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Conditions Affecting the Detection of Phytophthora megasperma in Soils of Wisconsin Alfalfa Fields. R. G. Pratt, Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; J. E. Mitchell, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 63:1374-1379. Accepted for publication 20 April 1973. DOI: 10.1094/Phyto-63-1374.

Phytophthora megasperma was detected by a seedling baiting technique in freshly collected soil samples from 18 of 100 alfalfa fields in Dane County, Wisconsin, in 1970. All samples were collected from low areas of fields which showed possible evidence of past or present root rot. When 52 samples initially rated negative were retested after planting to alfalfa for 8 weeks, P. megasperma was detected in 12. The pathogen was also detected in 48 of 109 samples collected throughout Wisconsin in 1971. Samples from 35 of the 54 counties sampled were positive. Soils commonly infested were clay loams and silty clay loams; sandy loams were less frequently infested. The baiting technique was most sensitive when 25 cc of soil was covered with 3-4 mm of water and the seedlings were added without any preflooding period. Infection of seedlings was greater at 15 C and 20 C than at 25 C; no infection occurred at 30 C. Planting naturally infested soils containing low levels of inoculum to alfalfa for 8 weeks, greatly increased the probability of detecting P. megasperma. Diluting samples with a steamed sand-sandy loam mixture, frequently increased seedling infection by P. megasperma, and also reduced the probability that seedlings would be infected by Pythium. It is concluded that P. megasperma is generally distributed throughout Wisconsin and may be an important pathogen of alfalfa in low-lying fields.

Additional keywords: Medicago sativa L.