In the English/British navy, it was well understood that fresh vegetables and greenstuff would immediately begin to reverse the causes of scurvy, and that preserved vegetables (sauerkraut, for example) could ward off the disease; but you are correct that the mechanism of "scurvy" per se (vitamin C deficiency) was not well understood, and that a whole host of marine disorders were lumped together as "scurvy." I should note that in the popular imagination, it's often thought that only fresh fruit and particularly citrus juice are proof against the disorder, but in fact just about any green vegetable and many fresh meats -- particularly organ meats -- will contain enough nutrients to cure the disease. John Narborough was using fresh meat and herbs to ward off scurvy in the 1670s.

I wrote about this here before in case you have follow up questions or comments.

Scurvy wasn’t just a problem for sailors. In North America, it could be a winter problem for indigenous peoples, and later fur traders, explorers, and settlers. Jacques Cartier’s 16th century experience with scurvy is a staple of the Canadian school curriculum. The French account of the exploration demonstrates the French crew’s unfamiliarity with the disease, the religious devotions undertaken for a cure, and the local native people’s effective medicine for scurvy.

In this article: Arginine, scurvy and Cartier's "tree of life" , Journal of Ethnobiology and Ethnomedicine, 2009 5:5, Don J. Durzan attempts to identify the species of tree in Cartier’s accounts. Durzan writes:

At Stadaconna (46° 49' N, 71° 13' N) and in November 1535, Canada's cold struck with its entire rigor, and ice thickened to two fathoms. In December, over 50 of the Iroquois died from an unknown sickness (scurvy).

Nota bene: If what they had was indeed scurvy, the local Iroquois did not at have a cure at hand to keep away scurvy or avoid casualties.

The sickness began to spread to Cartier's crews in all three of his ships. By mid-February 1536, of the 110 member crews, 8 were already dead and more than 50 past all hope of recovery. Excerpts from Burrage [1, p. 73] reveal that the unknown sickness in Cartier's crew "spread itselfe amongst us after the strangest sort that ever was eyther heard of or seene, insomuch as some did lose all their strength, and could not stand on their feete, then did their legges swel, their sinnowes shrinke as blacke as any cole. Others also had all their skins spotted with spots of blood of a purple colour: then did it ascend up to their ankels, knees, thighes, shoulders, armes and necke: their mouth became stincking, their gummes so rotten, that all the flesh did fall off, even to the rootes of teeth, which did also almost fall out".

Our Captaine seeing this our misery, and that the sicknesse was gone to farre, ordained and commanded, that every one should devoutly prepare himselfe to prayer, and in remembrance of Christ, caused his Image to be set upon a tree, about a flight shot from the fort amidst the yce and snow, giving all men to understand, that on the Sunday following, service should be said there, and that whosoever could goe, sicke or whole, should go thither in Procession, singing the seven Psalmes of David, with other Letanies, praying most heartily that it would please the said our Christ to have compassion upon us ...That day Philip Rougemont...being 22 yeeres olde, and because the sicknesse was to us unknown, our Captaine caused him to be ripped to see if by any meanes possible we might know what it was...he was found to have his heart white, but rotten, and more than a quart of red water about it: his liver was indifferent faire, but his lungs blacke and mortified, his blood was altogither shrunke about the heart, so that when he was opened great quantitie of rotten blood issued out from about his heart...Moreover, because one of his thighs was very blacke without, it was opened, but within it was whole and sound." Scurvy continued to spread until not more than three sound men remained in the ships. None were able to go under the hatches to "draw drink for himselfe, nor for his fellowes."

From this account, it appears that the largely Breton crew had not seen scurvy before, and had no idea of what may have caused it. There had been many previous Breton fishing trips to the Grand Banks off Newfoundland, but these voyages returned home in the fall. Cartier and his crew were the first Europeans to stay in the Canadian interior over-winter.

And then, At Stadaconna, Cartier encountered the native Domagaia, who "not passing ten or twelve dayes afore, had bene very sike with that disease, and had his knees swolne as bigge as a childe of two yeres old, all his sinews shrunke together, his teeth spoyled, his gummes rotten, and stinking. Our Captaine seeing him whole and sound, was therat marvelous glad, hoping to understand and know of him how he had healed himselfe...He answered, that he had taken the juice and sappe of the leaves of a certain Tree, and therewith had healed himselfe: For it is a singular remedy against that disease.”

Domagaia "sent two women to fetch some of it, which brought ten or twelve branches of it, and therewithall shewed the way how to use it... to take the barke and leaves of the sayd tree, and boile them togither, then to drinke of the sayd decoction every other day, and to put the dregs of it upon his legs that is sicke: moreover, they told us, that the vertue of that tree was, to heale any other disease: the tree in their language called Ameda or Hanneda..." Other translations refer to the tree as "Annedda", "Anneda" or "Hanneda" This sickness was treated with a boiled decoction from the bark and leaves of "a tree as big as any oak in France".

Cook [25] translates that "The Captain at once ordered a drink to be prepared for the sick men but none of them would taste it. At length one or two thought they would risk a trial. As soon as they had drunk it they felt better, which must clearly be ascribed to miraculous causes; for after drinking it two or three times they recovered health and strength and were cured of all the diseases they had ever had. And some of the sailors who had been suffering for five or six years from the French pox [syphilis] were by this medicine cured completely. When this became known, there was such a press for the medicine that they almost killed each other to have it first; so that in less than eight days a whole tree as large and as tall as any I ever saw was used up, and produced such a result that had all the doctors of Louvain and Montpellier been there, with all the drugs of Alexandria, they could not have done so much in a year as did this tree in eight days; for it benefitted us so much that all who were willing to use it recovered health and strength, thanks be to God."

The exact species of Cartier’s tree is a matter of debate: local varieties of hemlock, spruce, pine, and juniper are all candidate. It seems clear from this account that they got enough Vitamin C from this tree to improve, and the linked article surveys some other possible health benefits of this remedy. Cartier certainly talked it up like a complete cure-all, despite the fact that scurvy supposedly is also killing the local Iroquois. In fact it also takes away syphilis! Good for everything that ails you!

Lots of other French accounts follow of native people’s using evergreen needles and sap as medicine for scurvy. But French explorers also met native peoples who didn’t have such a medicine, and their own experiments at making the medicine with evergreens often times didn’t work out. This was the case with Samuel Champlain’s explorations in Acadia and Quebec. Champlain in 1613 complains:

1613 Champlain fac. ed. Voyages 53-55 trans]. [Winter at St. Croix Island 1604-05) "During the winter there was a certain sickness amongst several of our men, called sickness of the country, or Scurvy...There died 35...We could not find any remedy to cure this sickness. We had several cut open to try and find out the cause of the sickness...Our doctors could not do any better for themselves as they were sick like the others...One only has salt meat and legumes to eat during the winter, which engenders poor blood: this in my opinion causes in part this annoying sickness...65 We passed by a bay [on the Acadian coast) where there are a quantity of islands; and saw large mountains in the west, where is the home of a savage captain called Aneda; which I think is near the Quinibequy river. I was persuaded by this name that here was one of the race who found the herb called Aneda, that Jacque Cartier said had so much power against the sick-ness called Scurvy...which torments these men, savages as well as our own, when they arrive in Canada. The savages knew nothing about this herb, nor know what it is, even though their language contains the name.. .101 [Winter 1605-06 at Port Royal).

• p. 9, Medicinal and Other Uses of North American Plants: A Historical Survey with Special Reference to the Eastern Indian Tribes, Charlotte Erichsen-Brown

Captain Thomas James, trapped in James Bay in 1632, had his own ideas of what would cure scurvy.

I found the Land utterly barren, even of that which I thought easily to be found; which was Scurvy-grass, Sorrel, or some Herb or other, to have refreshed our sick people.”

• p. 53, The voyage of Captain Thomas James for the discovery of the northwest passage, 1631 ed. Bodilly, Ralph Burland, 1928