First Report of Black Stem Caused by Botryosporium longibrachiatum on Sweet Basil in Korea.

Abstract

Sweet basil, Ocimum basilicum L., is cultivated mainly for fresh consumption in Korea. In March 2009, in Icheon, Korea, several dozen plants showing symptoms of black stems were found in an organic farm that used polyethylene tunnels for production. The black stems were usually covered with a fungus that gave the appearance of hoar-frost on the stems, especially when plants were grown under a cool and humid environment. According to the farmer, black stems appear during the winter season of November to March when the tunnels were mostly closed. The relative humidity (RH) during that period was around 100% every night due to poor ventilation. Beginning the middle of April when both sides of the tunnels were open, providing good ventilation, no further disease development was observed. The fungus on the stems had an elongate, upright conidiophore, reaching 5 mm in length. At intervals along its length, the main axis of conidiophores produced lateral fertile branches in acropetal succession. Each lateral branch terminated in a cluster of four or five ampullae. Conidia were hyaline, oval, and 5.5 to 9.5 × 3.5 to 6 μm. The fungus was non-pigmented and colonies on potato dextrose agar were chalk white. Morphological and cultural characteristics of the fungus were consistent with the previous reports of Botryosporium longibrachiatum (Oudem.) Maire (3,4). Voucher specimens (n = 4) were housed at Korea University Herbarium (KUS). An isolate from KUS-F24010 was deposited in the Korean Agricultural Culture Collection (Accession No. KACC44849) and used for molecular analysis and pathogenicity tests. The complete internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced. The resulting sequence of 592 bp was deposited in GenBank (Accession No. JX666334). A BLAST search in GenBank showed that there was no comparable sequence of B. longibrachiatum and thus this was the first ITS sequence for the species submitted in GenBank. To confirm the pathogenicity, colonized mycelial plugs (3 mm in diameter) from 10-day-old PDA cultures were placed onto the stem apices (n = 10) of 2-month-old sweet basil pot plants, which were topped as normally harvested. Control plants were inoculated with uncolonized agar plugs. All plants were incubated at 22 ± 2°C in a humidified chamber with a 12-h photoperiod for 48 h, and then maintained in a greenhouse (22 ± 2°C). Three to four days after inoculation, necrotic lesions developed around the points of inoculation on all stems and expanded downwards, leading to black stems covered with the hoar-frost like fungus after 14 days. B. longibrachiatum was successfully reisolated from all inoculated stems, while control plants remained symptomless. The pathogenicity test was conducted twice with the same result. The association of B. longibrachiatum and sweet basil was previously reported (4). Several other plants including burley tobacco are also reported to be infected by this fungus (1,2). To our knowledge, this is the first etiological report of B. longibrachiatum on sweet basil globally as well as in Korea.