Cotton (Gossypium hirsutum L.) is one of the most important economic crops in China and fungal diseases are the major limiting factors in its production. In September 2013, cotton plants infected with leaf spots were observed in Sanya, Hainan Province, China. Initial symptoms developed as brick-red dots that led to the formation of irregular to circular lesions with gray centers surrounded by brown borders. Individual leaf spots formed concentric rings of alternating light and dark brown bands. Leaf tissue segments collected from the border between symptomatic and healthy tissue were surface disinfested in 75% ethanol for 1 min, then rinsed three times in sterile water with streptomycin sulfate. Fungal isolates obtained from these segments were purified by the single spore technique on potato dextrose agar (PDA) at 28°C. The initial color of the colonies was olivaceous, turning dark brown after 5 days. Conidiophores were scattered or clustered, brown, straight to curved, unbranched, and glabrous. Conidia had 4 to 12 pseudosepta and were 56 to 230 μm long and 5 to 15 μm wide, brown, straight to slightly curved, obclavate to cylindrical, glabrous, and apex obtuse. These characteristics were consistent with the description of Corynespora cassiicola (Berk. & M.A. Curtis.) C.T. Wei (3). A pathogenicity test was conducted with the four isolates on detached young cotton leaves (two to four true leaf stage). For each isolate, three slightly wounded and three unwounded leaves were inoculated with 5.5-mm-diameter mycelial plugs. For the control treatment, wounded and unwounded leaves were mock inoculated with sterile PDA plugs of the same size. The inoculated leaves were placed in a moist chamber and incubated with a 12-h photoperiod at 28°C. Necrotic lesions appeared on wounded spots after 2 days of incubation and on unwounded leaves 3 days after incubation. All symptoms were similar to those observed in the field. Symptoms were not observed on control leaves. The same fungus was always re-isolated from the diseased tissue according to Koch's postulates. To confirm the identity of the pathogen, DNA was extracted from a 1-week-old culture grown on PDA and the internal transcribed spacer region (ITS) of one isolate (GenBank Accession No. KF924624) was amplified using primers ITS1 and ITS4 (4) and sequenced. BLAST search in GenBank revealed 100% homology with sequences of C. cassiicola (EU364535.1, EU364536.1, FJ852574.1, and FJ852575.1). Based on the symptoms, fungal morphology, ITS sequence comparison, and pathogenicity test, this fungus was identified as C. cassiicola. Target spot of cotton associated with C. cassiicola has been reported in Georgia (2) and Alabama (1). To our knowledge, this is the first report that C. cassiicola can infect cotton in China inducing target spot of cotton (2). This report will establish a foundation for further study of C. cassiicola to aid disease measurement and control.
References: (1) K. N. Conner et al. Plant Dis. 97:1379, 2013. (2) A. M. Fulmer et al. Plant Dis. 96:1066, 2012. (3) J. Y. Lu. Page 407 in: Plant Pathogenic Mycology. China Agricultural Press, Beijing, 2000. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990.