The Rj2 gene (Caldwell, 1966), which conditions ineffective nodulation wtth strains of the cl and 122 serogroup of Rhizobium japonicum (Kirchner) Buhanan, and the Rj4 gene (Vest and Caldwell, 1972), conditioning an inef-fective nodulation response with rhizobial strain 61 of the Beltsville Culture Collection R. japonicum, were tested for linkage association with the gene L1 (black pod). The rj1 gene (Williams and Lynch, 1954), conditon-. ing a non-nodulating response with a broad spectrum of strains of R. Japonicum, was tested for linkage with the gene fr (roots nonfluorescent in UV light) and L1 (black pod). The gene y9 (chlorophyll defective) was tested for possible linkage with the genes ln (narrow leaf), and P1 (glabrous). The P1 gene was also tested for linkage with the gene fr.

All characters of living organisms. whether morphological, physiological, or biochemical, undoubtedly are genetically determined. The ability of a Eew microorganisms to attack a limited range of plant species and the within-species variation in disease development also are presumed to have genetic bases. Such a variation in disease expression was used to develop the gene-for-gene concept (Flor, 1955).

One method of determing field tolerance of soybeans to Phytophthora mega-sperma f. sp. glycinea (Hildeb.) Kaun & Erwin (Pmg) is to determine the percent of plant loss from emergence to maturity for cultivars grown in an infested field under conditions favorable for the disease (Buzzell and Anderson, 1982). Drawing upon the field results and the results of two unpublished greenhouse experiments (given below), a greenhouse technique similar to the field test was developed. 1962 Experiment : Cores of soil were obtained from a field known to be in-fested with Pmg race 1, the field having been used in a previous study (Ful-ton et al., 1961).