Maciej Ceglowski in Idle Words:
Scurvy had been the leading killer of sailors on long ocean voyages; some ships experienced losses as high as 90% of their men. With the introduction of lemon juice, the British suddenly held a massive strategic advantage over their rivals, one they put to good use in the Napoleonic wars. British ships could now stay out on blockade duty for two years at a time, strangling French ports even as the merchantmen who ferried citrus to the blockading ships continued to die of scurvy, prohibited from touching the curative themselves.
The success of lemon juice was so total that much of Sicily was soon transformed into a lemon orchard for the British fleet. Scurvy continued to be a vexing problem in other navies, who were slow to adopt citrus as a cure, as well as in the Merchant Marine, but for the Royal Navy it had become a disease of the past.
By the middle of the 19th century, however, advances in technology were reducing the need for any kind of scurvy preventative. Steam power had shortened travel times considerably from the age of sail, so that it was rare for sailors other than whalers to be months at sea without fresh food. Citrus juice was a legal requirement on all British vessels by 1867, but in practical terms it was becoming superfluous.
So when the Admiralty began to replace lemon juice with an ineffective substitute in 1860, it took a long time for anyone to notice.
More here. [Thanks to Cyrus Hall.]