Peat-Based Media as a Source of Thielaviopsis basicola Causing Black Root Rot on Citrus Seedlings. J. H. Graham, Professor, Citrus Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, 700 Experiment Station Road, Lake Alfred 33850. N. H. Timmer, Biological Scientist, Citrus Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 75:1246-1249. Accepted for publication 15 June 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1246.

Thielaviopsis basicola was identified as the cause of black root rot on citrus seedlings growing in soilless peat-based media in Florida greenhouse nurseries. Three of 14 samples of commercial bales of media and two of 12 samples of Canadian sphagnum peat yielded low densities of T. basicola (1–2 cfu/cm³ of medium) when wetted and plated on selective carrot-etridiazol-nystatin medium. T. basicola survived in peat debris and was detected from air samples in a greenhouse that had contained infected plants 2 mo previously. Under winter conditions, where media temperatures ranged from 18 to 27 C, isolates of T. basicola from peat and a peat-based medium caused severe root rot on rootstock seedlings of Cleopatra mandarin; moderate root rot on Ridge Pineapple sweet orange, sour orange, and Volkamer lemon; and mild root rot on rough lemon, trifoliate orange, Carrizo citrange, and Swingle citrumelo. Propagule density in the rhizosphere of all cultivars increased from the inoculated levels in the peat-based medium. Peat-based media may act as a source of T. basicola, a pathogen with a wide host range.