Dragon Disciple Prerequisite wording:
"Ability to cast 1st-level arcane spells without preparation. If the character has sorcerer levels, he must have the draconic bloodline. If the character gains levels of sorcerer after taking this class, he must take the draconic bloodline."
Is there any way a pure witch can cast spells without preparation? and does ability to cast a spell as a Spell-Like Ability count? (Minor Spell Expertise)
Minor Spell Expertise wording:
"Choose one 1st-level spell that you know. You may cast that spell twice per day as a spell-like ability. The caster level for this spell-like ability is equal to your caster level in the class from whose spell list the spell is taken."
So far the consensus seems to be that you need a Sorcerer, Bard or Summoner. Paizo staff members clarify that a Spell-Like Ability does not count when spells themselves are required for prerequisites.
Here's a quote on the matter, though the Witch itself wasn't out at the time.
Edit: the link might not stay at the relevant section so here's the text:
Q: With a wizards Arcane Bond's power to allow for casting arcane spells spontaneously can they qualify for the Dragon Disciple Prestige Class?
A: (James Jacobs 11/20/09) First off... I'm not a big fan of saying what you can and can't do in your home game. And as a result, I'm generally hesitant to lay down the law for things like this, since I can see the attraction to go either way. What it really boils down to is if the GM wants to let wizards qualify for the class... that's cool. Arcane bond's power to allow a wizard to cast an additional spell was not intended to let wizards "sneak" into prestige classes that do this, but there's no game balance reason as to why they shouldn't be able to do it. But for materials we're publishing, though, and for building NPCs who might have this class, being able to cast arcane spells spontaneously means "being a bard or a sorcerer" at this point.
Interesting, especially since the Witch can spontaneously heal now with the Hedge Witch archetype.
I would still say it is limited to bard, sorcerer, and red mantis assassin for the moment however (didn't see that last one comin', did ya?)
Nobody expects the Red Mantis Assassins! Our chief weapon is surprise... surprise and fascination. Fascination and surprise are our two weapons. And sawtooth sabres...