Grapevines can be injured or killed by a number of agents, including pests, diseases, frost and cold. A vine’s ability to withstand or recover from injury depends on which organs or tissues have been damaged and the health and physiological state of the vine at the time of injury.
As most vines have more leaf area than needed to produce sufficient sugar to ripen a moderate crop yield, perceptible amounts of insect damage to leaves can usually be tolerated and will have no effect on production or quality. However, if total leaf area is low relative to the crop level (C/P is high), vines are under stress from lack of water or nutrients or leaf function has been impaired due to disease, then small amounts of insect damage can impact fruit development and the carbohydrate available to be stored as reserves.
Other causes of leaf injury, including frost and foliar diseases, often damage a large portion of total leaf area which can significantly impact fruit development.
Growers should consider the extent of leaf damage and potential impact to the crop and stored reserves when deciding on controls to prevent leaf damage. When a large portion of leaf area has been damaged before the crop is mature, the crop should be thinned or removed to ensure there will be sufficient carbohydrate stored in reserves over winter to support early growth in the following spring.
Damage to vine buds, especially primary buds, can significantly impact the current year’s crop by reducing the number of shoots and/or clusters.
Cold temperatures in winter, or cutworm grazing in spring, can kill a significant number of primary buds. Often secondary buds will develop and produce clusters when primary buds have been killed, but secondary shoots usually bear fewer clusters, and these may mature later due to their delayed development. Despite these possible impacts to the crop, the development of secondary shoots can be important in maintaining the carbohydrate status of the vine. If winter temperatures reach below -22°C, substantial numbers of buds may be killed, including secondary and tertiary buds.
Bud hardiness, which is the ability of buds to withstand cold temperatures, increases as vines are exposed to subzero temperatures that become progressively colder over a number of days or weeks. Such well-acclimated buds can withstand temperatures as low as -23°C without perceptible damage.
Vines that have been exposed to warm temperatures near or above 0°C for a period can be inflicted with lethal damage to buds and vine tissues if exposed suddenly to temperatures near -15°C. Vine death can result from prolonged exposure to temperatures below -25°C. Susceptibility to cold injury depends on the level of acclimation, the variety, length of exposure and health of the vine.
