Pathogenicity to Carrots of Pythium Species from Organic Soils of North America

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Abstract
Carrot seedlings were grown in samples of organic soils from 9 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. P. irregulare, P. sulvatum and P. sylcaticum were isolated most frequently and re each found in soils from 6 or more regions. P. irregulare and P. sulcatum caused severe root dieback in artifically 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 fiedls in Wisonsin [USA] 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. The etiology of carrot root dieback is likely to be similar in organic soils throughout North America. P. irregulare and P. sulcatum are the primary causal agents.

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