Most fruit trees bloom in Missouri before the frost-free date. When a frost occurs, some or all of the fruit may be lost. The amount of fruit loss is dependent upon a number of factors including the stage of floral development, the actual low temperature reached, and the duration of time at that temperature. The temperatures for the days preceding the frost event and the general health of the plant also influence fruit loss.
As flower buds develop into blossoms, they become increasingly susceptible to frost injury. For example, apple buds at a silver tip stage can usually tolerate temperatures as low as 15°F, but fully open flowers (i.e., full bloom stage) may only avoid injury when temperatures are 28°F or above. Estimated temperatures at which spring frost injury occurs with photographs of the flower stages for apple, pear, peach, cherry, plum, and apricot can be found at: Picture Table of Critical Spring Temperatures for Tree Fruit Bud Development Stages - Fruit & Nuts (msu.edu).
An easy way to determine injury after a frost involves cutting twigs with flowers, bringing them indoors, and placing them in a container with cut ends of stems submerged in water. During the next two days, flower petals, stamen, and pistils that are frost-injured will turn brown and wither. Although all flowers on cut stems may be dead, there may be other, less developed flowers remaining on the tree that are uninjured which will produce a partial fruit crop during the growing season. Young apples and pears that are cold-injured often develop a brown-colored frost ring around the fruit later in the growing season but are still edible (Figure 1).
There are three important steps to follow after a frost kills flowers on a fruit tree. First, do not apply any nitrogen fertilizer during the rest of the growing season. When the fruit crop is lost, vegetative growth is promoted due to the reduced carbohydrate demand by fruit. The application of more nitrogen after a frost will also increase the amount of subsequent summer or dormant pruning.
Secondly, monitor plants closely for disease symptoms throughout the growing season. Some insects are attracted to damaged tissue and some diseases are favored by weakened, frost-injured tissue. For example, peach, nectarine, apricot, plum, and cherry are often susceptible to Luecostoma canker (also known as Cytospora canker) after cold injury. During the growing season, Luecostoma cankers often produce an amber-colored ooze (Figure 2). These cankers are very difficult to control and form on branches and trunks of trees once they enlarge. As the canker spreads, it can kill the affected limb or tree.
Lastly, minimize water stress during the growing season by irrigating fruit trees during dry periods. Apply water slowly beneath the canopy of the tree. Continue to irrigate until mid-November during droughty conditions. Because floral initiation begins in the growing season before the year of flowering, it is important to minimize stress to promote cropping in the subsequent year.
REApril 22, 2021ate -->