Susceptibility of Orobanche ramosa and potato tolerance to rimsulfuron

Field studies were conducted in Lebanon to examine Orobanche ramosa control, and the tolerance of six potato cultivars, to single or repeated foliar application of rimsulfuron. The effect of rimsulfuron was evaluated on a potato cultivar “Spunta” in 2000 and 2001, and on five new potato cultivars “Columbus”, “Farmer”, “Recolta”, “Score” and “Timate” in 2001. The results indicate that both single and sequential applications of rimsulfuron between 20 and 50 g ai/ha significantly reduced Orobanche shoot number and dry weight compared to the control. Repeated applications (three times) was the most effective in reducing Orobanche shoot dry weight in all tested cultivars in comparison to their respective controls. Total potato yield within a cultivar was not reduced by single or sequential application of rimsulfuron, but marketable yield and tuber quality were significantly reduced in all cultivars (except for Score and Timate). Most tubers were small, fissured, and had a high incidence of malformed popcorn-like tubers. The most susceptible cultivars to Orobanche and sensitive to rimsulfuron were Spunta, followed by Recolta, Columbus, Farmer, Score and then Timate. Single or sequential application of rimsulfuron at 20 g ai/ha could be used to control Orobanche in Timate and Score cultivars. Although both varieties were slightly susceptible to repeated application of rimsulfuron, total and marketable yield was not affected. Additional research is required under different locations in Lebanon before a recommendation to growers for the use of rimsulfuron can be made.