It must be patent law. This article in AgWeek says:
As the university’s lucrative patent on the Honeycrisp was about to expire, the school launched the SweeTango — a cross between the Honeycrisp and its Zestar! — to keep revenue flowing to support its cold-climate fruit research.
According to About.com, plants were not considered to be patentable until 1930, after Luther Burbank had already died.
But somehow he received a number of patents on his plants after he was dead.
Another article on About.com says that plant patents last for 20 years, and give the inventor:
the right to exclude others from asexually reproducing, selling, or using the plant so reproduced.
I guess I should stick to writing systems software :)