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The effects of herbicides on soil life
T
he use of herbicides for weed control has become common
practice in modern agriculture. The use of such chemicals
can, however, extend beyond the intended target species
and influence non-target organisms.
Fungi and bacteria are mostly known for, or associated with, the
diseases that they cause. Many of these diseases and their associ-
ated pathogens are soil-borne, and are accordingly subject to the
potential effects that pesticides such as herbicides might have.
Fungi – of the
pathogenic kind…
Disease development is favoured
by conditions that place the host
under stress. Several studies were
initiated internationally to establish
the potential effect that disease-
herbicide interactions involving
soil-borne pathogens could have
on certain crops.
A University of Missouri (Colum-
bia) research group
1
investigated
the impact of, amongst others,
2,4-DB, alachlor and glyphosate on
early stage development of char-
coal rot of soybean. They observed
that the actual stress induced with
2,4-DB in field studies, significantly
increased root colonisation by
Macrophomina phaseolina
(causal
organism of charcoal rot) on soy-
bean roots.
Alachlor, however, significantly re-
duced colonisation, whilst stresses
induced by glyphosate (applied at sub-lethal rates) had no effect
on colonisation of this pathogen. As the colonisation observed
was more due to the root injury caused as a result of herbicide ap-
plication than to herbicide stress per se, they concluded that crop
susceptibility towards plant pathogens can potentially be altered
with the application of certain herbicides.
Kremer and Means
2
, concluded that glyphosate application on gly-
phosate resistant soybean may enhance Fusarium root colonisa-
tion and potential disease through not only stimulating growth
of the fungal pathogen, but also suppressing bacterial antago-
nists, whilst another study reported that glyphosate can promote
Fusarium head blight of wheat and barley
3
.
No significant effect of glyphosate on either
Fusarium
spp abun-
dance or Fusarium-related disease levels was reported by other
studies
4,5
. The general trend of contradictory findings accordingly
continue.
During 2008 Powell and Swanton
6
published an article titled, ‘A
critique of studies evaluating glyphosate effects on diseases asso-
ciated with
Fusarium
spp.’ in the internationally accredited journal,
Weed Research
. Although they focussed on Fusarium related dis-
eases, their comments are applicable to all disease related research.
Their findings suggest that despite glasshouse studies demonstrat-
ing the stimulatory effect of glyphosate on
Fusarium
spp., few field
studies have demonstrated a link between glyphosate and crop
diseases associated with this fungal group.
They state that there is clearly disagreement internationally when
it comes to glyphosate use and enhanced plant disease associat-
ed with tolerant fungal species. The
authors attribute the lack of correla-
tion between glasshouse findings
and field studies to the possibility
that glyphosate stimulation of fun-
gal diseases is largely a glasshouse
phenomenon, where environmental
conditions can be easily controlled
to promote infection and advance-
ment of disease.
Powell and Swanton state that differ-
ences among field studies in overall
levels of variance and replication
could explain why some studies
observe significant treatment ef-
fects and others not. The authors,
however, concede that they cannot
disprove the linkage between patho-
genic fungi such as
Fusarium
spp.
and glyphosate use, but emphasise
that initial inoculum level, weed
abundance and community compo-
sition, fertility, agronomical prac-
tices, climate and soil chemical and
physical properties are all important
factors that interact with glyphosate and need to be considered
when designing field experiments.
They concluded that taken at face value, there is little evidence
from experimental field trials to support a causative link between
glyphosate and crop diseases associated with
Fusarium
spp.
Sanyal and Shrestha
7
similarly discussed the lack of consistent re-
sponses observed with studies investigating the effect of glypho-
sate in the severity of soybean sudden death syndrome caused by
Fusarium solani
.
Herbicides with antifungal properties
It is speculated internationally that the activity of herbicides can
potentially extend beyond their target organisms and inhibit spore
germination or mycelial growth, alter the phytoalexin levels, or
interfere with other physiological processes in plants.
An Australian research group initiated a short study on the effect
of glyphosate on
Sclerotium rolfsii
(a common soil-borne plant
pathogen known to persist in crop residues) after banana growers
ON FARM LEVEL
Fungi / Bacteria / Diseases
Integrated pest control
DR MARYKE CRAVEN, CHARNÉ MYBURGH, OWEN RHODE
and
DR JEANETTA SAAYMAN-DU TOIT,
ARC-Grain Crops, Potchefstroom
Part 3: Pathogenic fungi
“
...there is little
evidence from
experimental field
trials to support a
causative link between
glyphosate and crop
diseases associated
with
Fusarium
spp.
“