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Seasonal Colonization of Roots of Field-Grown Cotton by Verticillium dahliae and V. tricorpus. O. C. Huisman, Associate professor, Department of Plant Pathology, University of California, Berkeley 94720; Phytopathology 78:708-716. Accepted for publication 8 December 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-708.

Colonization of roots of field-grown cotton plants by Verticillium dahliae and V. tricorpus was examined by plating washed roots on a growth restrictive medium. The roots were collected biweekly, starting when plants were in the first true leaf stage. Colonies of Verticillium readily developed from these roots. The density of colonies on roots correlated with the inoculum density of Verticillium in the soil. Colonization of roots by the two fungi continued throughout the growing season. Seasonal changes in soil moisture and temperature had no influence on the colonization rate of roots by V. dahliae, whereas colonization of roots by V. tricorpus was affected by soil temperature, being greater at 2023 than at 2831 C. The mean length of colonies on roots was relatively constant (2.3 mm) during the season. Early in the season, colonies were randomly distributed along the length of the root, but, from mid-July onward and coincident with measured increases in inoculum densities in soil, there was evidence for clustering of colonies along the root. Colonization of roots was not related to pathogenicity since V. tricorpus readily colonized both cotton and tomato roots but only infected the vascular system of tomato.

Additional keywords: epidemiology, soilborne pathogens.