William Hupy has one of the sharpest eyes for quirks of language that I know of. Today it occurred to him that, while we skip the pronunciation of the L in salmon, we clearly pronounce it in salmonella. He wondered why.
Let me begin by saying that whether the L is pronounced in either word depends on where you are from: I’m from the South and we pronounce the L in both words sharply. I’ve been kidded about my pronunciation of salmon for decades in Pennsylvania, where I live now.
Among people raised in the North, however, unless L is followed by a vowel, it is pronounced [U] (the vowel sound in would and should), that is, before (voiceless) consonants and at the ends of words. My sons, who were born and raised in Pennsylvania pronounce milk [miUk] and hill [hiU]. This pronunciation is certainly common throughout PA, southern NY, and NJ. (In Serbian, by the way, L becomes other rounded vowel, O, in the same positions and is written that way. The past of biti “to be” is bila “she was” but bio “he was”.)
Now, since the L in salmon appears before a consonant, we would expect it to be pronounced [saumon] in these regions, as we hear almond sometimes pronounced. Most folks up North, however, have adopted the simpler pronunciation [sammon].
That leaves us with the question of salmonella. The problem here is probably what we might call ‘retroinfluence’. Though pronunciation is supposed to influence spelling (I’m not kidding; even in English), sometimes it works the other way around. We probably see salmonella written more frequently than we hear it spoken, so pronounce the L. We more than likely hear salmon more frequently than we read it, so the pronunciation change turns up there.