In August of 2008, leaves of hybrid corn (Zea mays L.) and popcorn from Pulaski and Jasper counties in northwest Indiana were submitted to the Purdue Plant and Pest Diagnostic Lab with symptoms characteristic of a bacterial disease. Symptomatic leaves had large, tan-to-gray necrotic lesions with dark freckling present within the lesions. Shiny bacterial exudate was present on the surface of many of the lesions. Microscopic observation revealed no fungal structures within the lesions, and bacterial streaming was observed from the cut edge of symptomatic tissue under ×100 magnification with phase contrast. A commercially available ELISA test (Agdia Inc., Elkhart, IN) determined that samples were negative for Pantoea stewartii, the causal agent of Stewart's bacterial leaf blight and wilt. A bacterial suspension was prepared from symptomatic tissue and streaked onto King's B medium and subcultured on semiselective CNS medium (1,2). Axenic, peach-colored colonies present on the CNS medium tested gram positive with a KOH test. Analysis of fatty acid methyl esters (MIDI Inc, Newark, DE) indicated that the strain was very similar (0.611) to Clavibacter michiganensis. Amplification of the 500-bp 16S rRNA region of the bacterial gene and subsequent BLAST alignments of the resulting sequence indicated a 99% match for C. michiganensis subsp. nebraskensis (GenBank Accession Nos. AM410697 and U09763; D16S2 gene bacterial library, version 2.10; MIDI Inc,). Koch's postulates were used to confirm pathogenicity of the isolated bacteria on corn inbred B73. Eighteen plants were mechanically inoculated at growth stage V1 to V2 with a bacterial suspension of approximately 1 × 108 CFU/ml prepared from cultures grown on CNS for 10 days at 28°C (2). Inoculum was rubbed onto leaves dusted with Carborundum and 0.1 ml of the bacterial suspension was injected into stems with a hypodermic needle. Nine control plants were inoculated with sterile water. Plants were kept at greenhouse conditions (24°C) with supplemental 400W high-pressure sodium light. Within 5 to 8 days, leaves and stems of all 18 inoculated plants developed water-soaked, necrotic lesions. No symptoms were observed in control plants. Bacteria were reisolated from symptomatic plants on CNS medium as described above, and gram-positive colonies were obtained. Reisolated strains were identical to C. michiganensis subsp. nebraskensis by D16S2 DNA sequence analysis, confirming the causal agent of the disease. Disease incidence in affected fields ranged from 20 to 60% and significant yield loss was reported. This confirmation is of regulatory importance because of potential export restrictions of Indiana-grown seed corn and popcorn to select countries. To our knowledge, this is the first report of Goss's bacterial wilt and leaf blight on corn in Indiana.
References: (1) D. C. Gross and A. K. Vidaver. Phytopathology 69:82, 1979. (2) L. M. Shepherd. M.S. thesis. Iowa State University. Ames, 1999.