Diagnosing group B herbicide damage in canola

Sulfonlyureas and sulfonamides are systemic herbicides that are used for pre and/or post emergent grass and/or broadleaf weed control in cerealsand are highly toxic in canola. Imidazolinones (Imi's) are toxic to non-Clearfield canola varieties. Damage can be caused by soil residue or spray contact.

What to look for

Paddock

Residual herbicide damage shows as pale stunted discoloured plants with worse damage on more alkaline (sulphonylureas) or acidic (imis) soils.
The persistence of TPS products is not influenced by soil pH (unlike IMIs and SUs).
Spray tank contamination causes similar damage that is more uniformly distributed but varies with spray coverage on overlaps or boom turns.
Spray drift damage is usually worse near the source, with similar symptoms before stem elongation. After stem elongation plants have reduced often extended flowering, and pod abortion.

Plant

Severely residue damaged seedlings emerge with thickened petioles and small cotyledons with blotchy pinkish-green surfaces and purple undersides.
Purple to brownish-yellow distorted or cupped emerging leaves develop small elongated blotchy yellow-green blades with elongated petioles.
The growing point may die, with subsequent plant death and plant can slowly die, or multiple new growing points produce bunchy narrow distorted leaves
Spray-affected plants stop growing and initially have pale brownish yellow emerging leaves and minor dead spots on older leaves.
Yellowing and/or reddening develops on leaves as does leaf spotting and leaf crinkling.
Mildly affected plants sprayed after stem elongation develop shortened internodes causing a bunchy appearance.
More severe damage causes flowering stem death. Plants repeatedly send up flowering side branches that also die until the plant recovers or dies.
Purple discolouration appears on the sunny side of stems, and plants can be more susceptible to disease.

Where did it come from?

Soil ph

Wet conditions

Chemical breakdown in soil is inhibited by high pH for sulfonylureas and low pH for imidazolinones.
Stressed plants are less able to metabolise herbicides and therefore more susceptible to damage. Stressors include drought, cold, wet weather, waterlogging, deep sowing, frost, nutrient deficiency and soil compaction.

How can it be monitored?

It is important to observe withholding periods for these herbicides to avoid damage to following crops.

Further information

Department of Environment and Primary Industries Victoria - Avoiding crop damage from residual herbicides [1]

Department of Environment and Primary Industries Victoria - Spraying, spray drift and off target damage [2]

APVMA registered products data base [3]

GRDC FactSheet In-crop Herbicide Use [4]

HerbiGuide [5]

Croplife Herbicide Mode of Action Group Table [6]

Links
[1] http://www.depi.vic.gov.au/agriculture-and-food/farm-management/chemical-use/agricultural-chemical-use/chemical-residues/managing-chemical-residues-in-crops-and-produce/avoiding-crop-damage-from-residual-herbicides
[2] http://www.depi.vic.gov.au/agriculture-and-food/farm-management/chemical-use/agricultural-chemical-use/spraying-spray-drift-and-off-target-damage
[3] https://portal.apvma.gov.au/pubcris
[4] http://www.grdc.com.au/GRDC-FS-InCropHerbicideUse
[5] http://www.herbiguide.com.au/
[6] http://www.croplife.org.au/downloadpdf.php?url=wp-content/uploads/2013/05/2014-Herbicide-MOA-Table.pdf