Root colonization by soil microorganisms has been shown to increase the
activity of glyphosate in resistant and susceptible biotypes of giant
ragweed and a susceptible common lambsquarters biotype, but not in horseweed
biotypes. The objective of this study was to investigate the colonization of
roots in glyphosate-resistant and -susceptible giant ragweed and horseweed
biotypes, and glyphosate-tolerant and -susceptible biotypes of common
lambsquarters after a sublethal glyphosate application. The three weed
species were grown separately in sterile and unsterile field soil and
treated with glyphosate at two sublethal rates. Soil microbes were isolated
from the roots onto sterile media 3 d after the glyphosate treatment. The
susceptible biotypes of giant ragweed and horseweed grown in unsterile soil
were colonized by more soil microbes at the higher rate of glyphosate,
compared to the resistant biotype grown in unsterile soil. Oomycetes were
isolated separately on a selective media and they were also more prevalent
in the roots of the susceptible biotypes of each weed species grown in the
unsterile soil when glyphosate was applied at the highest rate. Therefore,
the ability of these three weed species to tolerate a glyphosate application
may involve differences in the susceptibility to soil microbial
colonization, especially oomycetes.