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Population Dynamics of Clavibacter michiganense subsp. nebraskense in Field-Grown Dent Corn and Popcorn. Mary Smidt, Department of Plant Pathology, University of Nebraska, Lincoln 68583-0722. Anne K. Vidaver, Department of Plant Pathology, University of Nebraska, Lincoln 68583-0722. Plant Dis. 70:1031-1036. Accepted for publication 27 June 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-1031.

Clavibacter michiganense subsp. nebraskense (=Corynebacterium michiganense subsp. nebraskense), causal agent of Gossís bacterial wilt and blight of corn, was recovered from field-grown popcorn and dent corn plants and residue throughout 1982 and 1983. All collection sites were in fields near Imperial, NE. The population of the pathogen in residue was highest just after harvest in October and declined a total of four to five log10 units over the winter and summer. Conversely, bacterial populations from live corn plants increased three to four log10 units throughout the growing season. The pathogen was isolated from the surfaces of plants in early June before disease symptoms were observed, suggesting an epiphytic phase for the bacterium in the field. Symptoms were observed after mid-July, when the population of the pathogen exceeded about 107 cfu/g fresh weight of leaf tissue. Severity of Gossís bacterial wilt and blight symptoms was evaluated near the end of the growing season and compared with similar evaluations made in previous years. Disease severity appeared to be associated with daily mean temperatures during late May and June, which were 3Ė5 C cooler in 1982 and 1983 than in 1980 and 1981. Corn plants grown under controlled conditions were inoculated with Clavibacter michiganense subsp. nebraskense strain CN72-2 and grown at 12, 16, 21, 26, 32, and 38 C while the bacterial populations were monitored. The optimum temperature for bacterial growth in corn plants was 27 C. At lower and higher temperatures, the growth rate dropped until at 12 C, the doubling time was >9 hr compared with 3.5 hr at the optimum temperature. At 38 C, the bacteria died. When inoculated plants were shifted from a temperature regime that favored bacterial growth (day / night regime of 32 / 25 C) to one that retarded bacterial growth (day / night regime of 40 / 20 C), the growth rate rapidly decreased. Conversely, when inoculated plants were shifted from the restrictive temperature to the permissive one, bacterial growth increased and rapidly reached its maximal rate.