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Ministry of Agriculture, Food and Rural Affairs

Frost

Identification

  • When injury occurs before leaf emergence:
    • Leaf puckering along veins or margins, not necessarily necrotic.
    • Distorted leaf shape – half the leaf develops normally, the other does not.
  • When injury occurs after green tissue is present:
    • Brown, wilted shoot tips, leaves and flower clusters.
    • Stunted shoot growth.
    • Older leaves may remain alive and display a pattern of angular flecking with sectors of yellow, white and green.
  • When injury occurs after fruit set:
    • Mild frost may produce a corky ring around the surface of young pear fruit, usually near the calyx end; on pears it may produce vertical cracks that russet as the fruit matures
    • Frost occurring from petal fall to when fruit are about 8 mm in diameter may cause internal freezing of the fruit.
    • Externally there may be no symptoms, but as the fruit matures it becomes flattened or distorted by development of internal pockets or dead tissue.
    • On occasion, internally damaged fruit drop from trees a week or two after injury.
    • Sweet cherry fruit exposed to cold can be killed outright.
    • Injury to young peach fruit can result in fruit that never sizes beyond the size of a walnut

Often Confused With
Herbicide injury – contact herbicides applied; lower leaves with localized brown spots

Biology
Frost damage occurs when ice forms inside the plant tissue and injures the cells. This may have a drastic effect upon the entire tree or affect only a small part, which reduces yield or merely product quality. During cold periods, trees acclimate against freeze injury. As buds begin to swell and mature into blossoms, they become less resistant to frost injury. Fully dormant buds are most hardy while, fruit development beginning at  full petal fall are most susceptible. Buds which develop slowly tend to be more resistant.. As bud development starts  in the spring,  cold tolerance decreases.  Therefore, temperatures at which freezing occurs can fluctuate considerably depending on to what extent the trees have started to leave dormancy. This response to freezing temperatures also varies depending on cultivar. A bud that is swollen with the scales still closed may tolerate temperatures of approximately -5º C with little injury.  As soon as green tissue is exposed from the bud, it may be injured by exposure to temperatures of -3 to -1°C. Sensitivity to freezing temperature increases from bloom to fruit set, which is when the crop is most susceptible to frost injury.

Below some tables illustrating when frost damage occurs.
MSUE Table for Frost

Early frost in the fall can also severely impact yield and winter hardiness by prematurely  stopping leaf photosynthesis. This may result in leaf drop before the crop is mature and adequate carbohydrates and other  materials have been stored to promote acclimation.

Bud viability can be assessed by forcing buds to bloom indoors. Cut some shoots with flower buds on them, place them promptly into water and keep them at room temperature for a few weeks. Re-cut the shoot ends every five days or so because the vascular tissue tends to plug up in that length of time. Viable buds should bloom within 10 to 16 days when incubated in water at 21C.

Period of Activity
When trees are at bud swell or later and temperatures go below the critical point. This will vary with growth stage.  Green tissue may sustain injury at -1°C.

Scouting Notes
Look in low-lying areas or orchard locations that are close to obstacles that may impede or restrict air movement.  Cultivars with early budburst are more prone to injury.

Threshold
If less than 10% of the buds or shoots are killed, trees may respond by producing larger than average fruit. 

Management Notes
Choose sites with good air drainage.

Mow row middles; do not cultivate immediately prior to a forecast of freezing temperatures.

Delay pruning to delay bud burst.

Frost hazard is less with moist soil than dry soil.

Use wind machines to mix air, preventing the lower cold layer that can cause freezing of buds or flowers.

Some information included above excerpted from:

Frost ring on Bartlett pear Pear frost rings Frost damage to apricot shoot Frost damage to apricot shoot Frost damage to cherry blossom Frost damage to peach shoot Frost damage to peach fruitlet(note brown colour) Frost damage to pear blossom(note brown colour) Frost damage to pear fruitlet(note brown colour) Frost damage to pear blossom(note brown colour) Frost damage to plum shoot Comparison of healthy and damaged plum blossoms Poor fruit set due to frost in cherries Frost damage on nectarines Click to enlarge.