Effects of Maggots and Wounding on Occurrence of Fusarium Basal Rot of Onions in Colorado. K. L. Everts, Former Graduate Student, Department of Plant Pathology and Weed Science, Colorado State University, Fort Collins 80523. H. F. Schwartz, Associate Professor, Department of Plant Pathology and Weed Science, and N. D. Epsky, Research Associate, and J. L. Capinera, Professor, Department of Entomology, Colorado State University, Fort Collins 80523. Plant Dis. 69:878-882. Accepted for publication 27 February 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-878.

Inoculum densities of Fusarium oxysporum f. sp. cepae varied from 22 to 3,600 colony-forming units per gram of dried soil in onion production soils in Colorado. The percentage of F. oxysporum isolates pathogenic to onion increased from 25–56% at midseason to 62–90% later in the growing season. Seed-corn maggots (Delia platura) were commonly recovered from plants showing Fusarium basal rot symptoms. In preference tests, seed-corn flies oviposited eggs 78% of the time on diseased rather than on healthy bulbs. An ottidid fly (Euxesta notata) was most commonly recovered from cull onions, whereas no onion maggots (D. antiqua) were found in any field or infested bulbs in Weld County. Mechanical wounding significantly increased the incidence of Fusarium basal rot in Fusarium-infested soil, but seed-corn maggots generally appeared to be secondary invaders of diseased bulbs in onion fields.