First Report of Bacterial Leaf Stripe Caused by Acidovorax avenae subsp. avenae on Foxtail Millet in China

Foxtail millet (Setaria italica) is an important cereal crop widely grown in arid and semiarid regions of China. Each summer of 2015 to 2017, a reoccurring disease was observed in commercial fields of foxtail millet (cvs. Jigu 19, Yugu 18, and Henggu 13) located in Hebei Province, China. Symptoms on leaves included reddish-brown, necrotic stripes along the veins, especially displayed on the upper middle leaves of plants. The disease occurred with an incidence of about 30% of each affected cultivar. In some cases, infected plants became withered and lost productivity. In August of 2017, more than 10 symptomatic leaves were sampled from different millet fields in Shijiazhuang region. Bacterial colonies that were white-cream color were consistently recovered on nutrient agar from infected leaves. The isolates were gram negative, rod shaped, aerobic, and positive for oxidase and catalase. They induced strong hypersensitive response on tobacco 24 h after infiltration. Based on Biolog metabolic phenotype analysis (Biolog, Hayward, CA), the isolates were identified as Acidovorax avenae with similarity indices ranging from 0.65 to 0.78. To confirm the identity of the causal bacterium, two isolates from different fields, G1 and G3, were selected for further characterization. Polymerase chain reaction was used to amplify the 16S rDNA with primers fD1/rD1 (Weisburg et al. 1991) and the 16S-23S rDNA internal transcribed spacer (ITS) region with primers 1493f/23r (Schaad et al. 2008), respectively. Using BLASTn analysis of NCBI, the 16S rDNA and ITS sequences of the two isolates (GenBank accession nos. MH244342 to MH244343 and MH253879 to MH253880, respectively) shared 100 and 99% sequence identity to those of the A. avenae subsp. avenae (Aaa) strain ATCC 19860ᵀ (CP002521) in GenBank, respectively. Pathogenicity tests were performed by spraying foliage of 4-week-old millet seedlings (cv. Jigu 19) with bacterial suspensions (10⁸ CFU/ml). Five potted seedlings were inoculated with each isolate. The experiment was repeated three times. Similar treatments with sterile water served as a negative control. After inoculation, plants were covered with plastic bags and kept in a growth chamber at 28°C and 85% relative humidity. Brown stripes were observed on all inoculated leaves at 2 to 3 weeks postinoculation, but no symptoms were observed on the seedlings treated with water. The pathogen was reisolated from symptomatic tissues and confirmed as Aaa with 99 and 100% sequence identity to those of Aaa strain ATCC 19860ᵀ (CP002521), respectively, based on fragment sequences amplified by 1493f/23r and fD1/rD1 primers. No target bacteria were isolated from the control plants. A. avenae has been reported to cause disease in many economically important plants including hog millet (Myung et al. 2012) and corn (Ji et al. 2014). To our knowledge, this is the first report of bacterial leaf strip disease caused by A. avenae on foxtail millet in China. Further studies are required to understand potential prevalence and control strategy of the bacterial disease on foxtail millet.