SOYBEAN PHOTOSYNTHESIS AND YIELD AS INFLUENCED BY HETERODERA-GLYCINES, SOIL TYPE AND IRRIGATION

The effects of soil types and soil water matric pressure on the Hetero dera glycines-Glycine mm interaction were examined in microplots in 19 88 and 1989. Reproduction of H. glycines was restricted in fine-textur ed soils as compared with coarse-textured ones. Final population densi ties of this pathogen in both years of the study were greater in nonir rigated soils than in irrigated soils. The net photosynthetic rate of soybean (per unit area of leaf) was suppressed only slightly or not at all in response to infection by H. glycines and other stresses. Relat ive soybean-yield suppression in response to H. glycines was not affec ted by water content in fine-textured soils, but slopes of the damage functions were steepest in sand, sandy loam, and muck soils at high wa ter content (irrigated plots). Yield restriction of soybean in respons e to this pathogen under irrigation was equal to or greater than the y ield suppression under dry conditions. Although yield potential may be elevated by irrigation when soil-water content is inadequate, supplem ental irrigation cannot be used to circumvent nematode damage to soybe an.