The Rusty Plum is a small to medium-sized tree to 20 m high with a very fluted or irregular trunk. The young shoots have rusty hairs, and the shoots and stems exude white milky sap if cut. The leaves are alternate, 5 - 15 cm long and 2 - 5 cm wide with a firm texture and a 'quilted' appearance. The undersurface of the leaves is paler than the upper surface and has prominent raised veins, including 15 to 20 pairs of curved secondary veins. Creamy green flowers form in clusters of four to 15 on the stems in spring. The Rusty Plum has globular plum-like fruit, red turning black, 2 - 5 cm in diameter. The round seed inside is very shiny, with an elliptical scar on one side.

The idea of pitless, or stoneless, fruits is not new. In the early 1900s Luther Burbank, a prolific horticulturist, crossed a partially stoneless wild plum with California French prune varieties. These crosses led to commercial-quality fruit that almost completely lacked the stone, but still contained the seed. Burbank's work demonstrates that stoneless fruit can still hang onto the tree, reach appropriate size, and attain commercial quality.

To create truly pitless fruit, it is necessary to eliminate both the stone and the seed inside. "We have begun projects to address both issues. These take advantage of both traditional breeding-using Burbank's plums-and genetic engineering," says Dardick.