Freeze damage and frost tolerance thresholds for flowers of the lowbush blueberry (Vaccinium angustifolium Ait)

The wild lowbush blueberry is produced commercially only in the Canadian Atlantic Provinces, Quebec, and the state of Maine. The crop is highly susceptible to frost. Flowers develop at a time when frost events can result in a loss of production since even pollinated flowers fail to set fruit when critical temperatures are reached. Two year-old open-pollinated blueberry seedlings were subjected to various duration x temperature combinations under controlled conditions at both closed-flower and full-bloom stages to obtain quantitative information on cold tolerance. Following exposure flowers were hand pollinated to assess the effects of freezing on fruit set, size and apparent seed viability. Visible damage to flowers was inversely related to temperature and increased with duration of exposure. Damage varied with flower stage. For example, 4 hours exposure to -3.5degreesC resulted in 80% and 60% moderate or severe damage to open, and closed flowers, respectively. Subsequent development was severely affected; 75% of closed flowers exposed to -3.5degreesC for 4 hours failed to open. There was virtually no fruit set from flowers which had been exposed to temperatures below -3.5degreesC. Overall percentage fruit set from flowers that were closed at the time of frost was lower than that from open flowers due mainly to the inhibitory effects of sub-freezing temperatures on flower opening. Neither fruit size or apparent seed viability was affected by either temperature or duration of exposure.