Initial population density (Pi) estimates of root-knot nematode, Meloidogyne incognita, currently are made by counting second stage juveniles and/or eggs extracted from autumn to spring collected soil samples. These estimates are used to predict the damage potential of M. incognita from previously developed damage functions relating Pi to cotton relative yield (y). At several field sites in the San Joaquin Valley, California, quantitative relationships were developed of amount of root-galling visible on cotton at harvesttime with ensuing Pi in soil and with relative yield of a succeeding cotton crop. Root-galling assessments were transformed onto a weighted nematode rating (WNR) index as proposed by Jorgensen (1979). These relationships were adequately described by linear or more often quadratic functions. Values for the coefficient of determination (R²) ranged from 0.53 to 0.98 for both WNR(1) versus Pi(2) and WNR(1) versus y(2) relationships. These relationships are influenced by cultivar selection and presence of Fusarium wilt.

The results of these studies indicate that for a given set of conditions (e.g. sandy soils, cultivar, interacting organism present, etc.), root infection ratings based on weighted galling indices made in the field on plants at the end of Crop Year 1, can provide a reliable quantitative prediction of yield loss to a succeeding cotton crop grown in Crop Year 2. This system may reduce the need for soil sampling.