Where did the apples in Japanese Curry come from? Japanese curry (カレー, karē) is its own distinct style, made with a roux base, mild curry powder, and grated apples or apple puree.
It also has a well-documented origin, having been introduced by British/Indian sailors during the period of British-Japanese naval alliance.  This also includes an explanation for why Japanese curry uses a roux base (the navy wanted to add vitamin B1-bearing wheat to the sailor's diet).  What it doesn't include is an explanation for when, how, and why the apples became a key ingredient.
Per Chopstick Chronicles:

The ultimate Japanese curry rice secret ingredients “kakushi Aji”, which literally translates to “hidden taste”, are Apple and Honey. These are well-known Japanese curry rice ingredients among Japanese people so it’s not much of a secret anymore. Adding grated apple and honey gives the Japanese curry rice the signature sweeter flavour and is a staple for any Japanese mother’s home-cooked curry.

Apples are not used in Anglo-Indian curries that I've seen or been able to find online.  So this leads to several related questions:

Were apples added to standard Japanese curry when it was introduced to the 19th century navy?
If so, were they added for nutritional reasons?  Or were apples common in British curries of the time (the book &quot;Curry&quot; does not mention this, nor are apples mentioned in Mrs Beeton's)
If the apples were added later, how did that come about?  Was the Vermont Curry company responsible for it?

Thanks for any leads or ideas.
Curry and apples were both introduced to Japan in the late 19th century. Curry did not receive widespread popularity until the beginning of the 20th century and apples were not introduced to the curry rue until the 1960s (When the sweeter honey and apple curries increased their popularity with children). In 1963 House Foods Corp introduced House Vermont Curry made with honey and apples.
https://gogocurryfranchise.com/the-evolution-and-spread-of-japanese-curry-rice/