There is a great deal of debate about this... [From A Book of Gardening: Ideas, Methods, Designs, by Penelope Hobhouse, p. 205:] In 1985 the rose and lavender beds [at Castle Drogo] were renewed... The beds were treated with dazomet against rose replant disease...

Soil sickness and rose replant disease have been studied in Europe, where several plant species, including roses, suffer decline or death when grown repeatedly in the same soil. Leaving the soil fallow does not cure soil sickness. Plants that are failing because of replant disease can be moved to virgin soil and recover. More research has been done on apples than roses, but two microorganisms are being investigated as possible causes: nematodes and Actinomycetes. Certain rootstocks are much more resistant to nematodes than others. An interesting biological control of nematodes is a crop of marigolds. Summarized from Encyclopedia of Rose Science.