Bitter pit is a physiological (abiotic) disorder associated with low calcium levels in fruit.
Identification
Apples develop bitter pit while on the tree. Symptoms may appear on mature fruit before harvest, but symptoms most often develop during storage and become most apparent after 1 to 2 months of storage.
At the earliest stages of development, bitter pit lesions are small, water-soaked spots on the skin, commonly near the fruit calyx. These spots gradually become darker than the surrounding fruit skin. As the tissue underneath dies the spots turn brown. The spots can be from 1/16 to 3/8 inch (1.5 to 9 mm) in diameter, depending on the apple variety.
The shallow, dark brown areas beneath these spots resemble small bruises. Over time the spots become slightly sunken and the underlying dark brown tissue dries and becomes spongy.
Disorder Development
Development of bitter pit is influenced by the apple variety, amount of fruit set, climate, and tree vigor. Fruit on vigorously growing trees can have a high incidence of bitter pit because calcium is diverted away from the fruit to the rapidly growing leaves. Even when a tree is adequately supplied with calcium it can have calcium-deficient fruit if vegetative growth is too vigorous.
Low calcium levels in fruit eventually cause cells surrounding vascular bundles near the fruit surface to die and turn brown. The closer a fruit on the tree is to vigorous leafy growth the greater is its potential to develop bitter pit. As a result, fruit on upright, vigorous branches are more likely to develop bitter pit than fruit from spurs on horizontal wood or near the tree's main limbs.
Other conditions that cause calcium to concentrate in the foliage at the expense of the fruit include poor fruit set and hot, dry growing conditions. In hot, dry climates the diversion of calcium into the foliage is a result of high evapotranspiration rates. Once calcium gets into leaves, this mineral does not readily move to fruit.
Damage
Bitter pit causes brown spots on the skin and in the flesh of apples. Discolored tissue becomes brown and spongy. This makes the fruit unappetizing.
Apple varieties vary in susceptibility to bitter pit. In California, Gravenstein is highly susceptible. Golden Delicious, Granny Smith, Jonathan, and Yellow Newtown are also susceptible. Red Delicious is less susceptible and McIntosh and Rome Beauty are not very susceptible.
Granny Smith can also develop a form of bitter pit called lenticel blotch pit. The lesions are more prominent but generally smaller in lenticel blotch pit than in bitter pit and do not extend below the surface of the apple. However, the pattern of development and treatment for the two forms of the disorder are the same.
Solutions
Cultural practices can influence calcium availability to fruit, but no practice completely prevents bitter pit symptoms. Irrigate carefully to maintain a constant supply of adequate soil moisture.
Cultural practices that reduce excessive vegetative growth and increase the fruit-to-foliage ratio may reduce the development of bitter pit. Early thinning or thinning excessively may increase the abundance of bitter pit. However, delayed thinning can cause reduced fruit size.
High tree vigor that results in excessive vegetative growth commonly occurs in young trees that are just beginning to bear. It also develops in older trees that are heavily pruned during the dormant season, lost their crop due to frost or other conditions, or receive excessive application of nitrogen fertilizer.
Prune trees appropriately. Dormant pruning can result in fewer and larger fruit and this increases the potential for bitter pit. Summer pruning of vigorous young trees may help to control this malady by reducing tree vigor and the amount of foliage in relation to the number of fruit. However, summer pruning can increase the risk of sunburned fruit. In hot, dry areas sunburn poses a greater threat than bitter pit.
Do not fertilize trees excessively. At least part of the nitrogen fertilizer can be applied after harvest but well before leaves fall off trees. Heavy nitrogen applications while the fruit are on the tree or excessive use of potassium fertilizers may increase bitter pit.
Applying dilute sprays of calcium chloride three times at 1-month intervals beginning in mid-June can reduce the development of bitter pit. Follow the product label for how much to apply. If sprays are applied more frequently than three times per season, this increases the risk of fruit russeting (brown scarring of skins) and leaf burning (browning). Calcium chloride sprays must thoroughly cover the fruit to be effective.
Harvesting and storage practices also affect the development of bitter pit. Do not harvest fruit too early. Cool fruit promptly after harvest. If the fruit are cooled too slowly or are not stored at the proper temperature, bitter pit symptoms may increase. If possible, the storage temperature should remain constant. Except for McIntosh apples, which should be stored at around 36°F, most apples store the longest if held at 30° to 32°F.
References
Adapted from Integrated Pest Management for Apples and Pears and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).
