At this year’s All About Fruit Show, many home growers had bitter pit problems in apples and Asian pears. Bitter pit can affect all apple and Asian pear varieties but some are more prone than others. The main cause is a mineral imbalance, low levels of calcium.
Even if calcium is present in the soil, it does not readily move into the plant, it requires moisture and many trees are not irrigated. Once the calcium enters the plant by root tips, it does no move well in the plant. When calcium does move it is directed to the growing tips of the tree and fruit. All of the growing tips compete with the fruit for calcium, therefore, fewer tips more calcium for the fruit and less bitter pit. Remove excessive growth, such as suckers and water sprouts, as you see them all summer, until mid August.
Bitter pit starts internally and eventually causes external blemishes. Internal lesions occur anywhere in the tissue from the core line to the skin, but more common just below the skin. The lesions are small, brown, dry, slightly bitter-tasting and about 3-5 mm. in diameter. As the affected cells die, they lose moisture, become “corky” and the skin over the area sinks in a round or slightly angular pattern. The skin over the pits gradually becomes slightly brown or sometimes black. For the first time I saw one apple variety that had red circles above the bitter pit site. At first, I thought it was a San Jose scale infection by the color. However, the fruit had a sunken spot in the center of the red circle. When cut, the tissue was starting to turn brown beneath the skin.
Avoid and remove excessive tree growth, many times caused by excessive nitrogen fertilizer application and excessive pruning in winter this can minimize the frequency and severity of bitter pit. Avoid irregular watering, large fruit, light crop load and high levels of potassium and magnesium.
Add calcium in the form of agricultural lime, it does not contain as much magnesium as dolomite lime. The incidence of bitter pit may also be reduced and sometimes controlled by several applications of calcium post-harvest calcium drenches. One product, Nutra-Plus Calcium 8% can be applied every 10 to 14 days during the growing season. This is how commercial growers control bitter pit.