Effects of Inoculum Densities of Fusarium oxysporum f. sp. apii in Organic Soil on Disease Expression in Celery. Wade H. Elmer, Former Graduate Research Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Melvyn L. Lacy. Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 71:1086-1089. Accepted for publication 16 July 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-1086.

Populations of Fusarium oxysporum f. sp. apii in naturally infested organic soil were enumerated by conventional soil dilution methods and subsequent pathogenicity tests. Celery plants grown in naturally infested soil and either diluted or not diluted with autoclaved soil showed reductions in mean dry weights and increases in disease severity ratings that were proportional to estimated inoculum density in each dilution of soil. Infection incidence, correcting for multiple infections [log_c 1/(1 – X)], increased with inoculum density in a curvilinear fashion. An inoculum density of 42 propagules per gram of field soil was predicted to incite sufficient infections to cause vascular discoloration in every plant within 6 wk. A mean of 36.5 propagules per gram of soil was necessary to incite significant growth reductions within this period. Vertical samples of organic soil naturally infested with F. o. f. sp. apii contained the highest inoculum densities in the 15- to 30-cm profile, which corresponded to the zone of greatest root density. Propagule densities decreased significantly at soil depths below 45 cm.