I scoured the internet - well, I went through the first 3 or 4 pages of related search arguments about safety of fruits, veggies and greens for our horses - and came up with a lot of comments, lists and opinions but not one which was documented and referenced.

This downloadable pdf can be printed and stored in your feed room or kitchen - or wherever you might prepare fresh snacks for your horses.
While a few of the veggies listed in the "Avoid" group are considered "safe" in small amounts by some fresh veggie advocates, knowing that experts in toxicology consider them potentially poisonous for horses would have me leery of even considering them. But the list of yummy things your horse can enjoy is long enough that the "avoids" certainly won't be missed.
Best regards,
Patti in Vail AZ
where Dreamer thinks she heard someone say "Strawberries for dessert? Yum!"
Links:
http://www.shotgunranch.me/Fruits-vegetables-safe-unsafe.pdf
Shotgun Equine Nutrition http://www.shotgunranch.me/
Desert Equine Balance http://www.desertequinebalance.com/