08-16-2017, 08:32 PM
The most widely used pesticides utilized for the management of plant-parasitic nematodes
belong to the organophosphorus group. Their efficacy may be reduced in areas where adapted
microorganisms accumulate that are capable of rapidly degrading the active ingredients. The
enhanced biodegradation process of non-fumigant nematicides is of particular concern in
intensive agriculture. However, it remains unclear which microorganisms play the most
important role in the rapid metabolization and how and why this process develops.
Furthermore little is known as to whether the biodegradation process may be slowed down,
stopped or reversed. Studies using soils with different nematicide history collected in four
banana fields in the Atlantic region of Costa Rica demonstrated that the non-fumigant
organophosphate nematicide terbufos had lower levels of efficacy and shorter effective
activity against the burrowing nematode Radopholus similis when the soil had a prolonged
terbufos application history. Lower levels of efficacy were related to the microorganisms
capable of rapidly degrading the active ingredient. The analysis of soils collected in Germany
with different nematicide application history demonstrated that fenamiphos, another
organophosphate non-fumigant nematicide, was not rapidly biodegraded in soil with no
previous pesticide exposure. This study also demonstrated that Pseudomonas spp. does not
accumulate upon fenamiphos applications and may not be involved at all in fenamiphos
degradation. The lack of surfactant production of the isolated Pseudomonas spp. could be a
reason for their absence in the biodegradation process. Bacteria capable of degrading
fenamiphos were isolated from another German soil with a large fenamiphos-history. These
bacteria utilized fenamiphos as a sole carbon source. By comparison of the partial sequences
of their 16S rRNA coding genes with those genes present in the GenBank sequence database,
a fully resolved phylogenetic tree could be generated, showing that these fenamiphos
degrading (Fd) isolates belonged to closely related Microbacterium, Sinorhizobium,
Brevundimonas, Ralstonia, or Cupriavidus species. The Fd bacteria did not cross-degrade the
novel organophosphate nematicide fosthiazate, thus suggesting that they are fenamiphosspecific.
However, a combination of all microorganisms of the same soil from which the
fenamiphos-degrading bacteria was isolated, was capable of degrading fosthiazate, thus
demonstrating that there are other microorganisms capable of degrading nematicides even in
the absence of an application history. This also revealed that the nematicide-history of one
organophosphate nematicide does not intrinsically influence the degradation of another
pesticide of this same chemical group.
The application of plant revitalizers enhanced soil microbial biomass over time which
resulted in an enhanced biocontrol activity against the root-knot nematode Meloidogyne
incognita and a delayed biodegradation process of fenamiphos.
In conclusion, this research demonstrated that many different soil bacteria can adapt when
frequently exposed to a particular nematicide, thus offering them an alternative carbon source
to grow. This effect can be slowed down by altering the microbial soil diversity through the
application of natural plant enhancers that benefit nematicide non-degrading strains and
simultaneously reduce nematode damage.
