Morphological Comparison of Cork Spot and Bitter Pit of Apple Fruit

Abstract

Cork spot and bitter pit are the main spotting disorders of apple fruit in Japan, but it is difficult to separate these two disorders macroscopically, as intermediate symptoms are sometimes found. In this paper, the morphology of both disorders was compared microscopically and macroscopically.
Under climatic conditions of the Morioka district, cork spot appeared at the immature stage of fruit development, from mid-August to early September, while bitter pit appeared at the mature stage and during storage.
When analysed microscopically, it was found that cell collapse in the discolored cortical tissue of bitter pit was more severe than in that of cork spot.
When examined during the harvest season, intact cells in the discolored tissue of cork spot affected fruit were significantly smaller than those of the neighboring sound tissue. This may be due to a retardation of cell enlargement in the discolored tissue after the occurrence of the disorder. In contrast, no difference in cell sizee were found between the discolored and sound tissues in fruit affected by bitter p t which appeared after the mature stage.
Starch grains in the discolored cortical tissue of cork spot disappeared just after the occurrence of the disorder.
Small, specialized cells, approx. 50μm in diameter, appeared in the intercellular space of the discolored cortical tissue of cork spot. In rare cases, a layer of these small cells surrounded the discolored tissue. Formation of these small cells may have been stimulated by tissue injury caused by the disorder. A similar type of small cells was also found arround mechanically crushed cortical tissue.
No small cells were found in the discolored cortical tissue of bitter pit. The ability to induce the formation of small cells may have already ceased before the mature stage of fruit development during which this disorder appeared.
These microscopic differences between cork spot and bitter pit may be related mainly to the stage of fruit development at which each disorder occurred. The ‘intermediate’type was seldom found when using these differences as the basis of classification.
The occurrence of each disorder differed among cultivars and strains. Almost all spots found on ‘Tohko’, ‘Redgold’, ‘York Imperial’and ‘Aori No. 3’were cork spot, and those found on ‘Ohrin’ and ‘Tsugaru’ were due to bitter pit. Both disorders were found on ‘Starking Delicious’, ‘Megumi’, ‘Jonathan’ and ‘Fuji’.
Spots caused by cork spot were usually more than 5mm in diameter, while those caused by bitter pit were generally less than 5mm. However, spots of about 3mm in diameter caused by cork spot, and of about 10mm in diameter caused by bitter pit were sometimes found.