FROST
Identification
- When injury occurs before leaf emergence:
- leaf puckering, either 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:
- Wilted shoot tips and brown 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
- Undamaged shoots may occur next to damaged ones on the same cane
Often Confused With
Herbicide injury – contact herbicides applied; lower leaves with localized brown spots
Biology
As a bud develops in the spring,it's cold tolerance decreases. A bud that is swollen with the scales still closed may tolerate temperatures of approximately -5º 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. Cultivars will vary in their response to freezing temperatures.
Early frost in the fall can also severely impact yield and vine winter hardiness by stopping leaf photosynthesis and may result in premature leaf drop before the crop is mature and adequate sugars have been stored to promote acclimation.
Period of Activity
When vines are at bud swell or later and temperatures go below the critical point. This will vary with growth stage. Green tissue will sustain injury at -1°C.
Scouting Notes
Look in low-lying areas or vineyard locations that are close to obstacles that may impede or restrict air movement. Cultivars with early budburst (eg. Labruscana and hybrids- Baco Noir, etc) are more prone to injury.
Threshold
If less than 10% of the buds or shoots are killed, vines may respond by producing larger than average clusters. Some hybrid varieties (Marechal Foch, Seyval and Vidal) can produce marketable fruit from secondary buds so these cultivars can sustain significantly more primary bud injury and still produce adequate yields.
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. Retain ‘spare parts’ with extra canes or extra buds and adjust final bud number for proper vine balance once frost risk is passed.
When temperature inversions and frost occur in spring or fall, wind machines can increase temperatures around the vines above the critical temperature for tissue injury.
Frost hazard is less with moist soil than dry soil.
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