PRELIMINARY RESULTS WITH THE BORAX TREATMENT 
OF CITRUS FRUITS FOR THE PREVENTION OF BLUE 
MOLD ROT 1 
By Harry R. Fulton, Pathologist, and John J. Bowman, Junior Pathologist, 
Office of Fruit Disease Investigations, Bureau of Plant Industry, United States 
Department of Agriculture 
Recent studies by the senior author on the effect of ultraviolet light in dis¬ 
infecting the surface of citrus fruits, details of which will be published in the 
near future, have indicated that more than 99 per cent of blue-mold spores may 
be killed very quickly by light of short wave lengths, and at comparatively 
low cost, but that blue-mold rot is not proportionately checked. The two weak 
points in the ultraviolet treatment seem to be (1) the difficulty in killing the 
last small percentage of Penicillium spores, and (2) the lack of any later pro¬ 
tection against stray spores that remain unkilled or may reach the fruit. Being 
thus strongly impressed with the desirability of using some agent that would 
have a continuing effect, the writers have undertaken the testing of a large num¬ 
ber of mild chemical reagents with a view to finding inhibiting rather than 
killing substances. The stronger disinfectants would be objectionable, because 
their poisonous character would not permit residues to be left and because many 
of them would actually injure the surface of the fruit if not immediately washed off. 
The general procedure has been to select chemicals for preliminary tests largely 
on the basis of cheapness and availability. Spores of Penicillium were treated 
with relatively strong solutions of these chemicals for periods of 1 to 24 hours, 
and were then plated to determine the killing effect. Parallel tests were made 
for inhibition. Any reagent that seemed at all promising was then applied to 
a few citrus fruits to determine whether it would injure the skin. If it seemed 
safe to use, an actual test was made to determine its protective action on severely 
wounded and inoculated fruit held under conditions of temperature and moisture 
favorable for blue-mold development. Usually some one of the stronger fungi¬ 
cides was included in each test as a basis for comparison. 
Holding tests have been made thus far with fruit treated with solutions of the 
following chemicals: Sodium sulphate, sodium bisulphite, sodium borate, sodium 
fluoride, sodium perborate, sodium chloride, potassium permanganate, potassium 
borate, copper borate, bichloride of mercury, boric acid, salicylic acid, benzoic 
acid. Among these sodium borate, or borax, has seemed to give best protection 
against blue-mold rot. 
The results of one holding test, made March 14, 1924, which included borax 
along with certain other materials will serve as an illustration and is shown in 
Figure 1. California navel oranges purchased in the Washington, D. C., market 
were used. The fruit was soaked for 10 minutes in inoculum carrying about 
500,000 per cc. of mixed Penicillium digitatum and P. italicum spores. The 
fruits were dried, then atomized with the chemical solution (the control with 
water); dried again; punctured 100 times with pins to a depth of about half 
way through the peel. They were wrapped in waxed paper, packed in a tight 
1 Received for publication July 3,1924—issued Nov. 1, 1924. This paper has been referred to the Bureau 
of Chemistry, United States Department of Agriculture, for consideration regarding the use of borax. 
Journal of Agricultural Research, Vol. XXVIII, No. 9 
Washington, D. C. May 31,1924 
Key No. G-490 
96039—24f 
-9 
(961) 
