Diagnosing frost in cereals
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General symptoms
Frost damage is affected by many factors and depending on the particular situation symptoms may vary widely. Crops tend to be damaged unevenly and not all plants will show obvious symptoms.
When plant tissue is frozen, ice crystals rupture the cell walls and membranes. Alternatively the cells freeze without structural damage, but the cells can be killed by dehydration. When frozen tissues thaw they take on a dark, limp and water-soaked appearance, similar to frozen lettuce. Several days after the frost, the tissue starts to dry out and turns straw-brown prematurely.
Frosted plants remain high in sugars and nutrients because they have no grain to fill. As a result they are often infected by microbes, which can turn the crop black or brown.
Stem damage
Freezing damage to the stem can occur any time from stem elongation through to mid grain filling. One of the most characteristic symptoms of frost injury is the shrunken, distorted or blistered sections on the stem between nodes before heading or the 'white ring' seen on the peduncle (the stem above the highest node and below the ear). Freezing sometimes starts in the water that sits around the collar of the flag leaf, near the stem. For several days after the frost, the ring is pale-green rather than white. Stems can be damaged just above the nodes and this is usually seen as blistering, cracking or shrivelling. Peduncles damaged just above the highest node can be pulled out from the plant by the head more easily than normal stems.
Nodes themselves, can also be damaged. When nodes or stems are affected on one side, their subsequent growth can be distorted, causing the stem to bend or twist.
Stem damage may not completely disrupt flowering and grain filling. However, the stem is weakened at the point where it was frozen and if the crop is subjected to strong winds it may fall over and make harvesting difficult.
Head damage
The most visible type of head damage is the shrivelling, bleaching or dwarfing of florets.
If the head is partially emerged during the frost, part or all of the whole head may be blighted. Whether the tip, middle or base of the head is damaged depends on how far the head was emerged from the boot during the frost. Blighting usually affects only a small proportion of the heads and other head damage is much less conspicuous. The reproductive parts of the floret (the anthers and undeveloped grain) are the most susceptible. These are frequently frozen without any obvious external symptoms.
Damage can sometimes be gauged soon after the frost by looking at the crop into the sun. The frozen heads appear more translucent than normal and the glumes and awns are spread out like a fan. Several days after the frost, the colour of damaged heads begins to fade and several weeks later, the affected parts of the crop are easily recognised as lighter areas.
The only way to examine the reproductive parts of the head is to peel back the glume of an individual floret. A magnifying glass can help when inspecting the reproductive parts.
A normal undeveloped grain is white and it feels crisp when removed from the floret and squeezed between the forefinger and thumb. Damaged undeveloped grains appear dark green or brown, partially translucent and they feel spongy.
Frost damage to anthers is difficult to recognise. Anthers normally change from pale-green to bright yellow before they are expelled from the floret. They normally become pale-yellow or white a day or so after flowering. Not all anthers are expelled from the floret at flowering.
Frost damage during grain filling can cause grain shrivelling.
For more information refer to Diagnosing stem and head frost damage in cereals.
Other similar symptoms
Similar symptoms can result from causes other than frost:
Stem and head distortions can be caused by the ill timed use of hormonal herbicides. Check your spray records.
Copper deficiency causes grain infertility. In contrast to the sporadic nature of frost damage, copper deficiency affects all heads in the crop. Have the crop tissue tested if the copper status of the paddock is unknown.
Drought and hot dry winds can cause blighting at the tips of heads. Frost will damage different parts of the heads, not just at the tip.