REDUCTION IN THE STRENGTH OF THE MERCURIC- 
CHLORID SOLUTION USED FOR DISINFECTING 
SWEET POTATOES 
J. L. Weimer 1 
Assistant Pathologist, Office of Cotton, Truck, and Forage Crop Disease Investigations, 
Bureau of Plant Industry, United States Department of Agriculture 
INTRODUCTION 
The disinfection of seed sweet potatoes with mercuric-chlorid solution 
(HgCl 2 ) before bedding for the prevention of certain diseases has become 
a common practice since it was first recommended by Harter 2 in 1913. 
All investigators working on sweet potatoes recommend in general the 
same method of treatment, although they differ slightly on certain 
details of procedure. In fact it has never been satisfactorily deter¬ 
mined how many bushels can be treated in a solution of 1 to 1,000 
mercuric chlorid before it becomes so reduced in strength as to be no 
longer effective. 
Some determinations have been made of the amount of mercuric 
chlorid removed from the solutions used for treating Irish potatoes, 
the results of which, so far as the writer is aware, have never been pub¬ 
lished. 3 However, the data thus obtained have influenced some investi¬ 
gators studying Irish potato diseases to recommend either that the 
solution be discarded after having been used three or four times or that 
a definite quantity of mercuric chlorid be added from time to time to 
bring the solution back to approximately its original strength. 
Orton 4 states that the solution used for disinfecting Irish potatoes 
grows rapidly weaker, even losing as much as one-fourth of its strength 
during a single treatment. He recommends that this loss be offset by 
the addition of 1 ounce of mercuric chlorid to each barrel of solution 
after one batch of potatoes has been treated. It is suggested that after 
repeating this process three or four times the solution be discarded. 
This method of procedure for the treatment of Irish potatoes seems to 
be pretty generally agreed upon, since it is sanctioned by pathologists 
from several of the leading Irish potato growing States. 
It has been assumed that Irish and sweet potatoes probably produce 
a similar change in the solution in which they are treated. Hence, 
1 The writer is indebted to Dr. E. E. Harter for valuable suggestions and criticisms during the progress 
of the work reported in this paper. 
s Harter, E. E. control of the black-rot and stem-rot of the sweet potato. In U. S. Dept. 
Agr. Bur. Plant Indus. Circ. 114, p. 15-18. 1912. 
3 Since this article was submitted for publication results of a similar nature have been published. 
(Brann, J. W., and Vaughan, R. E. potato scab. Wis. Agr Exp. Sta. Bui. 331, 27, p., n fig. 1921.) 
* Orton, W. A. selection and treatment of seed potatoes to avoid diseases. U. S. Dept. 
Agr. Bur. Plant Indus. C. T.. and E. C. D. Circ. 3, 8 p., 2 fig. 1919. 
Journal of Agricultural Research, Vol. XXI, No. 8 
Washington, D. C. July 15, 1921 
yo Key No. G-236 
(575) 
