In the spring of each year from 2007 to 2009, a leaf blight of garlic (Allium sativum L.) was observed in more than 50 fields in Zhushan County of Hubei Province, China. Gray mold was observed on many of the blighted garlic leaves. The percentage of garlic plants with blight and gray mold symptoms ranged from 10 to 50% with one to three blighted leaves on each plant, which severely reduced the yield of young garlic plants (produced as a green vegetable). Ten strains of a Botrytis sp. were isolated from symptomatic garlic leaves collected from 10 different fields. These strains were inoculated onto potato dextrose agar (PDA) in petri dishes and incubated at 20°C for 3 to 15 days for observation of colony characteristics and morphology of sclerotia and conidia. All 10 Botrytis strains formed flat and “ropy” mycelia (mycelial strands) on PDA. Abundant sporulation with a gray powdery appearance was observed on the colonies after 6 days. Conidiophores were erect with alternate branches at the top and ranged from 907 to 1,256 μm high. Conidia were borne in botryose clusters on conidiophores, obovate, and 10.4 to 17.6 × 7.6 to 13.1 μm with an average length/width ratio of 1.36. Discrete sclerotia were produced on each colony after 15 days. Mature sclerotia were black, cerebriform and convoluted, and 1.9 to 9.1 × 1.6 to 6.5 mm. Morphological characteristics of the colonies, conidia, and sclerotia of these Botrytis strains were similar to Botrytis porri Buchwald (1,2). Strain GarlicBC-16 was selected as a representative for molecular identification. Genomic DNA was extracted from mycelia of this strain and used as a template for amplification of the internal transcribed spacer (ITS) region of rDNA using primer pair ITS1/ITS4. A 539-bp amplicon was obtained and sequenced (GenBank Accession No. EU519206). Excluding the flanking regions, the amplicon contained a 453-bp ITS sequence (ITS1 + 5.8S rDNA + ITS2) 100% identical to the ITS sequence of strain MUCL3234 of B. porri (GenBank Accession No. AJ716292). Pathogenicity of strain GarlicBC-16 was tested by inoculation of 10 young and fully expanded garlic leaves taken from 100-day-old garlic plants with mycelial agar plugs (three plugs per leaf and spaced by 5 cm). Ten garlic leaves inoculated with agar plugs of PDA alone served as controls. Inoculated garlic leaves were covered with a plastic film (0.1 mm thick; Gold Mine Plastic Industrial Ltd. Jiangmen, China) and incubated at 20°C with 12-h light/12-h dark. Control leaves remained healthy after 48 to 120 h, but gray, water-soaked lesions appeared on leaves inoculated with strain GarlicBC-16 after 48 h. The average lesion length reached 27.3 mm after 90 h and abundant sporulation was produced on necrotic leaf lesions after 120 h. Microscopic examination showed the shape and size of conidia that formed on garlic leaf lesions were similar to those formed by strain GarlicBC-16 on PDA. On the basis of the isolation, identification, and pathogenicity tests, B. porri was determined to be the causal agent of garlic leaf blight in Zhushan County. B. porri has been reported to cause neck rot of leek (A. porrum) (1) and clove rot of garlic (2), and has been isolated from asymptomatic foliage and seeds of A. cepa (3). To our knowledge, this is the first report of garlic leaf blight caused by B. porri in China.
References: (1) S. K. Asiedu et al. Plant Dis. 70:259, 1986. (2) F. M. Dugan et al. J. Phytopathol. 155:437. 2007. (3) L. J. du Toit et al. Plant Dis. 86:1178, 2002.