The six soup vegetables

I'm a great fan of Daniel Kevles' work on eugenics and breeding, but I must say that it was for rather frivolous reasons that I singled out this passage from the article he co-authored with Glenn E. Bugos on plants as intellectual property:

In its main features the Plant Variety Protection Act [signed into law by Nixon in December 1970] was a product of the UPOV [Union International pour la Protection des Obtentions Vegetales] model refracted through the demands of relevant American interest groups. It established a system of breeder's rights to be administered by the Plant Variety Protection OFfice (PVPO) at the Department of Agriculture headquarters in Beltsville, Maryland. The rights were given in the form of Plant Variety Protection Certificates (PVPCs) to be issued for seventeen years to new varieties that were distinct, stable, and uniform. Any plant could qualify, except the so-called soup vegetables--tomatos, okra, peppers, carrots, cucumbers, and celery--which were excluded from coverage at the urging of the Campbell Soup Company. Campbell feared that the PVPA would increase the costs of these vegetable[sic] and close off sources for hybridization programs, perhaps its own.

The six soup vegetables! Not the six I would have guessed, I must say--onions for instance would be a more obvious candidate than cucumbers...