In his book "Natures Garden--A Guide to identifying, harvesting and preparing Edible Wild Plants" Samuel Thayer states that all parts except the fruit are toxic. The unripe fruit is also considered toxic. "The ripe fruit turns to a light, dull yellow or brownish yellow. It has a delicious tropical scent..." "My favorite thing to do with mayapples is to tear a hole in the skin, suck out the pulp and enjoy its flavor as I spit out the seeds." He shows photographs of ripe and unripe fruit. Overall it is a great book that I have found very educational and recommend!
All parts of the Mayapple are listed as toxic except the ripe fruit. The ripe fruit looks like a small, slightly wrinkled, lemon and is quite delicious, if you can find one that hasn't been eaten by the local wildlife. Unfortunately the fruits are normally devoured by deer before they fully ripen. But they are easy to grow if you have a shady spot. Mayapples burn in too much sun, making the leaves turn red, and stunting the plant.
Only the two leafed plants flower and produce fruit, a single flower is produced, slung low in the V between the two leaves, the single leaf plants are sterile.