excellent scald control. DPA was approved for use on apples in the United 
States in 1962, with a residue tolerance of 10 p.pom. Im 1957, Smock (108) 
reported that ethoxyquin (6-ethoxy-1,2,-dihydro-2,2,4-trimethylquinoline) also 
had scald-reducing properties. Ethoxyquin was approved for use in 1960 and is 
sold commercially as "Stop-Scald."' A residue tolerance of 3 p.pom. was estab- 
lished for Stop-Scald. 
Since the early research on these two scald inhibitors, their effective- 
ness has been widely confirmed with other varieties and with various application 
procedures: United States-G, 34,35, '3%,,.94.1995; 83,,-955,.L08 09, nELG, LL le) 
Canada (18, 79, 85), Great pemeatal (16), redder (119, 120), Germany (42), Italy 
C25)7; Auseroraa (29, 43,50, 52, 122,, 129)... New Zealand (11,139, A070) 374), 
Switzerland (123), and South Africa | (23, 24). %In tests‘in which both DPA and 
Stop-Scald were included, DPA usually gave more complete scald control. Both 
materials usually provide much better scald control than is obtainable with oiled 
wraps, and are effective on most varieties. 
Sims (102) studied the influence of DPA on apple skin and found that DPA 
stimulated the production of yellow ground color on exposure to light and warm- 
ing. 
Yatsu (138) in studying the mode of action of DPA found that it did not 
inhibit cytochrome oxidase activity appreciably. DPA was able to act as an un- 
coupler of oxidative phosphorylation, and it inhibited the succinoxidase system 
of apple fruit mitochondria. Baker (3) noted that diphenylamine inhibits elec- 
tron transport in plant mitochondria, 
Residues of DPA were found to be mostly in the outer 2 to 4 mm. of the 
apple (39, 40). A dip in 1,000 to 2,000 p.p.m. DPA left:an initial residue of 
4 to 8 pepem., which declined to 3 p.pom. or less after 4 to 7 months' storage 
(5, 11, 23, 29, 40). Denmead, Vere-Jones, and Atkinson (11) reported that 
varieties differ by more than tenfold in DPA uptake for the same treatment. 
Hall, Scott, and Coote (29) stated that about 0.1 mg. of DPA was needed per 
average-sized apple to control scald. 
Several investigators reported that DPA used as a postharvest dip was an 
effective scald inhibitor in regular cold storage, in CA storage, and in poly- 
ethylene-lined boxes (29, 34, 37, 109, 122). 
Reports of injury from experimental use of DPA are fairly numerous (18, 
25, 34, 37, 70, 83, 109, 125). Sometimes this has been a scattered skin burning 
from DPA crystals picked up on the skin; sometimes the injury has been concen- 
trated in stem or calyx cavities from drainage of DPA solutions. The use of 
wettable powder formulations of DPA have since greatly reduced the chance of 
chemical injury (37, 55, 83, 109). An 83-percent wettable powder was the type 
approved for use in the United States. 
Stop-Scald used at 1,800 to 2,/00 p.p.m. usually has cuased no injury, 
except when it was applied as a preharvest tree spray when temperatures were 
above 80° F. This appeared as slight brown stains on the lower side of fruits 
(37, 54, 109). Stop-Scald at a concentration of 0.3 percent as a 15-second dip 
caused injury on several varieties tested in Switzerland (125). 
Both DPA and Stop-Scald gave the best scald control when used as a fruit 
dip (10, 23, 24, 25, 35, 37, 52, 55, 70, 72, 83, 108, 109, 116, 122). Spraying 
loose fruit on rollers, spraying filled field boxes, and dunking filled boxes 
are other application methods. Box sprays and box Pontles sometimes resulted in 
slight DPA injury to fruit in bottom layers. Spraying trees with scald inhibi- 
tors was one of the least effective application methods and was more wasteful of 
materials (10, 37, 55, 74, 108, 109, 117). However, tree sprays often gave 
better scald control than oiled wraps. Tree sprays to be effective must be 
applied within a day or two of harvest. 
202 
