Strawberries often bloom in Ontario before the last spring frost. Strawberry flowers are particularly susceptible to frost damage because the plant is low to the ground, and the blossoms open toward the sky. Straw mulch between the rows contributes to lower field temperatures, because it prevents the soil from warming very much during the day. There are three types of frost that can cause damage to strawberry plants: radiation frost, radiation freeze and advective freeze.
Symptoms vary with the stage of plant development, the actual temperature and the length of time damaging temperatures occur. Symptoms can include:
- Pistils in the center of blossoms turn black or brown.
- No fruit develops or misshapen fruit is produced.
- Tips and edges of new leaves may look water-soaked, and then turn brown and dry.
- Sometimes leaf tissue is damaged but not killed by frost; these leaves are misshapen, because the damaged tissue can not expand normally.
- Sometimes the lower leaf surface separates from the upper leaf surface, giving the leaf a crinkled appearance.
Often Confused with
Tarnished plant bug injury (Both tarnished plant bugs and frost damage can cause misshapen fruit. Although tarnished plant bug damage can be distinguished from poor pollination by the size of the seeds, it is not always possible to distinguish it from frost injury in this way. Consider the weather patterns, tarnished plant bug populations and the pattern of injury to distinguish these problems. Damage from frost will be more consistent than tarnished plant bug injury, and associated with a particular set of fruit (i.e. the primary fruit))
Cyclamen mite damage (Misshapen leaves can sometimes be confused with cyclamen mite damage, however, with cyclamen mite damage new leaves are affected, while cold temperatures only affect certain leaves present at the time of the frost event. Identifying the presence of mites also helps to distinguish the two problems.)
Strawberry buds, blossoms and immature fruit can all be damaged by cold. The critical temperature for injury depends on several factors, including the variety, the stage of development and climatic conditions such as temperature, wind speed and duration of adverse conditions. Strawberry flowers are most sensitive to frost injury immediately before and during opening. At this stage, temperatures lower than -2°C (28°F) will cause injury. When flowers are in tight clusters in the crown, they will tolerate temperatures as low as -5.5°C (22°F). The critical temperature for plant tissue at which injury occurs is difficult to measure and varies from variety to variety.
- Growers use irrigation to prevent frost damage to plant tissue. As water freezes, it releases enough heat to protect the plants from frost damage. A thin film of water must be constantly freezing during the frost event. If ice is cloudy , rather than clear, not enough water was applied and frost damage can still occur.
- Frost injury usually occurs to primary flowers because these are the first to open. Primary flowers also produce the largest and most valuable fruit.
- Frost-damaged tissue is very susceptible to Botrytis grey mould. In this case, Botrytis is a problem for a few pickings, as the fruit from frost damaged bloom develops.