17,4 Lee: Fungicides and the Citrus-canker Organism 339 
1 to 100 is entirely valueless, and spraying with such a solution 
might even serve to disseminate the canker bacteria and spread 
the disease. The present results would indicate that formalin, 
to be of any value, would have to be at a concentration of 1 to 
20. The use of formalin at a dilution of 1 to 20, or even 1 to 40 
or 1 to 60, is impossible on citrus trees, in as much as formalin 
at such dilutions burns the foliage. Mercuric bichloride at a 
dilution of 1 to 2,000 has also been known to burn the foliage 
of citrus trees. It would seem desirable, therefore, to use mer- 
curic bichloride at a dilution of 1 to 5,000 or 1 to 10,000. Such 
dilutions, as shown by these tests, are effective as bactericides 
against Pseudomonas citri. Commercial unslaked lime at a 
dilution of 1 to 500, as shown by these tests, would be effective 
against the canker bacteria; would have a more lasting effect 
than such disinfectants as lysol, formalin, etc.; and would be 
cheap and entirely safe for use on citrus trees. 
In orchard practice in countries where citrus canker is already 
generally prevalent and where spraying may be employed as an 
annual preventive measure, it may be desirable to combine such 
spraying measures with preventive measures against other dis- 
eases or insects. In such a case if a fungicide only is used, 
Bordeaux 4-6-50 would seem to be the only mixture having, 
at the same time, a proven fungicidal value and a bactericidal 
action against Pseudomonas citri. It has already been shown 
in field experiments not yet published that Bordeaux mixture 
4-4-50 is more effective against citrus canker than neutral Bor- 
deaux mixture. These field data in a way corroborate the pres- 
ent data in indicating that Bordeaux 4-6-50 would be the 
most effective Bordeaux mixture for use against citrus canker. 
In case a contact insecticide were desirable at the same time 
as a spray against citrus canker, lime sulphur, as shown by the 
foregoing tests, would be effective at any of the dilutions de- 
sirable for insecticidal use. 
The tests presented here suggest many possibilities in con- 
nection with citrus-canker control by spraying. It is very evi- 
dent that the bactericidal action exhibited by Bordeaux and by 
Burgundy mixtures as used in orchard and field work is due 
to the excess of the negative ion rather than to the copper. 
It was shown in the tests with copper sulphate that as strong a 
concentration as 1 to 200 was necessary to exert any effect upon 
P. citri in 2\ minutes. Consideration of the conditions incident 
to the weathering of Bordeaux mixture on the foliage leads to 
the conclusion that rarely or not at all will such a concentration 
