Leaf Spot of Field Corn Caused by Pseudomonas Andropogonis

A leaf spot of field corn was shown to be caused by the bacterium Pseudomonas andropogonis. Leaf spot symptoms were observed over 3 years in several States. Inoculation of corn by vacuum infiltration of the bacteria was necessary to reproduce field symptoms. Plant Dis. Reptr. 62: 213-216. A bacterial leaf spot disease of different cultivars of field corn was observed in early summer of 1973, 1974, and 1975. Leaves with essentially the same symptoms were obtained from South Dakota, Iowa, Kansas, Nebraska, and Michigan. The same symptoms were also seen in Wisconsin. Symptoms consisted of circular to ellipsoidal, tan to brown spots, with irregular margins. The spots were 1 to 4 mm in diameter, with one or more darker brown rings within the lesions. Some spots were surrounded by a chlorotic ring 1 mm wide. All spots tended to have a slightly sunken appearance. Occasionally the spots coalesced into irregular, somewhat elongated blotches. Water-soaking of young lesions was also seen. Bacteria were routinely isolated from the lesions. Because the symptoms did not correspond with previous reports of bacterial diseases of corn, experiments were undertaken to determine the causal bacterium and compare it with known bacterial pathogens of corn. The pathogen was subsequently identified as Pseudomonas andropogonis, previously reported to cause bacterial stripe disease of sorghum, sudangrass, teosinte, johnsongrass, field corn, broomcorn, and sweet corn (3,7,8,11). As P. stizolobii (syn. P. andropogonis), the bacterium causes leaf spot diseases of Bougainvillea, clover, and other leguminous plants (1, 3, 4). MA TERIALS AND METHODS Isolations: Bacteria were isolated from lesions by the direct puncture technique (2) onto NBY agar (9). After 3 to 5 days at 24 to 28° C, transfers from single colonies were made twice more to obtain purified clones. Eight strains were obtained in different years from various locations in the Midwest. Cultures were maintained on NBY agar, including reference strains of K. andropogonis NCPPB 933, NCPPB 934, K. stizolobii NCPPB 1024, and NCPPB 1127. Cultures were stored at 2 to 5° C and also lyophilized to serve as reference throughout this study. Bacteriological observations: Tests for oxidase, arginine dihydrolase, levan production, and fluorescence were carried out by the procedures of Lelliott, et al. (6). Sudanophilic granules were examined by the method of Hugh and Gilardi (5). Flagella of all ten P. andropogonis and two P. stizolobii strains were examined by electron microscopy after negative staining of bacteria with a 3 : 1 mixture of potassium phosphotungstate and vanadatomolybdate (10). The bacteria were treated previously with 40/0 glutaraldehyde for 10 min. Sodiumdodecyl sulfate slab gel electrophoretic patterns of total proteins were obtained for six strains, including all four reference strains; details of this procedure will be reported elsewhere. Pathogenicity tests: Inoculum for pathogeniCity tests was prepared by suspending bacteria from 2to 3-day-old plate cultures into sterile distilled water. For vacuum infiltration the suspensions were made up in 0.1"/0 Triton X-100 (Rohm & Haas Co.). The A420 nrn was adjusted to 0.3 (approximately 4 x 108 CFU/ml) and dilutions were made as necessary. Sweet corn ('Golden Cross Bantam'), field corn (A619 x A632), and sorghum (RS626) were inoculated initially by needle puncture into the stems. White sweetc10ver (Melilotus alba) was inoculated by aerosol spray under pressure into the stomates. Corn and sorghum plants were 6 to 8 inches high (3 to 4 leaf stage) and clover plants were about 6 weeks old (4 to 6 leaf stage). 214 Vol. 62. No. 3--PLANT DISEASE REPORTER--March 1978