My "guess" on mastering this mode is likely to first learn any piece that
you can find that is played in this mode and then creating your own
variations.
Steve Riley had once told me that he likes to tinker around (not his words,
verbatim) on the accordion in order to discover licks and techniques. So,
he's a proponent of experimentation.
Or, if it's easier to find melodies this way, create them on piano first and
re-locate them on the accordion keyboard?
I recall Steve demonstrating playing "Oh, Mam/Oh Mom" on accordion using this
cross playing technique. I know that Steve also likes the D-Scale for blues
and such because so many blue notes fall into place.
I think that is actually the Dorian mode. If I remember correctly, the Phrygian mode would be if you began the scale on E with a C accordion.
But, that does not answer your question...
When I have had to deal with weird modes, it is usually because of a specific song or songs that I want to play. So, I find the notes, and play it 500 times until I have it down. Eezy peezy!
Both BG and David are correct. The Pine Grove Blues are played in dorian mode, or Key of D on an C accordion. It is also similar to the Blues scale. I have played bass with both Chris Miller and Steve Riley playing Tous les temps en temp using this same scale. I have actually developed a couple of tunes using it, but it isn't an easy scale. Technically, you can play it over a major, or minor chord progression, but I don't like it over minor.
Phrygian mode is actually E minor on the C accordion. It is really hard to play any type of melody in this position, but I have heard a couple of Zydeco songs that just have a couple of licks from this position.
I was just working out Boozoo's "keep your dress tail down" and I think it's the same fingering as Pine Grove, more or less.
I had to use Amazing Slow Downer to change the key so I could work it out on my C box(the only one with me for the winter months until I get back home and start gigging again).
I think Boozoo must have used the A key. I just bumped the pitch up.
I do play Pine Grove and it does get easier with time. After a while you'll get to where you can jam/innovate with it.