Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Cork spot and bitter pit are disorders in apples that occur when the calcium levels in the fruit are low. Cork spot is seen on the outer surface of the fruit as depressed green spots that enlarge to form corky dark colored areas about ¼ inch deep in the fruit. Bitter pit is found as brown areas of collapsed tissue under the apple skin on the blossom half of the fruit about 1/8 to ¼ inch in size. Bitter pit can occur at harvest but more often develops in storage.
Calcium is important for the development of cell walls and the “cement” between fruit cells. Low calcium in soils is rarely a cause in properly limed orchards. More commonly, it is conditions that impact root activity and that impact the movement of calcium in the soil and in the plant. Dry soils and drought conditions can be a major factor restricting calcium uptake. However, a wet year, such as we are having in 2013, can also affect calcium uptake. Root activity in saturated soils is reduced and cloudy conditions limit transpiration and the movement of calcium in the water flow in the tree. Most of the calcium transport to the fruit occurs in the 4 to 6 week period after bloom. Wet (or dry) conditions during this period will increase the potential for cork spot or bitter pit.
Varieties differ in their susceptibility to bitter pit and cork spot. Some susceptible varieties include York, Red and Golden Delicious, Granny Smith, and Jonathan.
Control of these disorders often requires the use of foliar calcium sprays to the fruit starting after petal fall and continuing through fruit development for 5-8 applications. Other cultural practices that will reduce these disorders include improving orchard drainage, good irrigation management, managing nitrogen (not over-fertilizing), and managing fruit loads with good thinning and pruning practices.