sterilized water (as controls) for about 1 min. One leaflet was covered by a cotton ball and was kept for 48 h (four leaflets for each plant, three plants for each method). The first symptoms were observed after 1 month on treatments, whereas controls remained healthy. Pathogen reisolated from diseased leaflets was identical to the inoculated isolates by morphology and 18S rDNA analysis. It has been reported that C. tenuissimum caused leaf spot of banana (Pandey and Gupta 1983) and leaf blight in watermelon (Narain et al. 1985). The disease was not previously reported in castor. Thus, this is the first report of C. tenuissimum causing leaf spot in castor. This study will provide an important reference for the control of the disease. The epidemiology of C. tenuissimum on R. communis should be investigated further.