Scales. That is a big subject and often a subject of contention. Many grassers use their ears and learned licks to carry them through the process of taking a break (solo). Some folks (like myself), listen to those common licks and determine, through music theory, what scales/notes are being used and how those scales/notes relate to the chord/harmony being played. In addition, bluegrass is very melody oriented, so the break the soloist takes must point to or fit the original melody closely. You can see why many folks learn bluegrass by ear exclusively. And, in fact, asking even some of the top grassers what they are doing specifically (scale and harmonically) often is met with a annoyed, quizzical look and a response like, “I just play what fits.”
If you are trying to learn how to decipher the musical language of bluegrass, then you can begin to choose and experiment with scales that fit each chord. If you want to play solos/breaks that are diatonic in nature (without a blues tonality), you can stick with scales that fit the chords/tonal center of the moment. For example, over a G major chord you would play a G major scale. And in fact, over any set of chords that fit within the key of G major (G, Am, Bm, C, D7, Em, F#m7b5), the G major scale will work nicely. However, if you would like to deal with each chord as it’s own harmonic entity, you can add blues notes and strong blues movement to that chord. For example, in the key of G major, you could add the Bb, Db and F to a riff while playing over the chord G. However, in the same key of G major if you are playing over the chord C, you could add the notes Eb, Gb, and Bb to a lick over the C chord.
Keep in mind that adding these ‘blue’ notes needs to be done in a proven and thought out manner. Random playing of blue notes will sound just that, random. However, in a major tonality, following the b3 with a natural 3 will give a strong bluesy sound as will following the 1 with a b7 or a b5 with a natural 5.
As to actual ‘patterns’ there are many places on the net where you can find fingerings for major (Ionian), minor (Aeolian or Dorian) and dominant (Mixolydian) scales. These 4 scales make up a large majority of the notes you might use in any given situation. Adding blue notes (b3, b5, b7) to any of these scales can make them sound more “bluesy”.
If you would like to see some simple scale patterns, I can post some that I use if you like. I have a system for learning scales that requires the memorization of just four forms/patterns. From those four forms, you can play in any key (12) in any mode (7) in any fret position (12) for a total of 1008 different combinations of fret positions, keys and modes. However, memorizing those 4 patterns is not the same as learning those patterns. But it is a good start and once you learn those scales, things start to make sense (at least to me).
I was able to locate the scale forms I mentioned above in one of my online storage sites.