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Ecology and Epidemiology

Interaction Between Fusarium oxysporum f. sp. medicaginis and Corynebacterium insidiosum in Alfalfa. L. E. B. Johnson, Research fellow, Department of Agronomy and Plant Genetics, USDA, ARS, University of Minnesota, St. Paul 55108; F. I. Frosheiser(2), and R. D. Wilcoxson(3). (2)(3)Professors, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Phytopathology 72:517-522. Accepted for publication 21 July 1981. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1981.. DOI: 10.1094/Phyto-72-517.

The development of Corynebacterium insidiosum was adversely affected by the presence of Fusarium oxysporum f. sp. medicaginis in the field, the glasshouse, and in vitro. In the field, bacterial wilt symptoms were less severe in roots of alfalfa plants inoculated with a mixture of C. insidiosum and F. oxysporum than in plants inoculated with C. insidiosum alone. This effect was greater in cultivars susceptible to bacterial wilt than in resistant cultivars. In the glasshouse, C. insidiosum developed more slowly in alfalfa roots inoculated with a mixture of both pathogens than in roots inoculated with C. insidiosum alone. Only occasional bacteria were found in various cells of vascular bundles when vessel elements contained F. oxysporum. Eighteen of 19 Fusarium isolates including F. oxysporum f. sp. medicaginis, killed before being placed on agar media seeded with C. insidiosum or Rhizobium meliloti, inhibited the development of both bacterial species. The sterile filtrates of four isolates of F. oxysporum f. sp. medicaginis grown in a nutrient broth in still and in shake culture were added to agar medium seeded with C. insidiosum. Filtrates from the still cultures (mostly mycelium) inhibited C. insidiosum, but filtrates from the shake cultures (mostly conidia) had no effect on the bacterial growth. F. oxysporum apparently produces a substance that inhibits C. insidiosum and reduces bacterial wilt severity in alfalfa plants.