Effects of VA Mycorrhizae and Metalaxyl on Growth of Alfalfa Seedlings in Soils from Fields with “Alfalfa Sickness” in Alberta. S. F. Hwang, Plant Pathologist, Alberta Environmental Centre, Vegreville, Alberta, Canada T0B 4L0. . Plant Dis. 72:448-452. Accepted for publication 8 December 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0448.

The causes of poor alfalfa seedling growth in “alfalfa sickness” (AS) soils were studied in a growth chamber. The growth of alfalfa seedlings was increased in sterilized AS soils but not as vigorously as seedlings grown in “healthy” or sterilized “healthy” soils. Of the Pythium spp. isolated from stunted alfalfa seedlings, P. paroecandrum caused preemergence damping-off and stunting similar to that observed in AS fields. Seed treatment with metalaxyl increased seedling survival in AS soils but did not improve plant growth. Seedlings inoculated with VA mycorrhizae, however, grew more rapidly than nonmycorrhizal seedlings. Metalaxyl had a negligible effect on VA mycorrhizal infection of alfalfa root segments but reduced the VA mycorrhizal colonization of root length. This study suggested that a combination of metalaxyl and VA mycorrhizae could be used to produce healthy alfalfa seedlings in AS soils.