PURPOSE:To enable individual control and effectively multiply disease-free seedlings of sweet potatoes by planting the disease-free seedlings of the sweet potatoes or disease-free tuberous roots thereof in a rearing greenhouse, then covering the peripheries of planted places with lids using growth apparatuses in the longitudinal direction and growing vines, etc., to grow therefrom in the longitudinal direction. CONSTITUTION:Disease-free seedlings of sweet potatoes or disease-free tuberous roots 10 thereof are individually planted in soil 24 in a rearing greenhouse 22 for agricultural crops. The peripheries of the planted places are then covered with lids using growth apparatuses 20 in the longitudinal direction to grow vines, etc., growing vines from the aforementioned disease-free seedlings of the sweet potatoes or disease-free tuberous roots thereof in the longitudinal direction with the above-mentioned growth apparatus 20. Thereby, extension of affected seedlings by spread of viruses is suppressed to remarkably improve the yield efficiency of multiplied seedlings. As a result, production cost is remarkably reduced.
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