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The Effect of Time of Exposure to Inoculum, Plant Age, Root Development, and Root Wounding on Fusarium Yellows of Celery. L. P. Hart, Former graduate research assistant, Department of Plant Pathology, University of California, Riverside 92521, Present address of senior author: Assistant Professor of Plant Pathology, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824; R. M. Endo, professor, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 71:77-79. Accepted for publication 9 June 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-77.

Fusarium yellows of celery was more severe as the time of exposure to inoculum of Fusarium oxysporum f. sp. apii in soil was increased from 4 to 14 days. Young, 2- and 4-wk-old, plants were more susceptible than were 6-and 8-wk-old plants. Root tips were important sites of infection, but wounded roots were not. Experiments in which celery roots were inoculated by the root dip method or the soil infestation method indicated that there was a finite number of susceptible infection sites (ie, apices of young roots) on celery roots at any one time, and that large numbers of infections were required for severe disease development. Subsequent disease severity was not as great after roots were dipped into inoculum as after roots grew through soil infested with the pathogen. This suggested that there was a limited number of infection sites during inoculation with the root dip method and that subsequently formed infection sites were not exposed to the inoculum.