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The Influence of Races of Heterodera glycines on Nodulation and Nitrogen-Fixing Capacity of Soybean. P. S. Lehman, Former Graduate Student, Department of Plant Pathology, North Carolina State University, Raleigh 27607, Senior author is now Assistant Professor, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30601; D. Huisingh(2), and K. R. Barker(3). (2)(3)Associate Professors, respectively, Department of Plant Pathology, North Carolina State University, Raleigh 27607. Phytopathology 61:1239-1244. Accepted for publication 21 May 1971. DOI: 10.1094/Phyto-61-1239.

Three races of Heterodera glycines were compared for their effects on nodulation and nitrogen(N)-fixing capacity of Lee soybean. Alteration in N-fixing capacity was quantitatively determined, using an acetylene-ethylene gas chromatographic assay. Inoculum densities of 100, 200, and 400 crushed “cysts”/pot of race 1 of H. glycines plus Rhizobium japonicum caused a significant decrease in nodules per gram of root tissue and N-fixing capacity per plant as compared with R. japonicum check plants. The same inoculum densities of races 2 and 4 did not decrease nodules per gram of root tissue or the N-fixing capacity per plant. Nodule number and nodule weight were inversely correlated with increasing densities of race 1. Infection by race 1 initially increased nodular efficiency of the few nodules present; i.e., increased N-fixing capacity per milligram fresh weight of nodules. However, as race 1 multiplied, nodular efficiency and total N-fixing capacity declined. Race 1 caused severe chlorosis of soybean, whereas races 2 and 4 did not. This was correlated with the greater effectiveness of race 1 to decrease nodulation and N-fixing capacity.

Additional keywords: nitrogen metabolism, nematode, Glycine max.