Greenhouse and field trials were conducted at the Aberdeen Research and Extension Center to determine the effect of hairy nightshade competition on two potato varieties with different growth habits. Greenhouse replacement trials included treatments of three plants total in each pot with potato : hairy nightshade ratios of 3 : 0, 2 : 1, 1 : 2, or 0 : 3. Varieties tested were ‘Russet Burbank’ and ‘Russet Norkotah’. Greenhouse-grown hairy nightshade (cotyledon to one-leaf stage) were transplanted into pots earlier than or at the same time as potato plant emergence. As the number of Russet Burbank plants per pot decreased, potato plant biomass dry weight (average per plant) increased, regardless of hairy nightshade number or transplant time. When hairy nightshade were transplanted before Russet Norkotah emergence, potato plant biomass dry weight per plant was similar, regardless of potato : nightshade ratio. Field trials were conducted with Russet Burbank and Russet Norkotah in 2004 and 2005. At potato emergence, greenhouse-grown hairy nightshade plants (one- to two-leaf) were transplanted in between potato rows at densities of 0, 1, 2, or 3 plants m⁻¹ row, and solid-seeded at approximately 100 plants m⁻¹ row. Hairy nightshade biomass, stem and berry number, and seeds per berry were reduced by competition from Russet Burbank due to the amount and duration of shading, as compared with Russet Norkotah. Russet Burbank U.S. No. 1 and total tuber yields in plots with 1 hairy nightshade plant m⁻¹ row were similar to weed-free control yields, whereas yields in plots with 2, 3, or 100 m⁻¹ row were reduced in comparison. In contrast, Russet Norkotah yields were reduced when only 1 hairy nightshade plant m⁻¹ row was present. Overall, Russet Norkotah were less competitive with hairy nightshade than Russet Burbank in both the greenhouse replacement and field trials.

Nomenclature: Hairy nightshade, Solanum sarrachoides auct. non Sendtner SOLSA, Solanum physalifolium Rusby; potato, Solanum tuberosum L.