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First Report of Onion Bulb Rot Caused by Botrytis aclada in China

During the spring of 2006, onion bulbs with gray mold symptoms on the surface were observed in a few supermarkets in Wuhan, China. Onions mummified as they decayed. Further surveys of five randomly selected batches of onion bulbs in one of the supermarkets indicated that the disease occurred in all batches and the disease incidence ranged from 6 to 50%. Eight diseased onion bulbs were collected arbitrarily and isolations were made using homemade potato dextrose agar (PDA). Single-spore cultures of the isolated Botrytis sp. were established and maintained on PDA plates at 20°C. The 10-day-old PDA cultures of all of these isolates were gray and covered with abundant beige, ovoid- or oblong-shaped conidia, which were budded from terminal ampullae formed on dichotomously branching conidiophores. Conidia from these isolates measured 7.6 to 10.4 μm long and 4.2 to 5.6 μm wide, with an average of 8.4 × 5.0 μm. No sclerotia were produced from any of these PDA cultures after incubation at 20°C for 30 days. Morphological characteristics of colonies and conidia of these isolates were similar to Botrytis aclada according to the description made by Yohalem et al. (3). Inoculation of healthy onion bulbs with one of the eight fungal strains, OnionBc-15, resulted in gray mold symptoms similar to those observed in the supermarkets. Microscopic examinations showed that the size and shape of conidia that formed on the surface of diseased bulbs of onion were identical to the size and shape of conidia of OnionBc-15, indicating that this isolate can cause onion bulb rot. The isolate OnionBc-15 was further characterized by molecular techniques. Genomic DNA was extracted from mycelia of this strain and used as a template for amplification of two previously reported DNA regions, the internal transcribed spacer (ITS) region of the ribosomal RNA genes and the L45-550 sequence (1), which can be used to distinguish B. aclada and two closely related species, B. allii and B. byssoidea (3). Universal primers ITS1 and ITS4 were used to amplify the ITS region (2). A 539-bp DNA sequence was generated, cloned, and sequenced (GenBank Accession No. EU093077). The sequence contained two SphI restriction sites and was 99% identical in nucleotides to that of B. aclada strain PRI006 (GenBank Accession No. AJ716295). It is different from B. allii and B. byssoidea, which have only one SphI restriction site for the ITS1/ITS4-amplified DNA sequence (2). The Botrytis-specific primers, BA2f and BA1r, were used to amplify the L45-550 sequence (2). A 413-bp DNA sequence was generated, cloned, and sequenced. The sequence did not contain any ApoI restriction sites. This is also similar to B. aclada, but different from B. allii and B. byssoidea, which contains one ApoI restriction site in the BA2f/BA1r-amplified DNA sequence (2,3). On the basis of morphological characteristics and the two molecular features, it is concluded that the isolate OnionBc-15 belongs to B. aclada. To our knowledge, this is the first report on the occurrence of B. aclada causing onion bulb rot in China.

References: (1) K. Nielsen and D. S. Yohalem. Mycologia 93:264, 2001. (2) K. Nielsen et al. Plant Dis. 86:682, 2002. (3) D. S. Yohalem et al. Mycotaxon 85:175, 2003.