Diagnosing halo blight in oats

There are two types of bacterial disease which infect oat foliage; halo blight (Pseudomonas syringae pv. coronafaciens) and stripe blight (Pseudomonas syringae pv. striafaciens).

Straw or brown colour spots surrounded by a yellow water-soaked halo than may resemble septoria
Lesions turn brown and join together to form irregular blotches
Close up image of oat leaf showing symptoms of halo and stripe blights
Image of disease spread by vehicle movement through an oat crop

Bacterial blight is most common in continuous oaten hay crops and is prevalent with extended periods of moist weather which facilitates splash of bacteria and provides suitable conditions for infection. The disease is favoured when crop density is high and there has been a high input of nitrogen making the plants soft. As the growing season progresses, plants generally grow away from this disease. Warm dry spring conditions will rapidly reduce spread of this disease.

What to look for


  • Diseased plants may occur in patches, and is worse in wetter areas and wet seasons.
  • Vehicles may spread the disease by driving through wet infected plants.


  • Initially small light green oval water soaked spots on leaves and leaf sheaths up to 10mm in diameter.
  • Centres of spots change to a straw or brown colour surrounded by a yellow water soaked halo.
  • The lesions turn brown and join together to form irregular blotches.
  • Heavy infection leads to withering and death of leaves often from the tip.

What else could it be

Condition Similarities Differences
Diagnosing stripe blight in oats Red-brown leaf lesions; worse in patches, wetter areas Starts in parallel streaks instead of oval lesions with a paler halo
Septoria avenae blotch
Red-brown leaf lesions and old leaf death Lesions are more uniformly distributed on affected leaves, and the disease is more uniform across the paddock

Where did it come from?

Contaminated stubble
Contaminated stubble
Insect vector
Insect vector
  • Blight bacteria survive on seed and crop debris.
  • They are spread by rain-splash or leaf contact.
  • Insects, particularly aphids, also spread them.

Management strategies

Stubble management
Stubble management
Clean seed
Clean seed
  • Unless infection is very severe, grain losses are insignificant, but hay quality is reduced.
  • Fungicides are not registered or effective against bacterial diseases.
  • Avoid sowing into infected stubbles and burn or incorporate stubble if the problem is widespread.
  • The disease can be seed borne, do not re-sow seed from infected crops.
  • Avoid paddock operations when leaves are wet to prevent disease spread.
Page last updated: Friday, 19 February 2016 - 12:10pm