Diagnosing stem and head frost damage in cereals

All winter grains susceptible to frost. Wheat is more susceptible then barley at flowering, but it is not known if barley and wheat have different frost susceptibilities during grain fill. The order of susceptibility for cereals is (most to least); triticale, wheat, barley, cereal rye and oats.

Image 1: Frost damage at booting vs healthy head
Image 2: Frost damage of the peduncle showing a pale green ring
Frost affected anthers are white and distorted banana shaped) turning a dull brown colour
Image 4: Frost affected ovaries turn a dull brown colour and are spongy when squeezed. A frost affected stigma takes on a crumpled appearance
Image 5: Unfrosted to frosted grain comparision
Frosted barley head - floret sterility can be determined by raising the head so the sun is behind it. Light comes through florets where grain is absent.
Image 7: Frost damaged oat head showing aborted floret damage
Peel back the husk to inspect the condition of the floral organs in the head

What to look for in wheat

Paddock

  • Symptoms may not be obvious until 5-10 days after a frost.
  • Heads on affected areas have a dull appearance that becomes paler as frosted tissue dies.
  • At crop maturity, severely frosted areas remain green longer as its likely the plants have reshot to compensate for the damage.
  • Severely frosted crops crop have a dirty appearance at harvest due to blackened heads, stems and discoloured leaves.

Plant

Leaves, stems, florets and grain can all be affected by frost. In some circumstances, a plant may suffer both stem, flowering and grain frost, especially if a series of frost events occurs. The nature of frost damage depends upon the plant development stage at which the frost occurs.

Head emergence (Z31-45)

When the head is developing and  emerging from the boot, cold air or water is caught next to the flag leaf, or travels down the awns into the boot, freezing the sensitive tissue. This can cause pollen abortion, resulting in undeveloped florets or bleached florets (Image 1).

Stem

Stem frost occurs when a small amount of water settles inside the leaf sheath above the penultimate node and adjacent developing tissue of the boot. This is likely to happen if a light shower of rain occurs at dusk and then a frost follows. The damaged section of the stem then moves up, as the peduncle continues to elongate with flowering. Frost damage of the peduncle will have a pale green ring. Physical damage can be seen as a flattened peduncle or felt by running a finger down the stem.

Flowering

A frost event during flowering can cause sterilisation of the floret, as the pollen, ovary or both are damaged. The result is that the head will be underdeveloped and/or have bleached florets with no grain developed in that floret.

  • Healthy anthers before flowering are green to yellow in colour. After flowering (anthesis), they are yellow turning white with age.
  • Frost affected anthers are white and distorted (banana shaped) turning a dull brown in colour (Image 3).
  • Heathy ovaries are bright white in colour when squeezed. As the grain develops it turns green in colour and begins to fill the floret. 
  • Frost affected ovaries turn a dull brown and are spongy when squeezed. They begin to shrivel as no grain develops (Image 4).
  • A healthy stigma is white and feathery until after pollination.
  • A frost affected stigma takes on a crumpled appearance (Image 4).

Grain

Frosted grain at the milk stage is white eventually turning brown, with a crimped appearance. It is usually spongy when squeezed and does not exude milk/dough. Frosted grain at the dough stage is shrivelled and creased along the long axis, rather like a pair of pliers has crimped the grain in the middle (Image 5).

What to look for in barley

Flowering occurs close to the boot, which offers protection to anthers against frost exposure. As in wheat, when a frost occurs it can cause sterile florets leading to absence of grain. Floret sterility can be determine by raising the head so the sun is behind it. Light comes through florets where grain is absent (Image 6). As grain develops, the florets remain closed.

Stem frost in barley is not as common as in wheat but when it does occur it has similar symptoms.

What to look for in oats

Oats are susceptible to frost at stem, pre-flowering, flowering and grain filling albeit less than wheat and barley. Florets hang downwards allowing warm air to be trapped inside, protecting them from frost.

Sterility of florets sometimes occurs as the pannicle is emerging (Image 7).

What else could it be

Condition Similarities Differences
Diagnosing copper deficiency in wheat White, rat-tail heads, shrivelled grain delayed maturity Copper deficient plants are paler with distorted flag leaves. Plants with grain have weak straw
Diagnosing molybdenum deficiency in cereals White, rat-tail heads, shrivelled grain delayed maturity Molybdenum deficient plants are paler, poorly tillered and and often result in plant death
Diagnosing spring drought in wheat and barley White, rat-tail heads, shrivelled grain Differences include death of older leaves rather then plants
Diagnosing take-all in cereals White, rat-tail heads, shrivelled grain Differences include death of older leaves rather then plants

Where does it occur?

Temperature
Temperature
Wet conditions
Wet conditions
  • This can depend on crop type, stage of development and if canopy is wet. It also depends on how low the temperature gets and for how long. Frost is a three stage response, with damage increasing for each stage.
  • 1. Cold damage: occurs when plants are exposed to temperature less than 10°C down to -2°C. If this occurs during pollen development (Z39-45) it can cause spikelet damage.
  • 2. Desiccation damage: when ice formation occurs on the outside of the leaves at temperatures from 0°C to -2°C. Moisture is drawn from the leaves leaving them dry and brittle, subsequently dying at the tips.
  • 3. Freezing damage: usually occurs at temperatures below -2°C when there is rapid ice nucleation and ice crystals form within the leaves. The ice crystals physically rupture cell walls and membranes within the cells causing physical damage. Damage can be seen once thawed as dark green water soaked areas, 10 days after a frost bleached heads might be evident.
  • A canopy that is wet from a light shower of rain is often more prone to frost damage. This is because rain contains ice nucleators such as bacteria or dust. These ice nucleators raise the freezing point of water. As a result, a slightly wet canopy may get frosted at warmer temperatures compared to a dry canopy.
  • Position in the landscape influences temperature variations and the severity of frost damage, with low lying areas more likely to experience cooler temperatures for longer.

Management strategies

  • A comprehensive frost management strategy needs to be part of annual farm planning. It should include pre-season, in-season and post frost event management tactics.
  • 1. Identify frost prone paddocks – with topographic, electromagnetic, yield maps and paddock history.
  • 2. Consider enterprise in a zone – cropping/sheep balance.
  • 3. Review nutrient management – targeted nitrogen, potassium, copper inputs.
  • 4. Modify soil heat bank – stubble levels, crop canopy.
  • 5. Select appropriate crops – oats, barley, wheat, canola.
  • 6. Manipulate flowering times – stage sowing time, mix long and short season varieties.
  • 7. Fine tune cultivar selection – wheat, barley susceptibility during flowering.

How can it be monitored?

  • When: Inspect crops when they are between ear-emergence and grain-fill, and when the temperature drops below 2°C (screen temperature). Damage is usually most evident 7-10 days after a suspected frost event.
  • Where: Examine the crop in more susceptible lower parts of the landscape first and if the crop is damaged proceed to higher ground.
  • How: Walk through the crop and examine a whole plant every 10-20 paces.
  • If the head has not emerged from the boot, check that the head has not been damaged. You will need to carefully dissect the plant from the top down to find the head of the plant.
  • If the crop has flowered, open the florets to check if the grain is developing (Image 8).
  • After a frost event, tag a few heads with tape and note the stage of grain fill. Return a week later to determine if grain development and grain filling is continuing.

Where to go for expert help

Ben Biddulph
+61 (0)8 9368 3431
Page last updated: Monday, 17 July 2017 - 3:43pm