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A Sorbose-Based Selective Medium for Enumerating Propagules of Fusarium oxysporum f. sp. apii Race 2 in Organic Soil. R. T. Awuah, Graduate research assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853; J. W. Lorbeer, professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Phytopathology 76:1202-1205. Accepted for publication 13 March 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-1202.

A selective medium incorporating l-sorbose (20 g), Bacto agar (20 g), dl-asparagine (2 g), chloramphenicol sulfate (600 mg), pentachloronitro-benzene (PCNB as Terraclor 75WP, 120 mg a.i.), and sodium p-(dimethylamino) benzenediazo sulfonate (Dexon 5% granular, 120 mg a.i.) in 1 L of distilled water distinguished among four main types of colonies of Fusarium on soil dilution plates after 5-6 days. Colonies of Fusarium oxysporum f. sp. apii race 2 on the sorbose-based medium (SBM) exhibited a creamy to white color, smooth margins, compact mycelium, and slightly raised centers. The other three colony types of Fusarium were nonpathogenic to celery. Growth of colonies of Fusarium on SBM generally was slow, permitting colony counts up to 7 days after culturing. Background colonies of bacteria, mucoraceous fungi, and pythiaceous fungi were completely inhibited. Growth of Penicillium spp. was limited to whitish pinhead colonies. The efficiency of recovery of F. o. f. sp. apii race 2 with SBM was 78 and 85% when autoclaved organic soil was artificially infested with 10,000 and 5,000 conidia per gram of dry soil, respectively. Populations of the pathogen detected with SBM at five sites in a field with a history of severe Fusarium yellows ranged from 1,751 to 2,469 propagules per gram of air-dried soil. In another field with a history of a moderate incidence of the disease, the vertical distribution of the pathogen was 1,125 and 900 propagules per gram of air-dried soil in the top 0-20 and 20-40 cm, respectively. The pathogen was not detected at a soil depth of 40-80 cm. Celery plants grown in soil at a depth from 0-20 and 20-40 cm developed Fusarium yellows symptoms. Plants grown in soil 40-80 cm deep remained healthy.