Diagnosing contact herbicide damage in cereals

This category includes contact herbicides from a range of chemical groups that require uniform spray coverage to be fully effective.

All exposed parts are affected, more on one side with spray drift
Spot damage on leaves caused by contact herbicide - paraquat (left) carfentrazone (centre) and bromoxynil (right)
Leaf kinking and tip scorch may result from herbicide droplet damage
Sprayed plants rapidly become limp, bleached and scorched

Herbicides in this category include:

  • Bromoxynil and carfentrazone that are used for post-emergent broadleaf weed control in cereals. These chemicals cause mild symptoms such as necrotic (tissue death) leaf spots and streaks.
  • Non-selective knockdown herbicides that cause symptoms ranging from leaf streak and spotting, to death depending on rate, coverage and plant age.
  • Symptoms appear within hours (Group L), or one or two days (Group G, bromoxynil) of application.

Chemical name Example trade name Chemical name Example trade name
Group L - Bipyridyls Group G - Aryl triazalinone
Diquat Reglone Cafentrazone

Affinity®, Hammer®

Paraquat

Gramoxone®

Group G - Pyrimadindione
Diquat+paraquat

Spray Seed®

Butafenacil Knockdown component of Logran

® B-Power®

Group C - Nitrile Group G - Diphenyl ethers
Bromoxynil

Buctril®

Oxyfluorfen

Goal®

 

What to look for

    Paddock

  • Widespread plant bleaching and limpness.
  • Wind drift damage is worst on paddock edges, temperature inversion drift occurs in low lying areas.

    Plant

  • The pattern of damage reflects spray coverage and may be worse on one side of the plant. Plants die or recover with fresh new shoots appearing.
  • Numerous pale necrotic spots sometimes with a brown edge or streaks on the leaves and stems and occasional leaf tip death.
  • More serious damage includes leaf wilting, interveinal chlorosis, marginal leaf burn and leaf death.
  • Severely damaged plants shrivel up within four days of application.

What else could it be

Condition Similarities Differences
Diagnosing abiotic leaf spots in cereals Pale leaf lesions, tip death, leaf kinking Vegetative frost damage have generally less severe symptoms that occur in different parts of successive leaves and may be more likely in frost-prone areas
Diagnosing foliar nutrient damage in cereals Pale leaf lesions, tip death, leaf kinking Plants with foliar nutrient damage recover quickly
Diagnosing group F herbicide damage in cereals Pale leaf blotches and streaks on sprayed leaves Leaf blotches caused by Group F herbicide damage fade without causing lesions

Where does it occur?

Spraying herbicide
Spraying herbicide
Temperature
Temperature
  • Serious damage is generally caused by direct spraying. Contact herbicide spots may be caused by spray drift, but plants will generally recover. Damage declines with distance from intended spray area and spotting will be worse on the side of plants facing the source of the spray.
  • Group G damage symptoms are greater when spraying occurs under humid, high temperature, high moisture, dewy and high sunlight conditions. Plant symptoms will also be greater where adjuvants such as oils are used. There has been no reported yield loss associated with these symptoms.
  • Plants stressed by frost are more susceptible to damage by bromoxynil.
  • Bipyridl herbicides have some movement within a plant if sprayed at night, and consequently cause more damage than when sprayed in daylight.

Management strategies

  • There are no treatment options, plants either die or completely recover. As a precaution, spray when the risk of drift is low or when environmental conditions are safe.

Where to go for expert help

Page last updated: Friday, 26 June 2015 - 8:11am