Accurate measurement methods for the determination of severity classes of fungal infection are necessary to improve the evaluation of management inputs and resistance testing because of the high environmental impact on disease development. Fungal population density determinations from soil and plants have primarily relied on the use of a semi-selective modified Nash and Snyder’s medium. However, dilution plating can be tedious, and generating accurate and precise numbers is challenged by the shortcomings in selectivity of the medium and slow growth habit of F. virguliforme. Because of similar problems in other pathogen systems, PCR methods have been developed for improved detection and quantification, and they have been used to generate information on infection levels for plant disease development and predictions. An example of coupling quantitative real-time PCR assays with extraction of DNA from soil is given with a DNA-based testing system offered in Australia to quantify soilborne pathogens and predict risk for plant disease. Several sets of PCR primers aiming at amplification of a mitochondrial gene or a tox gene of F. virguliforme have been developed,. These primers and the accompanying qPCR assays have primarily been used for specific amplification in plants grown under controlled conditions, but those available at the time this research was done were not specific for F. virguliforme. A more robust and specific method for detection and quantification of DNA from F. virguliforme for field research was urgently needed. The primary objective of this study was to determine the interactive contributions of H. glycines and F. virguliforme on SDS development and severity and soybean seed yield. The secondary goal was to develop a qPCR assay for specific detection and quantification of F. virguliforme in field-grown soybean roots and soil for this and other studies. The interactive contributions of F. virguliforme and H. glycines on foliar SDS development, root disease severity, and on soybean seed yield were modeled. Combinations of DNA of F. virguliforme and egg counts of the nematode in soil at planting predicted the risk for severe SDS in this study, although the environmental conditions during the growing season also can influence disease development. Management recommendations for plant-parasitic nematodes have relied heavily on the use of the action threshold level concept. No attempts to determine threshold levels of SDS under field conditions have been published. In related studies of Fusarium wilt of chickpea, caused by Fusarium oxysporum f. sp. ciceris, a steady increase of inoculum potential led to a maximum plateau at an optimum temperature and inoculum densities. Bhatti and Kraft reported this optimum to be at inoculum densities of 500 to 1000 propagules g-1 of soil and around 25 to 30uC. Constraints of the current study are the limited quantitative variability of the different factors applied to the disease system.