Seed tubers of ‘Yukon Gold’ potato (Solanum tuberosum L.) exhibit strong apical dominance, resulting in low stem density after planting. After emergence, the terminal buds reimpose apical dominance on the rest of the canopy leading to production of few secondary branches. Therefore, several experiments were conducted at the Virginia Polytechnic Institute and State University Kentland Research Farm, Blacksburg in 1992, 1993, and 1994 to evaluate the effects of topping [(excising 2-3 cm from the terminal buds at 1 to 2 weeks after the date of plant emergence (WAE)], stage of vine cutting at ground level, and main stem density on canopy growth, tuber bulking rate, and tuber yields of potato. Topping (3 years) and vine cutting (2 years) at 1 WAE resulted in more uniform plant stands and increased average yield of US No. 1 tubers of ‘Yukon Gold’ by 16 and 14%, compared to untopped and uncut plants, respectively. When plants of ‘Kennebec’, ’Superior’, and ’Yukon Gold’ were topped in 1994, yield of US No. 1 tubers increased by an average of 10%, indicating that topping could be used to improve tuber yield of potato cultivars exhibiting different growth habits. The increased tuber yields following topping resulted from a combination of increased leaf area duration and increased period of tuber bulking. Cutting vines of ‘Yukon Gold’ at 2, 4, and 6 WAE in 1992 and 1993, decreased yield of US No. 1 tubers by 11, 69, and 38% and by 19, 56, and 48%, respectively. Yield of US No. 1 tubers increased with increased stem density and increased in-row spacing. Interaction effects between in row spacing and topping were significant. Control plants of ’Yukon Gold’ produced higher tuber bulking rates and yield of US No. 1 tubers than topped plants at 15 cm in-row spacing, while topped plants had higher tuber bulking rates and yield of US No. 1 tubers than control plants at 20, 25, and 30 cm in-row spacings. Neither topping nor in-row spacing increased the incidence of hollow heart of tubers in 1994. The results showed that the difference in tuber yield between topped and control plants was greater during long growing season and topped plants should be spaced at 25 cm in-row in order to obtain maximum tuber yield. / Ph. D.