See:
Apple (Malus spp.) - Bitter Pit
Cause Nonparasitic. Cork spot is a physiological disorder resulting from calcium deficiency in the fruit. It can develop within the first month or two of cold storage as well as on the tree before harvest. Low levels of calcium in the fruit are due to competition with shoots or high fruit load for calcium, which may be aggravated by weather conditions. Hot, dry weather in July or August tends to increase the incidence. Irregular irrigation may also increase cork spot, however, summer deficit irrigation reduced size and bitter pit in 'Honeycrisp'. Heavy dormant-season pruning, overthinning, and excessive nitrogen fertilizer promote the problem. Injury to trunks, such as winter freezes, interferes with calcium movement.
Symptoms Small sunken areas that are greener than the surrounding tissues, especially near the calyx end. The d'Anjou pear gets a bumpy uneven appearance on the lower half of the fruit. These areas may be more yellow than other areas. Brown corky lesions can be seen in the flesh when cut but may be symptomless on the surface. Depth and shape of the corking spots will give diagnostic clues as to the cause. Cork spot (calcium deficiency) has spots deeper into the fruit than a brown marmorated stink bug could reach with its mouthparts (1 cm or less from the surface). Stink bug caused cork spots are shallow and have irregular (but not square) shape. The boron deficiency spots tend to be square and flat-bottomed.
Cultural control For Anjou, lower calcium rates are recommended than those for correcting bitter pit in apple because there is a risk of marking the fruit.
- Prune lightly by thinning out branches rather than by heading.
- Head new shoot growth in summer on an overly vigorous tree. This may be done up until a month before harvest.
- Apply five (5) summer sprays about 1 month apart of calcium chloride or calcium nitrate at not more than 5 lb/100 gal. Add a surfactant to all sprays to reduce the possibility of burning. Higher rates will burn foliage and sometimes fruit. Sometimes calcium nitrate interferes with fruit color formation. Do not apply in hot weather. Spray must cover the fruits.
- Where possible, use overtree sprinklers to cool trees in unusually hot weather.
- Bring soil pH into optimal range of 6 to 6.5 and avoid excess nitrogen.
- Crop load management is important such as the use of chemical thinning to avoid alternate-year bearing.
- Avoid damaging trunks.
- Maintain a constant soil moisture supply during initial fruit development. Reduce irrigation 60 days after full bloom to avoid excessive fruit growth.
- To avoid the condition in stored fruit, rapidly cool fruit after harvest then store at low temperatures, high humidity and under controlled atmosphere.
Reference Ferguson, I.B., and Watkins, C.B. 2011. Bitter pit in apple fruit. Horticultural Reviews, 11:289-355.