for 5x20, two rows of six evenly spaced plants would eventually fill in (3-5 years?). Plant more densely if the area will not get much water in summer when plants are young. Fruit is currently at its peak in Portland, does not compare to other raspberries but is pleasant enough, quite similar to salmonberry.

I haven't noticed any diseased plantings, but am unfamiliar with the disease transfer subject.

Some chatter from 2011:

http://forums.gardenweb.com/discussions/2054683/rubus-calycinoides-fruiting?n=16

Name that seems to have been settled on is Rubus rolfei. The superior, uniformly branching, abundantly fruiting clone that has come to dominate commercial offerings came from the plant selection program of the U.B.C. in Vancouver, as 'Emerald Carpet'.

Disease problems in R. idaeus cultivars is probably best approached by planting resistant selections. In soils where the water table is at least 3 ft. below the surface. You'll also need a support system to tie them to - every year.