Field experiments were continued in the same plots for a third year in 1985 at Shafter, CA, in order to further define the root knot nematode-Fusarium wilt interaction on cv. Acala SJ-2 cotton. The experiments were conducted in two adjacent fields that had similar ranges of M. incognita (Mi) population densities, one field with high F.o. vasinfectum (Fov) inoculum densities (Field A) and one with only trace Fov inoculum densities (Field B). Details of the experimental methods and plot design were described previously (1). One quarter of the plots in each field were preplant fumigated with 1,3-D to reduce Mi densities and enable an estimate of yield potential in the absence of disease (ie. 100% relative yield).

In field A (high Fov) mean relative yield in non-fumigated plots was 56.1% in 1984 and 40.0% in 1985, whereas in Field B (trace Fov) equivalent values were 76.1% in 1984 and 67.9% in 1985. Damage functions (based on the Seinhorst model (2) relating relative yield to Log 2 Mi initial density) for the two fields in 1985 revealed a similar tolerance limit (T) for each field. However, minimum relative yield (m, at high Mi initial densities) was 16% lower and occurred at a lower Mi initial density in the presence of Fov.

The changing preplant Fov inoculum densities from 1984 to 1985 was significantly negatively correlated with the 1984 preplant Fov inoculum density. Changes in Mi initial densities from year to year were similar in the presence (Field A) or absence (Field B) of Fov.

The results of these studies are useful for the further development of nematode and disease management strategies based on soil sampling and extraction procedures.