R.-S. Chen, Department of Biochemical Science and Technology (DBST), National Chiayi University (NCYU), Chiayi 60004, Taiwan;
W.-L. Wang, Department of Horticulture, Chia-Yi Agricultural Experiment Station, TARI, Chiayi 60044, Taiwan;
J.-C. Li and
Y.-Y. Wang, Department of Microbiology and Immunology, NCYU, Chiayi 60004, Taiwan; and
J.-G. Tsay, Department of Nutrition and Health Science, Toko University, Puzih City, Chiayi County 61363, Taiwan
In March of 2008, a leaf scab disease was observed in a papaya (Carica papaya L.) orchard at Guoshing, 24.03°N, 120.51°E, in Nantou County, Taiwan. Infected papayas developed symptoms of numerous, pale green, water-soaked areas, 0.5 to 1.5 mm. Infected leaves gradually turned white to gray on the upper surface and small, circular swellings were observed on the abaxial surface. Lesions may coalesce to cover more than 50% of the leaf, rendering them to fall prematurely. Lesions on the lower surface of the leaves were covered with olive-gray patches of mycelia and abundant conidia. Pieces (~2 × 2 mm) of diseased leaf tissue from margins of individual lesions were surface disinfected in 1% sodium hypochlorite solution for 1 min, rinsed in sterile water, plated on water agar, and incubated at 25°C. After 4 days, mycelium was isolated and transferred to potato dextrose agar (PDA). Five isolates (Cc-5 to Cc-9) were isolated and identified as Cladosporium cladosporioides (Fresen.) de Vries based on the velvety, olive-brown with almost black reverse colony color and dimensions and color of conidia and conidiophores (1). Conidia formed in long branched chains that readily disarticulate, single celled, elliptical to limoniform, 2 to 9 (4.6) × 2 to 3 (2.2) μm. Conidia were pale-to-olive brown and smooth to verruculose. Ramoconidia were 0 to 1 septate, 6 to 14 (9.4) × 2 to 4 (2.7) μm, smooth or sometimes minutely verruculose. Conidiophores were pale-to-olive brown, macro- and micronemateus, smooth or sometimes verruculose, 68 to 244 (141.7) × 3.2 to 4 (3.9) μm. To confirm the identity of the fungus, the internal transcribed spacer (ITS) 1 and 4 regions and mitochondrial small subunit (mtSSU) rDNA were sequenced (GenBank Accession Nos. EU935608 and FJ362555), which had 99% homology to the ITS and mtSSU rDNA of C. cladosporioides (GenBank Accession Nos. EU497957 and AY291273, respectively). Pathogenicity tests were conducted in the greenhouse at 25°C with natural daylight conditions. Fungal isolate Cc-6 was used; it was grown on PDA for 6 days and a spore suspension was made (106 spores/ml). Three papaya seedlings (cv. Horng-Fe) were sprayed with the spore suspension and covered with plastic bags. Control treatments were sprayed with sterile water. After 2 days, the bags were removed. Symptoms developed on all inoculated seedlings 4 days after inoculation. In all cases, the typical scab symptom, pale green, water-soaked areas on the lower leaf surface, were observed. C. cladosporioides was reisolated from inoculated leaves following the procedure used for the original isolation. Control seedlings developed no symptoms. The five isolates are being maintained at the DBST, NCYU, Taiwan. Previously, papaya scab reported in China was caused by C. cariciolum Corda (2), C. caricinum C. F. Zhang et P. K. Chi (3), and C. cladosporioides (4). To our knowledge, this is the first report of C. cladosporioides causing papaya scab in Taiwan.
References: (1) M. B. Ellis. Dematiaceous Hyphomycetes. CMI, Kew, Surrey, England, 1971. (2) H.-H. Peng and Z.-Y. Zhang. J. Yunnan Agric. Univ. 12:23, 1997. (3) C.-F. Zhang. Ph.D. thesis. South China Agricultural University, Guangzhou, P.R.C., 1995. (4) Z. Y. Zhang et al. Flora Fungorum Sinicorum 14:1, 2003.