long period of rain in early spring

Panax ginseng (Chinese ginseng)

種が島: matchlock / a kind of fragrant aloeswood used for incense / (slang, archaic) fugu (so named for the way that poorly-prepared fugu is deadly, like a matchlock)

多禰島, 多禰嶋: Tanegashima (an island in Kagoshima Prefecture) / a surname

revealing the secret

to reveal the secret (of a sleight of hand, etc.) / (figuratively) to explain; to make clear

koji starter (a preparation containing koji mold, used to produce a large batch of koji)

historic, historical / already-happened, past / former, old, traditional

historian

study of history