Damage processes and symptoms

Losses due to spring frosts are a critical factor in fruit crop production in temperate countries like New Zealand.   Sensitivity of crop plants to spring frosts depends on:

• genetic make up
• stage of development of the plant and its parts
• impact of environmental conditions
• impact of cultural practices on the plant

Frost damage symptoms are expressed in a variety of responses of buds, flowers and fruits, ranging from lethal effects causing abscission to modifications of the characteristics of the ripe fruit. This factsheet outlines these responses to frost and details the main areas to watch for symptom expression in your crop.

Mechanisms of frost injury

Frost injury to apple flowers

Frost injury results from ice crystal formation in bud, leaf, flower and fruit tissues - the low temperatures that cause a frost event do not directly cause injury.  Ice crystal formation can occur either within the cells of these tissues or outside (between) the cells.  Ice formation within cells occurs if cooling is rapid and inevitably results in cell death due to the rupturing of the cell membrane and other cellular structures.  In contrast, ice formation that occurs outside the cells can protect them from injury.  As ice crystals form between the cells, water is drawn out of the adjacent cells.  This increases the concentration of soluble solids and decreases the freezing point of the remaining liquid within these cells.  The protection provided by this process is, however, only temporary and if too much water moves out of the cells, they become dehydrated and die.  Tissue injury resulting from spring frosts usually arises through this mechanism.  As a rule of thumb, drought tolerant plants are most likely to also prove to be relatively frost tolerant.

Plant sensitivity to frost

For a given severity and duration of a frost event, the damage sustained depends upon:

• type and cultivar of the crop
• type and stage of development of the frosted tissue
• the weather conditions that the plants have experienced prior to the the frost

For example, the temperature at which fruit buds on deciduous crops are injured by frost often depends strongly on their stage of development. They are most resistant to frost injury during the winter when they are fully dormant, but as they begin to swell and expand into blossoms in the spring, their resistance declines markedly, primarily due to the increased level of water in their tissues.   Within a single tree, slowly developing buds are more resistant to frost than buds that are growing quickly.  Plants that have been hardened off by experiencing days or weeks of cold conditions prior to a frost are less likely to suffer injury than plants that are actively growing.

Actively growing grape shoots

The amount of water contained in tissue has a substantial influence on its sensitivity to frost.  For a given crop at a given time, new growth is more sensitive to frost injury than less developed tissues.  The more developed tissue tends to have a higher water content and will freeze at a higher temperature. Hence, shoot tips, emerging leaves and developing flowers are more likely to be injured by frost and at higher temperatures than dormant buds.   Dormant buds of deciduous crops can tolerate temperatures as low as -19°C, primarily due to the inability of ice crystals to form in the liquid in the cells, a phenomenon called supercooling.  As the bud develops further, however, its moisture content increases and as a consequence, its capacity for supercooling is lost.  The bud will be sensitive to temperatures only slightly below 0°C and will be killed should temperatures fall to or below that level.

If plants experience cold conditions for a period of days or weeks, the temperature at which freezing occurs is often depressed the plants 'harden off' and solute levels in cells increase and other physiological changes occur (note that little or no hardening occurs in many C₄ plants, palms or tomatoes). A few warm days will remove cold hardiness far more quickly than it is regained.

Like other fruit crops, frost susceptibility of avocado trees is influenced by the environmental conditions in the orchard prior to a frost event with a warm autumn or winter followed by severe cold spells increasing susceptibility to spring frosts. Similarly, if the general health status of the tree is poor due to stress, low nutrition, root rot, heavy crop load or sixspotted mite, susceptibility increases.

Trees with predominantly Guatemalan parentage, i.e. Hass and Reed, should not be planted in areas where the temperature is likely to drop below -2ºC as these trees are less cold tolerant than trees of mixed Mexican and Guatemalan parentage, i.e. Bacon, Fuerte and Zutano.

Within a tree, susceptibility of different tissues follows a consistent sequence:

Bud damage

Even if the trees show no obvious sign of frost damage the new flowering buds can have been killed by the frost. Buds affected by frost will have a brown centre indicating they are dead. Frost damaged trees will drop leaves and fruit. These trees should not be pruned until spring when the full extent of damage will be known. The exposed branches in defoliated trees should be whitewashed as soon as possible to avoid sunburn.

Light frost damage with mild bronzing of leaves, flowers are unaffected deep within the canopy.

Light frost damage (≈ -1°C) typically results in light bronzing on leaves, but fruit stalks light yellow or green (healthy). Flower buds are likely to be damaged but the tree will continue to carry the current crop.

Browning of leaves

Moderate frost damage (≈ -2°C) results in bronzing of leaves, browning of fruit stalks, and some bronzing of the fruit surface. Trees are likely to drop most of their fruit, rendering the fruit unmarketable. Flower buds are damaged and the level of spring flowering will be very poor. Trees should be given a light prune in spring.

Severe frost damage (lower than -3°C) results in severely bronzed leaves, browning of fruit stalks, fruit with some bronzing, and damaged stems. All flower buds will been killed and there will be no flowering likely in spring. Young, 1 year old wood is likely to have suffered frost damage. Trees should be pruned back to 2 year old wood in spring. The trees will be out of production for at least two years.

Damage to fruit and small branches, young trees, and bud damage respectively

Damage symptoms

Frost injury to grape vine shoots and leaves

Symptoms of frost damage do not appear until after the ice formed in the affected tissue has melted.   Leaves show an immediate and dramatic response to frost damage, with a wilted and dark green, slimy appearance immediately after thawing that changes over subsequent days to a desiccated, brown appearance.  For several crops, the immediate external symptom of flower buds killed by a spring frost is a general browning followed by desiccation and bud abscission.  

Injury to other parts of the flower can often go unnoticed due to an absence of external symptoms.  For example:

• Frost injury to the style, ovaries and/or stamens can occur without external symptoms.  Although sepals and petals may continue their development, pistil development ceases and the fruitlets abscise and drop.
• Similarly, frost damage to seeds is often not apparent until the fruitlets start abscising two or more weeks after the frost event.  Seeds produce hormones which are required for the normal development of the fruit.  If the seeds are damaged, those hormones are not produced at their normal levels and the developing fruit will either die and fall from the plant or
• the surviving fruit may be misshapen, small and non-marketable.

The extent of damage to flowers depends on their stage of development and the severity of the frost.

• A severe frost can kill the ovules (immature seeds) and ovaries, resulting in rapid abscission of the flower or developing fruit.   
• If the frost event occurs well before flowering, long term effects of injury may be ameliorated by new cells that arise from the rapid cell division that naturally occurs during this growth phase, with no tissue damage visible.
  

  Frost rings on a pear fruitlet

• Damage immediately prior or during flowering will have a lasting detrimental impact on fruit growth and quality, with callus growth scarring the fruit tissue.  Anecdotal experience from apple growers indicates that frost protection during low risk frost events can significantly increase fruit packouts.

Next steps

This overview will have given you insight into how frost damages plant tissues. You should also have some feeling for the cold temperature conditions that different plant tissues need protection from.

It could be useful to look at the potential timing and magnitute of frosts at your site (HIP32, HIP36) and to review the different passive (HIP33) and active (HIP50) frost protection methods that may be available to you.

This factsheet may be cited as:
Anon. Damage processes and symptoms. NZ Avocado Industry 26 Aug 2009. (accessed 4 Feb 2026) <http://www.hortinfo.co.nz/factsheets/fs85-41.asp>.