A great deal of intensive work must be performed in a commercial apple orchard. Each year the trees should be pruned and fertilized, with nitrogen ＿＿（15）＿＿ , to grow well and bear properly. Other elements such as potassium and magnesium are also needed. Frost, insect pests, and fungus diseases are among the most serious problems apple growers face. Open apple blossoms are killed when the temperature drops a few degrees below freezing, and ＿＿（16）＿＿ are often lost because of spring frosts. Commercial orchards generally are planted on slopes or high ground to reduce this danger, since cold air tends to sink. Some apple growers protect their orchards with heaters ＿＿（17）＿＿ frost occurs in blossom time. Many insects attack the apple tree. One of the worst is the codling moth, which lays its eggs on or near the fruit. When the eggs hatch, the larvae bore into the fruit and a “wormy” apple ＿＿（18）＿＿ . Diseases caused by fungi and bacteria can also infect the flowers, fruit, and leaves of the apple tree. Repeated spraying is often necessary to keep the fruit ＿＿（19）＿＿ insects and diseases. Because pests appear at different times of the growing season, some areas may need 12 to 14 sprays per year. New varieties of apples are now being bred that are resistant to certain kinds of insects and fungi. This reduces the need to spray so often.