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.

Sulfonylureas (SUs)		Imidazolinones (IMIs)		Sulfonamides (TPS)
Chemical name	Example trade name	Chemical name	Example trade name	Chemical name	Example trade name
Triasulfuron	Logran®	Imazapyr	Arsenal®	Flumetsulam	Broadstrike®
Chlorsulfuron	Glean®	Imazapic	Flame®	Metosulam	Eclipse®
Sulfosulfuron	Monza®	Imazamox	Raptor®	Florasulam	Paradigm®
Metsulfuron	Ally®	Imazethapyr	Spinnaker®	Pyroxsulam	Crusader®
Iodosulfuron	Hussar®	Imazapic+Imazapyr	Intervix®
Mesosulfuron	Atlantis®	Imazapic+Imazapyr+MCPA	On Duty®

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.

What else could it be

Condition	Similarities	Differences
Diagnosing glyphosate damage in canola	Discoloured distorted young plants with distorted or dying growing point. Plant death or multiple growing points. Flowering spike death.	Group B herbicide damage can be from soil residue or trace contamination. Also causes less purple-red colours, internode shortening, and bunchy pod spikes
Diagnosing Group A herbicide damage in canola	Distorted flowering spike	Plants dark green, and don't die back. High rates needed.
Diagnosing calcium deficiency in canola	Flowering head death	Calcium deficiency has no leaf symptoms and plants usually recover
Diagnosing waterlogging in canola	Seedlings with pale to reddened, cupped cotyledon	Growing points in Group B herbicide affected plants die before the rest of the plant

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.