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Ecology and Epidemiology

Pathogenicity to Carrots of Pythium Species from Organic Soils of North America. R. J. Howard, Former Research Assistant, Department of Plant Pathology, University of Wisconsin-Madison, Madison, WI 53706, Present address: Alberta Horticultural Research Center, Brooks, Alberta T0J 0J0; R. G. Pratt(2), and P. H. Williams(3). (2)(3)Former Research Assistant and Professor, Department of Plant Pathology, University of Wisconsin-Madison, Madison, WI 53706; (2)Present address: USDA-ARS, Department of Plant Pathology and Weed Science, P.O. Drawer PG, Mississippi State, MI 39762. Phytopathology 68:1293-1296. Accepted for publication 3 March 1978. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1293.

Carrot seedlings were grown in samples of organic soils from nine carrot-growing regions of North America at a uniform moisture tension. Pre-emergence damping-off and root dieback were evaluated, and Pythium spp. were isolated from diseased roots. Twelve species were identified among 518 Pythium isolates. Pythium irregulare, P. sulcatum, and P. sylvaticum were isolated most frequently and were each found in soils from six or more regions. Pythium irregulare and P. sulcatum caused severe root dieback in articifically infested soils, whereas P. sylvaticum and most other species caused only slight root dieback. Most species caused severe pre-emergence damping-off. Root dieback occurred in soils from all regions and was greater in samples from fields in Wisconsin that had been cropped to carrots than from virgin fields. Root dieback was correlated with the presence of P. irregulare and P. sulcatum but was not consistently correlated with total Pythium populations in soils. Results of this study indicate that the etiology of carrot root dieback is likely to be similar in organic soils throughout North America, and that P. irregulare and P. sulcatum are the primary causal agents.