LAUSR

Maize (Zea mays L.) is one of the most important crops in China. Since 2020, a new leaf spot disease has occurred in southwest China in areas such as Yunnan, Sichuan and Hubei provinces. Typical symptoms appeared after tasseling. The spots are scattered on the leaf surface, round to oval in shape with diameter range 3-20 mm. Spots are initially water-soaked, gradually turning yellow or white. In 2021, diseased leaf samples with typical white spot were collected for pathogen isolation and identification in Qujing, Yunnan province. Four small pieces of leaf tissue (about 0.25 cm2 in area) were excised from the edge of the necrotic lesion of each plant, surface sterilized with 75% ethanol for 1 min, rinsed three times with sterile distilled water, and soaked in sterile distilled water for 5 min. The solution was plated on Luria Broth medium (LB) plate (Shin et al. 2022) . After incubation at 28°C for 24 h, round, smooth-edged yellow colonies appeared in the LB plate. The bacterium isolated was gram-negative, negative for oxidase, positive for peroxidase, indole, citrate (Wells et al. 1987). Three strains (PA21QJ01, PA21QJ02 and PA21QJ03) showed identical colony morphology. PA21QJ01 was used for further molecular analyses. DNA was extracted from fresh colonies cultured in LB(Shin et al. 2022), and the fragments at the 16S rDNA, gyrB and rpoB loci were amplified using primers 27F/1492R (Galkiewicz and Kellogg 2008), UP-1/UP-2r (Yamamoto and Harayama 1995) and rpoBCM81-F/rpoBCM32b-R (Brady et al. 2008), respectively. The sequences of fragments of 16S rDNA, gyrB and rpoB from isolate PA21QJ01 were was deposited in GenBank (accession number: OM184301.1, OM302500, OM302499). A search for homologous sequences using BLAST resulted in 99.9, 99.6 and 99.8% identity of 16S rDNA, gyrB and rpoB, respectively, with sequences from the NN08200 of Pantoea ananatis (GenBank accession numbers: MK415050.1 for 16S rDNA, CP035034.1 for gyrB and CP035034.1.1 for rpoB). Above molecular results indicated that PA21QJ01 isolated from maize white spot is P. ananatis. Pathogenicity tests were performed on tasseled plants of the suscptible maize variety Wugu1790. After culture in LB medium plate at 30°C for 12 h, the bacterial solution was used for inoculation at a concentration of 1 × 108 CFU ml-1. After 7 days of inoculation, the leaves of the plants appeared water-soaked. After 10 days, white spot developed with brown margin. In contrast, the control plants remained healthy and symptomless. The same P. ananatis was reisolated in the inoculated maize plants, fulfilled Koch's law. In the last decade, P. ananatis has been reported to cause maize white spot in South Africa, Mexico, Poland, Argentina, Brazil (Sauer et al. 2015), and Ecuador (Toaza et al.2021). It has caused crop diseases with other crops, such as onion, rice, pineapple, melon, and sorghum, and others (Sauer et al. 2015). It caused leaf blight and leaf steak in rice in China (Yu et al. 2021). This is the first report of maize white spot caused by P. ananatis in China. However, to our knowledge, this is the first report of maize white spot disease in China. Attentions should be paid to screening for disease-resistant resources and breeding disease-resistant hybrids. Reference: Wells, J. M. et al. 1987. Int. J. Syst. Bacteriol. 37:136-143. Shin, G. Y. et al. 2022. Plant Dis. Doi: 10.1094/PDIS-08-21-1810-SC. Brady, C., et al. 2008. Syst. Appl. Microbiol. 31:447. Galkiewicz, J. P., and Kellogg, C. A. 2008. APPL ENVIRON MICROB, 74.24: 7828-7831. Toaza, A. et al. 2021. Plant Dis. Doi:10.1094/PDIS-02-21-0298-PDN Yamamoto, S., and Harayama, S. 1995. APPL ENVIRON MICROB, 61:1104.L. Sauer, A. V. et al. 2015. Agronomy Science and Biotechnology. Doi:10.33158/ASB.2015v1i1p21 Yu et al. 2021. Plant Dis. Doi:10.1094/PDIS-05-21-0988-PDN.