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Populations of Fusarium oxysporum f. sp. cepae in Organic Soils in New York. G. S. Abawi, Former Research Assistant (now Research Associate), Department of Plant Pathology, Cornell University, Ithaca, New York; J. W. Lorbeer, Associate Professor, Department of Plant Pathology, Cornell University, Ithaca, New York. Phytopathology 61:1042-1048. Accepted for publication 22 March 1971. DOI: 10.1094/Phyto-61-1042.

A selective agar medium was developed for quantitative isolation of Fusarium oxysporum f. sp. cepae from organic soils in New York. Martinís rose bengal medium was supplemented with 2 ppm chloro-tetracycline HCl (Aureomycin), 1 ppm thiram, 100 ppm sodium-p-(dimethylamino) benzenediazosulfonate (Dexon), and 100 ppm pentachloronitrobenzene (PCNB). Plate counts indicated that F. oxysporum f. sp. cepae is unevenly distributed in New York organic soils cropped to onion, and is isolated from as deep as 30.5 to 45.7 cm. Average populations ranged from 300 to 6,500 propagules/g oven-dry soil. Soils with a long history of Fusarium basal rot of onion caused by this fungus had higher populations of the fungus than soils with histories of low levels or no known occurrence of the disease. However, several organic soils with high populations of F. oxysporum f. sp. cepae did not have histories of a high incidence of Fusarium basal rot. Only the sporodochial form of the fungus was isolated from naturally infested soils.

Additional keywords: Allium cepa.