while the fruits are on the tree or after harvest, but the variety is not highly 
susceptible. The York variety is highly susceptible, however, to a disorder 
that has been known for many years as York spot. Unlike bitter pit, which at 
times develops while apples are on the trees, York spot develops entirely while 
the fruits are on the trees. This disorder has been considered by some to be 
bitter pit, but it will be discussed separately under the subheading Cork Spot 
(York Spot). 
Next to scald there has probably been more research on bitter pit than 
any other physiological disease of apples, Martin (93) made a comprehensive 
review of publications, between 1935 and 1959, which pertained to bitter pit. 
He divided the research into four groups. In 1961 van Schreven (171) also pre- 
pared a digest of literature. She followed Martin's plan with some elaboration. 
These reviews have been fully utilized here. 
In 1934 Barker (12) reviewed 209 papers published during the previous 65 
years. He concluded that there was no decisive information on the causal fac- 
tors of bitter pit, and that only certain characteristics of the disorder were 
known, Some predisposing factors that were almost universally accepted were: 
(1) liability to develop bitter pit after harvest decreased with increasing 
maturity at harvest (without an equivalent amount of bitter pit developing while 
the fruits were on the tree); (2) susceptibility increased with increasing fruit 
size; and (3) fruits from light crops, from young and vigorous trees and those 
given heavy nitrogen dressings had high susceptibility. Less conclusive re- 
sults were obtained on factors such as rootstock, soil conditions, manuring 
(other than nitrogen), pruning, thinning, and climatic and storage conditions. 
Research continued along these lines for a time after 1935, confirmed previous 
research, but made no major advances. This area of research constitutes 
Martin's first group. 
The second group of research involved greater precision than the first 
and also the use of new techniques. Sod culture or mulch gave less bitter pit 
than clean cultivation (68, 78, 88, 113, 159). Fruit size was closely corre- 
lated with bitter pit (2, 27, 90, 91, 92). 
The histology of bitter pit lesions was studied and compared to those of 
boron deficiency. Skin coatings (63), prompt storage, and controlled-atmosphere 
storage (95) were reported to reduce bitter pit, but a careful evaluation indi- 
cated that these treatments delayed the appearance of bitter pit but did not 
necessarily reduce the final incidence. After 1935 there was more general 
agreement that water stress, particularly during the latter stages of develop- 
ment and on larger fruits was an important factor, Experiments with irrigation, 
mulching, and humus supply gave evidence that a regular water supply lessened 
the incidence of bitter pit (73). 
The competition for water between leaves and fruits was demonstrated and 
the osmotic values of each were studied. 4/ In 1950 these studies were sum- 
marized by Heinicke (66) in considering the predisposing factors of apples to 
bitter pit. In 1959 Woodhead (180) also summarized conditions that influence 
bitter pit development. 
In Martin's third group of research, he reviewed literature that reported 
tests with boron as a possible control of bitter pit. Results between 1935 aid 
1945, where boron was added to the soil or injected in the trees, were mostly 
negative (8, 84, 139). 
7/ ‘Smock, R. E. Studies on Bitter Pit of the Apple. N. Y. (Cornell) 
Agr. Expt. Sta. Memoir 234, 45 pp. 1940. 
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