Identification of bitter pit in apples
What causes bitter pit?
Most of the research in this area attributes bitter pit to a lack of sufficient Ca (calcium) (2; 1), but
there are reports indicating the probability that bitter pit is due to excess Ca in the cell (8).
A deficiency in Ca in the fruit can be due to cultural practices, hormonal regulation via fruit-shoot
competition, as well as morphological factors and Ca transport in the plant. This has been reported on
comprehensively by various researchers (2; 11).
When is bitter pit initiated?
•
Initiation of bitter pit in of ‘York Imperial’ apples was recorded early, 4 – 6 weeks after full
bloom, in the season (6),
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Where as others (9) reported initiation late in the season in tissues just beneath the peel.
Where to find bitter pit lesions:
•
Corking disorders can occur in the epidermis, hypodermis and outer cortical regions of an
apple, but bitter pit is usually found in the cortex (flesh), under the epidermis (peel) (3).
•
Lesions may be few in number and dominant on the calyx end or numerous and extend over
much of the surface of the fruit (9).
•
Development of the disorder is characterised by breakdown of the cuticle and changes in the
cells of the epidermis, hypodermis and cortex of the fruit (5; 6).
•
Simons (3) reported a prominent lenticel exterior to a breakdown in the outer cortical region
of a bitter pit fruit.
•
Research (5) showed that early stages of abnormal cell development in corking disorders like
bitter pit in ‘York Imperial’ apples were always associated with senescing vascular bundles.
How to identify bitter pit against similar disorders:
The detection of symptoms accompanying bitter pit-like physiological disorders are often masked by
the degree of severity, time of initiation of the tissue injury and external appearance. Calcium related
deficiencies include amongst other,
Jonathan spot (Jonathan apples)
Cork Spot
Lenticel spot
Jonathan spot (only in skin, aggravated by maturity), cork spot (internal lesion associated with boron
deficiency), lenticel ‘blotch’ spot (begins in lenticel, only in skin, associated with Ca deficiency) and
bitter pit (skin and flesh, Ca deficiency aggravated by immaturity). To ensure the correct disorder is
identified, it is essential to include an internal, additional to the visual, external evaluation.
To illustrate this, consider the following characteristics exhibited by Jonathan spot, cork spot and bitter
pit (4):
•
Extreme tissue abnormalities,
•
cell proliferation in localised areas of cortical tissue,
•
secondary vascular tissues in the outer cortex are areas of tissue, degeneration coinciding with
isolated cell abnormalities originating deep within the cortex,
•
cell proliferation and degeneration always occurred adjacent to vascular bundles,
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formation of cork (phellem) on the external surface of fruit,
•
starch grain accumulation in areas affected by tissue degeneration which indicated localised
immaturity of tissues and
•
continued development and breakdown in storage
Characteristics of bitter pit fruit:
Morphology
•
The epidermis of the apple usually appears normal, except for a slight indentation that is
slightly darker than the surrounding tissue.
•
Skin over the depression changes to a deeper green and finally, it appears as a small, brown,
desiccated pit.
•
Bitter pit fruit had marked depressions with a sharp distinction between normal and affected
tissue, apparently occurring around a lenticel.
•
A transverse section of the lesion will reveal brown, dry, spongy tissue just beneath the skin.
•
It is possible to distinguish bitter pit from cork spot in that the bitter pit lesion is softer than
the surrounding tissues, whereas the cork spot lesion is harder.
Anatomy
•
Early stages of development shows fragmentation or breakdown of the cuticle (K) and
changes in the cells of the hypodermis (HP) and cortex (KO) of the fruit within the pit area.
•
The outer hypodermal layer of cells is compressed with concurrent lack of cuticle development.
•
The cuticle of affected fruits shows longitudinal ridging and extends into the outer cell walls of
the epidermis (EP).
•
The hypodermis is abnormal, with more cell layers. The hypodermal cells are characterised
by thick walls and contain cellular deposits which are absent in normal tissues.
•
The cortex is affected with necrotic tissues extending to the core line. Pit tissues are manifest
in the vascular tissues of the outer cortex where the cells develop thick cell walls with apparent
necrosis. The cortical cells are disrupted and malformed close to the bundles.
Thus
abnormal cell development occurs at the site of vascular tissues, apparently from tissue injury
occurring early in the life of the fruit. Vascular bundles and the areas surrounding bitter pit are
injured early in fruit development (3).These cells often are cork like. Browning of this tissue
occurs.
•
Adjacent to this abnormal development, extreme cell proliferation occurred. Cortical cell
proliferation was intense 60 days before fruit maturation and cytoplasmic breakdown of the
larger cells was also apparent at this stage of development. (6).
•
Starch accumulated on the walls of the larger cortical cells (10), as well as in these minute
cells. This indicates immaturity with a lack of starch conversion to sugar in the pitted area.
•
According to some researchers (9), the discoloured tissues of bitter pit are higher in N
compounds and Mg, K and P contents than the surrounding tissue with a higher transpiration
rate than the unaffected tissue.
•
In ‘Golden Delicious’ citric acid was the only organic acid in the affected tissue, whereas
malic acid was the major constituent in unaffected tissue.
•
Ca is the only element that was lower that in adjacent healthy tissue, with a large part of the Ca
in soluble form, in sharp contrast with the unaffected tissue. Bitter pit was found to develop
in an area of the cortex low in Ca, but not in B. These morphological and anatomical
changes as well as mineral trends, were confirmed with ‘Starking Delicious’ and ‘Spigold’
apples 60 days before harvest (7).
Acknowledgments
Ester Lotz at ARC-Infruitec for additional photographic material.
References
1. Bramlage, W.J., Drake, M. & Baker, J.H., 1974. Correlations of calcium content of ‘Baldwin’
apples with leaf calcium, tree yield and occurrences of physiological disorders and decay. J. Amer.
Soc. Hort. Sci. 94(4), 379-380.
2. Ferguson, I.B. & Watkins, C.B., 1989. Bitter pit in apple fruit. Hort. Rev. 11, 289-355.
3. Simons, R.K., 1962. Anatomical studies of the bitter pit area of apples. Amer. Soc. Hort. Sci. 81,
41-51.
4. Simons, R.K., 1968. The morphological and anatomical comparison of some physiological disorders
in apples. Amer. Soc. Hort. Sci. 93, 775-791.
5. Simons, R.K., Hewetson, F.N. & Chu, M.C., 1971. Sequential development of the ‘York Imperial’
apples as related to tissue variances leading to corking disorders. J. Amer. Soc. Hort. Sci., 96(2),
247-252.
6. Simons, R.K. & Chu, M.C., 1980.Scanning electron microscopy and electron microprobe studies of
bitter pit in apples. Acta Hort. 92, 57-69.
7. Simons, R.K. & Chu, M.C., 1982. Cellular image profile analysis of apples exhibiting corking
disorders as related to calcium and potassium. Sci. Hort. 16, 217-231.
8. Steenkamp, J., & De Villiers, O.T. , 1980. The role of organic acids and nutrient elements in
relation to bitter pit in Golden Delicious apples. Acta Hort. 138, 35-42.
9. Faust, M. & Shear, C.B., 1968. Corking disorders of apples: a physiological and biochemical
review. Bot. Rev. 441-458.
10. Wooldridge, L.G. & Terblanche, J.H., 1978. A new development in bitter pit research. Dec. Fr.
Gr., Jan, 10-12.
11. Wooldridge, J., 2004. Bitter pit DFPT bulletin…