Each autumn of 2011 to 2013, a disease was observed on corn (cv. Huameitian 8) in the Experimental Farm of South China Agricultural University, Guangzhou, Guangdong Province, with an incidence ranging from 20 to 65%. Initial symptoms appeared as small, water-soaked lesions on the leaves. Under warm and humid conditions, lesions expanded along the veins producing necrotic stripes. Stripes later became dry and brown, often with shredding of the infected tissue. Bacterial ooze was observed coming out of sections of leaves showing stripe symptoms. Diseased leaf tissues were surface disinfected in 75% ethanol for 1 min followed by three rinses with sterile distilled water. Bacteria were isolated from symptomatic tissues. Colonies were creamy white, circular, convex, smooth with entire margins on nutrient agar, and non-fluorescent and slow-growing on King's medium B. The strains were gram-negative, rod-shaped with a unidirectional flagellum, aerobic, positive for gelatin liquefaction and catalase, amylase activity, and negative for nitrate reduction. The strains utilized mannose, fructose, and citrate, but not lactose and maltose. 16S rDNA genes of two strains, GDHN01 (Accession No. JQ904301) and GDHN02 (JQ904302), were amplified using universal primers (3). A 1,433-bp amplification product was obtained. Using BLAST analysis of NCBI GenBank, these sequences showed 99 to 100% sequence identity with the type strain Acidovorax avenae subsp. avenae ICMP 3183 (NR041757). These two strains were identified as A. avenae by Biolog metabolic phenotype analysis (Biolog, Hayward, CA) with similarity indices ranging from 0.550 to 0.823. The strains induced hypersensitive response on leaves of 2-month-old tobacco plants within 24 h. The pathogenicity of GDHN01 and GDHN02 was tested three times by needle puncture with bacterial suspensions containing of 6 × 108 CFU/ml on five-leaf-stage healthy corn seedlings (cv. Huameitian 8) (n = 5 seedlings/isolate/experiment). Positive controls were inoculated with A. avenae reference strain Yu13 (obtained from Nanjing Agricultural University, China) (1) and negative controls were inoculated with sterile water. Plants were kept in the greenhouse at 28°C and 80% RH. Yellow to brown stripes were observed on all inoculated leaves at 4 to 6 days post-inoculation; similar symptoms were caused by the reference strain, but no symptoms developed on the negative control plants. Re-isolated bacteria from symptomatic tissues were confirmed to be A. anevae based on morphology, physiological assays, and 16S rDNA sequence analysis described above. No target bacteria were isolated from the control plants. A. avenae was previously identified as the causal agent of bacterial leaf stripe on corn in Jiangsu Province (1) and rice in Zhejiang Province (2). To our knowledge, this is the first report of A. avenae naturally infecting corn in Guangdong Province. Guangdong is the largest sweet corn-producing and consuming province in China. This seed-borne pathogen may pose a threat to corn production in Guangdong Province.
References: (1) Y. Gao et al. Jiangsu J. Agric. Sci. 23:22, 2007. (2) L. H. Xu et al. Chinese J. Rice Sci. 22:302, 2008. (3) N. W. Schaad et al. Laboratory Guide for the Identification of Plant Pathogenic Bacteria, 3rd edition. APS Press. St. Paul, MN, 2001.
