ISIS Report 26/05/10
Glyphosate Tolerant Crops Bring Diseases and Death
New research reveals disastrous ecological
impacts of the world’s top herbicide and GM crops made tolerant to it Dr. Mae-Wan Ho and
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Glyphosate tolerant (GT) crops and glyphosate herbicide (commercial
formulation, Roundup) poison nitrogen fixing and other beneficial soil
bacteria, increase fungal pathogens, undermine plant immunity to diseases, decrease
plant micronutrients available in the soil, and more.
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Research findings over the past decades paint a damning
picture of the cropping system that has taken over 85 percent of the 134
million hectares of global agricultural land now growing genetically modified
(GM) crops (see  Scientists Reveal Glyphosate
Poisons Crops and Soil, SiS 47). The unprecedented rise in GT crops
has been accompanied by a sharp increase in the use of the glyphosate
herbicides worldwide, especially in the US  GM Crops Increase
Herbicide Use in the United States, SiS 45).
The ecological disaster has been unfolding amid mounting
evidence of the herbicide’s adverse impacts on human and animal health [3, 4] (Glyphosate Herbicide
Could Cause Birth Defects, Ban Glyphosate
Herbicides Now, SiS 43), and the breakdown of the Roundup Ready (RR)
cropping system as weeds and superweeds become resistant to the herbicide [5, 6]
(GM Crops Facing
Meltdown in the USA, Glyphosate
Resistance in Weeds - The Transgenic Treadmill, SiS 46) .
How glyphosate works
(Figure 1) is a broad-spectrum herbicide initially patented by Monsanto in the
1970s under the trade name Roundup.
Figure 1 Glyphosate
plants by binding to and inhibiting the enzyme
5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) of the shikimate pathway
for the synthesis of the aromatic amino acids, phenylalanine, tyrosine and
tryptophan. These amino acids are essential building blocks for all proteins,
and also precursors for growth factors and phytoalexins, compounds involved in
the plant’s defence against diseases [8, 9]. Animals do not have the shikimate
pathway and depend on getting the essential amino acids from their diet.
GT plants depend
on incorporating an EPSPS from the soil bacterium Agrobacterium tumefaciens
(which causes crown gall disease) that is insensitive to glyphosate, and hence
not killed by the herbicide.
For a long time,
glyphosate has been promoted as the safest and most environmentally benign
herbicide available. But glyphosate has many other effects that act
synergistically on crop health and productivity that extends well beyond the
plant into the soil ecosystem and the wider environment.
The first hints
of these effects came from observations that glyphosate application greatly
increases the severity and incidence of plant diseases, not just in the GT
crops, but also in subsequent crops grown in the same soil.
increases plant disease by several mechanisms that weaken the plant and its defences
against disease, and at the same time boosts the virulence of pathogens and
their populations in the soil. What makes glyphosate such a strong herbicide is
that it is translocated throughout the plant to the growing points of shoots
and roots to make the susceptible plants stop growing. In the roots, glyphosate
is exuded into the rhizosphere (the soil surrounding the roots) where it exerts
powerful effects on the microbial community and soil chemistry.
Glyphosate robs plants of nutrients
Chemically, glyphosate is a strong chelator
(binder) of metal ions, rendering these essential nutrients unavailable in the
soil and within the plant. Glyphosate was patented as a strong chelator in
1964. In contrast to many chelators that bind specific metal ions, glyphosate
is a broad spectrum chelator that binds both macro and micronutrients such as
Ca, Mg, Cu, Fe, Mn, Ni and Zn. It is that which makes it a broad spectrum
herbicide as well as a potent antimicrobial agent, as the function of numerous
enzymes depends on specific metal co-factors .
The EPSPS enzyme, for example,
requires manganese (Mn) as co-factor, as does 25 other plant enzymes, and
glyphosate reduces the availability of Mn for all of them. In addition, because
the herbicide also chelates other metal ions, it interferes with a wide range
of biological functions, thereby weakening the plants, making them more
susceptible to diseases, and reducing productivity. This applies to GT as
well as non-GT plants. Thus, GT plants are weakened, though not killed by
the chelating action of glyphosate, which accounts at least in part for the
reduced yield of GT crops.
As glyphosate is exuded through
the roots of GT crops or dying weeds that have been sprayed, the herbicide is
rapidly taken up by other plants or immobilized in the soil by binding to metal
ions, so that the ions are no longer available for uptake by the plants.
Glyphosate can remain in the soil for a very long time, which is just as well,
because its degradation products are toxic to GT plants and non-GT plants. But
that means crops subsequently grown on the same soil will still be exposed to
high levels of the herbicide, and the effects accumulate as more herbicide is
Glyphosate reduces nitrogen fixation
Glyphosate reduces nitrogen fixation by
several mechanisms. Nitrogen-fixing bacteria such as the soybean symbiont, Bradyrhizobioum
japonicum, possess a glypohsate-sensitive EPSPS, and hence fail to grow
when exposed to glyphosate. This may be another significant factor in the reduced
growth and yield of GT soybean.
N-fixation is also affected
indirectly via the physiology of the host plant. Glyphosate inhibits the
formation of the growth factor Indoylacetic acid IAA in GT soybeans, which
leads to lower root nodule formation by the symbiont. Glyphosate forms several
metabolites such as aminomethlphosphonic acid (AMPA), sarcosine and glycine.
Chlorotic symptoms of GT soybean following glyhosate application have been
attributed to the accumulation of AMPA.
Nickel (Ni) is involved in N
fixation via the Ni-requiring hydrogenase activity that recycles hydrogen to
provide energy for nitrogen fixation. So lack of Ni in soils due to glyphosate
chelation can limit the sympbiotic bacteria’s hydrogenase activity. When
hydrogenase is inhibited, some 30 to 50 percent of the energy supplied to
nitrogenase can be lost as H2, greatly diminishing the efficiency of
Tests were carried out at the
State University of Maringa, Parana, Brazil, in both a clayey and sandy soil
with seeds of near-isogenic normal and GT soybean varieties treated with
fungicide . The tests showed that glyphosate reduced N fixation independent
of soil type and cultivar.
Shoot and root dry weights were
both reduced by glyphosate application. AMPA damage to chlorophyll could reduce
shoot growth. Glyphosate treated plants exhibited chlorotic symptoms (yellowing)
compared to plants without glyphosate, reflecting damage to chlorophyll and
decreased photosynthetic rate.
Chlorophyll content in leaf was
indeed lower, probably due to direct damage by AMPA or the chelation of Mg
components of chlorophyll or Mn involved in electron transfer during
Glyphosate makes crops more susceptible to
disease by reducing Mn availability
Herbicides are known to increase specific
plant diseases since the 1970s.
Glyphosate inhibits EPSPS both
directly and via Mn chelation (see above). Plants with a compromised shikimate
metabolism are predisposed to various plant pathogens, and glyphosate is
actually patented as a synergist for mycoherbicides to enhance the virulence
and pathogencitiy of fungi used for biological weed control. The synergistic
activity of glyphosate weed control in predisposing plants to infectious
organisms has been observed for many diseases and the extensive use of
glyphosate in agriculture is a significant factor in the increased severity or
“re-emergence” of diseases once considered efficiently managed .
The toxic effects of glyphosate
are cumulative, and become worse with continued use, so Mn deficiency is now
observed in areas that were previously Mn sufficient, partly because of the
accumulation of glyphosate itself in the soil and also because glyphosate poisons
and depletes the populations of Mn-reducing soil organisms. Reduced Mn is taken
up by plants, whereas oxidized Mn is not; hence the balance of Mn-reducing
microorganisms in the soil is crucial to Mn availability to plants. The
presence of glyphosate-tolerance gene also reduces Mn uptake and physiological
efficiency (through the accumulation of glyphosate in the plant), and
concomitantly, increases disease severity.
The virulence of some pathogens
such as the fungi Gaeumannomyces, Magnaporthe, Phymatotrichum
and Corynespora, and the bacterium Streptomyces, involves Mn
oxidation at the site of infection (Mn2+ to Mn4+), which compromises
the plant’s resistance via the shikimate pathway; as oxidized Mn is of no use
for the enzymes in the pathway that uses Mn as co-factor.
Some 40 diseases are known to
be increased in weed control programmes with glyphosate and the list is growing
[1, 12], affecting a wide range of species: apples, bananas, barley, bean,
canola, citrus, cotton, grape, melon, soybean, sugar beet, sugarcane, tomato
Corynespora root rot of
soybean, previously considered minor, may become economically damaging in RR
soybean. This fungal root rot is more severe when glyphosate is applied to
soybeans under weedy conditions even though the weed may not be hosts for the
fungal pathogen. The reason is that the weeds serve to translocate and release
more glyphosate into the rhizosphere, to deplete the populations of Mn-reducing
organisms, and diminish manganese availability for plants to shore up their
defence. All that acts synergistically to boost the increase of Corynspora
and its ability to cause disease.
“Take-all” disease of cereal
crops increases after a pre-planting “burn-down” of weeds with glyphosate; and
this has been recognized for more than 15 years. The disease is also increased
when glyphosate is applied to RR soybeans the preceding year compared with the
use of a non-glyphosate herbicide. Again, this is due to reduced availability
of Mn; all factors that reduce Mn availability, such as low soil pH, overuse of
N fertilizers, also increase the severity of the disease. Microorganisms
proposed for biological control of the disease such as the bacterium Bacillus
cereus and fungus Trichoderma konigii are all strong Mn reducers
that increase Mn availability in the rhizosphere. In contrast, addition of Mn
oxidisers increases take-all.
addition to its chelating action that makes Mn unavailable, glyphosate poisons
Mn-reducing and N-fixing organisms in the soil, so the availability of both N
and Mn for crop plants could be markedly compromised.
take-all root, crown, and foot rot of cereals following application may result
from the synergistic effects of reduced resistence due to Mn deficiency,
inhibition of root growth from glyphosate accumulation in root tip, and increase
in Mn-oxidizing organisms and diminished Mn reducing organisms in the soil
increase Fusarium diseases and Fusarium pathogens in soil
Diseases caused by the fungus Fusarium
have increased with the extensive use of glyphosate . For example, glyphosate
use predisposes tomatoes to Fusarium crown and
root rot. Cotton growers in Australia and the Western USA have seen a
resurgence of Fusarium wilt since the introduction of Roundup Ready cotton,
and previously high levels of wilt resistance appear to be less effective under
glyphosate management Fusarium head scab of cereals and other Fusarium
diseases increase follow glyphosate applications. Head scab and the mycotoxins
produced by the causal fungi are now prevalent in cooler areas where they were
rare before the extensive use of glyphosate.
The Palouse area of Washington, Idaho, and Oregon in the US has had a long history of pea, lentil, and wheat production on the deep loess soils
characteristic of the area. However, pea and lentil yields have been in slow
decline as symbiotic nitrogen fixation is reduced and Fusarium diseases
increased with the extensive use of glyphosate for no-till wheat production. Pea
and lentil production are now uneconomical in some farms, and production is
rapidly moving from the Palouse to Montana where glyphosate use has been more limited.
A new Fusarium wilt of canola caused by F. oxysporum and
F. avenaceum has severely reduced yields in the nutrient poor soils of Alberta and Saskatchewan in Canada since 2000.
Sudden death syndrome (SDS) of soybean reached “epidemic
proportions” in North and South America in the late 1990s . It remains
widespread in the soybean-growing regions in the US, Argentina and Brazil, and is caused by two distinct species: F. viruliforme in North America and F.
tucumaniae in South America. The
increased use of glyphosate was identified with SDS, especially in years of
increased rainfall .
dating back to the early 1980s revealed that the herbicidal efficacy of
glyphosate is largely due to the colonization of roots of affected plants by
soil-borne pathogens (rather than the inhibition of aromatic amino acid
synthesis as originally thought). The two most important pathogens in this
regard were the fungi Pythium and Fusarium, both ubiquitous in
Robert Kremer, a microbiologist
with the USDA, and his research team has conducted studies in Missouri from
1997 to 2007 to assess the effects of glyphosate on GT soybean and GT maize
root colonisation, and soil populations of the fungus Fusarium along
with other rhizosphere bacteria . They found that the roots of GT
maize and soybeans treated with glyphosate were heavily colonised by Fusarium
compared to GT and conventional crops not treated with glyphosate. Their
findings suggest that glyphosate exuded by the plant roots may even serve as a
nutrient source for fungi and stimulate the germination of Fusarium
spores, as Fusarium dominated the fungal community of the rhizosphere after
long term exposure to glyphosate.
increase in Fusarium was detected in just two weeks after the application
of glyphosate at recommended doses, and was two to five times higher in GT
soybean treated with glyphosate than non-treated GT or conventional soybean (Figure
1, top). GT maize treated with glyphosate had Fusarium colonisation three
to ten times higher than when the herbicide atrazine was used instead (Fig. 2,
Fusarium colonisation of
glyphosate-resistant (GR) crops with and without glyphosate applications 
Another study based on crop field surveys and large-scale
experiments in Saskatchewan, Canada, singled out glyphosate application as the
most important factor in the development of crop diseases. Head blight in
barley and wheat crops caused by Fusarium graminearum was especially
widespread. Fusarium graminearum was the most common pathogen isolated
in the four year study, infecting 41.3 percent of the common and durum wheat
analysed. According to the study , “...growing susceptible crops under
minimum-till management in fields where glyphosate has been previously applied,
resulted in the most damage from FHB (Fusarium Head Blight) in years conducive
to disease development.” This is especially worrisome as Roundup crops were
designed specifically for farmers to avoid tilling and due to this “convenience”
they may actually be inadvertently preparing their crops for head blight.
personally went to USDA administrators urging the agency to publish a news
release on the numerous studies confirming the environmental impacts of GT
crops and glyphosate, but was ignored. “Their thinking is that if farmers are
using this (Roundup Ready) technology, USDA doesn’t want negative information
being released about it,” he said .
Glyphosate kills beneficial
microorganisms and increases pathogens
Research published in 1979 already showed
that glyphosate absorbed through plant foliage after application was
transported systemically toward the roots and eventually released into the
rhizopshere  where it changes the whole ecology of the soil, resulting in increased
colonisation of plant roots by pathogenic species such as Fusarium and Phytophorthora,
as well as Pythium in bean plants.
In addition, glyphosate
increases excretion of substrates from roots that may be selectively
metabolized by pathogens, such as amino acids and glyphosate itself, thus encouraging
these pathogens to flourish. In the meantime, glyphosate and its breakdown
products such as AMPA are poisonous to susceptible organisms, many of which are
important multifunctional bacteria in the rhizospere that produce numerous
secondary metabolites that suppress harmful microbes such as fungal pathogens,
including Fusarium, and contribute to reducing Mn, thus making it available to
the plants. Glyphosate and GT soybean significantly decrease these beneficial Pseudomonas
species in the rhizosphere, thereby further encouraging the growth of the
fungal pathogens by suppressing their bacterial antagonists.
oxidation and reduction are primarily carried out by bacteria in the
rhizosphere, and have a major impact on plant nutrient availability and
metabolism. A low ratio of Mn reducers to Mn oxidizers was found for GT soybean
treated with glyphosate compared with non- GT soybean, suggesting less Mn is
available for the plant. In addition to suppressing Mn reducing Pseudomonas,
glyphosate enhances Mn oxidizing bacteria, most likely Agrobacteria that
typically form biofilms on the soybean root surface. The oxidized Mn are
retained within the biofilm. These Agrobacteria probably have glyphosate-insensitive
EPSPS, similar to the Agrobacterium tumefaciens that supplied the enzyme
for GT soybeans.
As mentioned earlier,
glyphosate also inhibits the growth of the N-fixing symbiont of soybean.
Glyphosate poisons the soil for all
Glyphosate released through the roots of dying
plants (weeds) is transferred to living plants not treated with glyphosate via the
roots, suggesting that glyphosate applied to weeds or other vegetation in
orchard alleys may be similarly transferred to trees causing disease and yield
losses . There is evidence that such transfer to other plants via the roots
of dying plants is much more effective than direct spraying into the soil.
A green house experiment showed
that Roundup Ultramax (from Monsanto) sprayed on ryegrass at killing doses was
much more effective in inhibiting the growth of sunflower seedlings sown in the
same soil than the same amount of herbicide mixed directly into the soil .
The growth inhibition was most pronounced when there was no lag time between
the application of the herbicide and the sowing of the sunflower seeds. The
growth inhibition effect wore off between 7 to 21 days after herbicide
application, though it did not disappear entirely. With no lag time before
sowing the sunflower seeds, growth inhibition was 90 percent when herbicide was
applied to ryegrass, compared to 50-70 percent when herbicide was mixed into
The increase in
shikimate with glyphosate application through ryegrass weed was 10 to 100 fold
that of direct application to soil. Glyphosate poisoning of sunflower seedling
was associated with an impairment of the manganese-nutritional status, which
was still detectable after a waiting time of up to 21days (see Figure 3).
3 Effect of glyphosate application through weed compared with application
directly to soil 
In most plant
species, glyphosate is not readily metabolized and is preferentially
translocated to young growing tissues of roots and shoots, where it can
accumulate at substantial concentrations. This can easily create hotspots in
the soil containing high levels of glyphosate. The findings of the study are in
line with field observations of plant damage in winter wheat in no-till systems
after glyphosate applications before sowing and lag times shorter than two
It is clear that GT
crops and accompanying glyphosate use are serious threats to the sustainability
of agriculture and food production. These extensive ecological impacts of
glyphosate and the Roundup Ready cropping system should be seen in the light of
accompanying threats to human and animal health that already justifies a global
ban on the use of the herbicide . It is indeed time to stop both
glyphosate and glyphosate GT crops, and indeed all GM crops and concentrate
resources on localised organic agricultural systems that have proven to work
sustainably and more productively that chemical industrial agriculture (see  Food Futures Now: *Organic
*Sustainable *Fossil Fuel Free).
There are 7 comments on this article so far. Add your comment
|Dean Mindopck Comment left 27th May 2010 12:12:46|
work sustainably and more productively that chemical industrial agriculture (see  Food Futures Now: *Organic *Sustainable *Fossil Fuel Free).
Typo in above fragment. Suggest replace "that" with "than".
|Peter Brenton Comment left 30th May 2010 12:12:49|
Now lets get everyone concerned about the use of Round-up and the fact it is produced by the organic world's biggest threat Monsanto , to campaign strongly against its presence in foodstores and garden centres
|Ed flugel Comment left 26th November 2010 08:08:01|
I wish I knew about this in the spring, yes I have a well. I will need to change weed practices.
|Mae-Wan Ho Comment left 9th June 2010 08:08:49|
John wilson, yes, glyphosate can become unbound when the soil is rich in phosphate, as when manure or artificial fertilizer has been added, then glyphosate leaches from the soil to contaminate ground/drinking water.
|John Wilson Comment left 9th June 2010 08:08:23|
Many thanks for this extremely thorough and hard hitting article. One query.
I have read that glyphosate can become unbound from soil particles. i.e. It may not be permanently immobilised by metal ions in the soil. Could the glyphosate then enter soil water and eventually migrate into streams and rivers ?
|Ai Wai Comment left 19th August 2011 09:09:19|
Oh dear... I should have read this a year ago. I sprayed my durian orchard with Roundup to get rid of 'lalang' (Imperata cylindrica) a noxious weed that spreads through underground rhizomes. Then I observed defoliation, death of branches, and phytophtora infestation on some trees. The problem is there don't seem to be other weedkillers than can effectively eradicate the particular weed - slashing makes it even worse.
|Abdur Rashid Comment left 1st December 2011 18:06:56|
I am presently doing some experiments and I require time to comment here.