R. Pan, and
X. Chen, Department of Plant Pathology, South China Agricultural University, Guangzhou 510642, China; and
M. Deng, Plant Protection Research Institute, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, China
In July 2010, a serious disease of peanut (Arachis hypogaea) resembling Cylindrocladium black rot (CBR) was found in Longnan County, Jiangxi Province, China. Symptoms included chlorotic, yellowish and blighted leaves, and wilting of the plants. Taproots and hypocotyls were blackened and rotted. Clusters of reddish orange spherical fruiting bodies appeared in the lesions present on basal stems, pegs, pods, and roots of peanut. Disease incidence reached as much as 50% in some patches of the field. Plants with symptoms were sampled from fields. Microscopic examination revealed that the reddish orange, spherical fruiting bodies were the perithecia and measured 461.6 (337.5 to 609.4) × 395.5 (309.4 to 496.9) μm. With gentle pressure, asci and ascospores were exuded from perithecia. The asci were hyaline, thin walled, and long stalked. Ascospores were hyaline, falcate with one septum, and measured 43.5 (27.3 to 54.5) × 5.6 (4.1 to 6.8) μm with a length/width (L/W) ratio of 7.8 ± 1.3. A fungus with white-to-pale buff border mycelia and yellowish brown pigment was consistently isolated from the edge of basal stem lesions on potato dextrose agar at 25°C. Mycelia grew at temperatures ranging from 8 to 32°C and the optimum was 25 to 26°C. To determine the species, single-conidial isolates of the fungus were cultured on carnation leaf agar for 7 days at 25°C and 12 h of light/dark conditions. Conidia were hyaline, cylindrical with one to three septa (mostly three septa), and measured 49.3 (27.3 to 70.9) × 5.9 (4.1 to 6.8) μm with L/W ratio of 8.4 ± 1.6. Vesicles were globose and measured 5.5 to 10.9 μm in diameter. The fungus was identified as Cylindrocladium parasiticum (teleomorph Calonectria ilicicola) (1,2). A PCR assay was conducted on one representative isolate (JXLN32) by analyzing multilocus sequences of the TUB2 (coding β-tubulin protein), ACT (coding actin), and CaM gene (coding calmodulin protein) and were amplified and sequenced using the primers reported by Crous et al. (3). Sequences of the studied DNA regions were submitted to GenBank (Accession Nos. TUB2: JF429649; ACT: JQ070809; and CaM: JQ070808). BLAST searches with the existing sequences in GenBank showed that there was 99 to 100% identity with the existing sequences of C. ilicicola (GenBank Accession Nos. TUB2: AY725643; ACT: GQ280446; and CaM: GQ267402). To complete Koch's postulates, inoculum was prepared by mixing the microsclerotia (MS) suspension of the isolate (JXLN32) with soil at a proportion of 10 MS per g of soil. Ten replicate plastic pots containing five peanut seeds (cv. Yueyou 7) each were planted and placed in a glasshouse at 25 ± 2°C. The same number of peanut seeds was used as an uninoculated control. Typical basal stem and roots rot symptoms of CBR were observed in 2 months and C. parasiticum was reisolated from these inoculated diseased plants. No symptoms were detected on the control plants. To our knowledge, this is the first finding of Cylindrocladium black rot in Jiangxi Province, which is the main peanut-producing area in China. The disease has been previously reported in Guangdong Province in southern China but is not known elsewhere (4). Because of its ability to spread through seed and soil and its destructive potential, this pathogen may pose a serious threat to peanut production in China.
References: (1) D. K. Bell and E. K. Sobers. Phytopathology 56:1361, 1966. (2) P. W. Crous et al. Mycol. Res. 97:889, 1993. (3) P. W. Crous et al. Stud. Mycol. 50:415, 2004. (4) R. Pan et al. Plant Pathol. 58:1176, 2009.