Lasting Effects of Soil Fumigation on Nematodes?

I am often asked about the duration of control that pre-plant soil fumigation has on

parasitic nematodes. My typical response is "Probably about 3-4 years, or once the roots move outside of the treated area." This generalization appears to be true in
many orchards as symptoms of nematode parasitism, which include stunting, bacterial canker, and bud drop, often do not develop until the fourth or fifth year.

This past fall, we performed our annual nematode sampling of all of our research plots. One of the plots was our almond replant plot located near Ballico. This orchard is located on a sandy soil with a history of almond trees and nematodes. In the fall of 2010, it was fumigated with four fumigant treatments, including methyl bromide row-strip, C35 row-strip, Telone II rowstrip, and Telone II broadcast, a control plot, and a spot steam treatment. In the spring of 2011, the trees were planted.

In 2011, although stunting was observed across treatments (table 1), we did not find any nematodes in the fumigated soils (Figure 1). In this year's sampling, however, we detected a large increase in the numbers of all three nematodes of concern for almond in all treatments (Figure 2). Please note the scale change with the y-axis. To me, this is a bit surprising as I thought that we would see 3-4 years of "relief" from nematodes after pre-plant fumigating. Even more concerning is that the numbers detected are high enough in some plots to cause economic damage and would possibly warrant post-plant treatment (i.e. Movento). This data suggests that nematode sampling may be needed as early as two years after fumigation in order to catch a developing population.

A quick thanks to the grower hosting this plot (I didnt know if you wanted to be named), USDA-ARS Area-Wide Methyl Bromide Fumigant Alternatives Project, and The Almond Board of California.