SA Graan November 2018

November 2018

reported little mycelial growth and fewer sclerotia on rotting resi-

dues sprayed with glyphosate

Their laboratory-based research findings concluded that glypho-

sate sprays might inhibit the growth of

S. rolfsii

affecting banana

material in the field. Other herbicides such as diuron, atrazine,

simazine and metribuzin were found to have inhibitory effects on

Sclerotinia sclerotiorum

(causal organism of Sclerotinia stalk rot

in soybean) at a certain concentration tested in petri dish studies

Acetochlor treatment was shown to decrease vascular wilt due to

Fusarium oxysporum

in melon

The pre-plant application of glyphosate reportedly reduced the

incidence of red crown rot in soybean

, whilst reduced conidial

germination, mycelial growth and sporulation were found for

F. solani

(causal pathogen of sudden death syndrome). For every

study that points to a potential inhibitory effect of a fungal pathogen,

one would, however, almost always find a different article that was

unable to observe any effect at all. As already stated throughout our

series of articles, care should be taken in extrapolating what hap-

pens in the laboratory to what is observed in the field.

Some studies have indicated that glyphosate can affect growth and

reproduction in vitro, meaning where fungi or bacteria were tested

on liquid or a solid medium in a glass container, but showed adverse

effects in the field

– accordingly a similar situation to the glass-

house phenomenon that Powell and Swanton

referred to. In vitro

studies do, however, have an important role to play as far as research

goes, as they assist with establishing whether the observed effect is

due to a direct herbicide-pathogen interaction, or whether it is rather

due to an indirect effect, which results in the plant becoming less or

more resistant to the pathogen.

Two authors, Sanyal and Shrestha

, concluded in their article pub-

lished during 2008 titled, ‘Direct effect of herbicides on plant patho-

gen and disease development in various cropping systems’ that the

mechanisms of all possible interactions between herbicides and

plant pathogens are clearly not yet well understood. Taking

Rhizoc-

tonia solani

as an example, they state that based on what has been

published internationally, herbicides might have an important role to

play regarding managing this pathogen in crops. It was found that

R. solani

has the capability to utilise some herbicides as an energy

source

, whilst herbicides such as trifluralin, atrazine, paraquat and

alachlor can actually reduce

R. solani

15,16

They, however, continue to state that the contrasting results ob-

tained internationally suggest that the direction of interactions ob-

served between disease severity and herbicides are specific to each

crop-herbicide combination, with various other factors including

crop variety, pathogen strain, environmental conditions and adju-

vants contributing to the observed effects.

Altman and Rovira

probably summarised the situation best dur-

ing 1989. They concluded that it is beyond question that herbicides

affect plant diseases, but since so many factors are altered when

herbicides are used, it remains extremely challenging to identify

specific effects on a particular pathogen or host-plant relationship.

In part 4 of this series, focus will be placed on research findings

published on the impact of glyphosate and other herbicides on

earthworms. Readers are welcome to contact the

authors at ARC-Grain Crops with any enquires they

might have at 018 299 6100.

References

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Effects of herbicide-induced

stress on root colonisation of soybeans by

Macrophomina phaseolina. Plant Dis.

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Kremer, RJ and Means, NE. 2009.

Glyphosate and glyphosate-resistant crop in-

teractions with rhizosphere microorganisms

. Eur. J. Agron. 31, 153 - 161.

Fernandez, MR, Selles, F, Gehl, D, DePauw, RM and Zenthern, RP. 2005.

Crop

production factors associated with Fusarium head blight in spring wheat in east-

ern Saskatchewan

. Crop Sci. 45, 1 908 - 1 916.

Njiti, VN, Myers, OM, Schroeder, D and Ligthfoot, DA. 2003.

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spp

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