George O. Kegode , Department of Plant Sciences,
NDSU, Fargo, ND and Brian M. Jenks , North Central
Research Extension Center, NDSU, Minot, ND
Biennial wormwood has become one of the important weeds in dry bean in North Dakota, Minnesota, and South Dakota. In 1978 and 1979 surveys of North Dakota weeds, biennial wormwood was not identified in 2,819 fields sampled. Since the early 1990s, however, many farmers have reported severe infestations of biennial wormwood that were not controlled by their current herbicide programs.
It is suspected that biennial wormwood became a serious problem in dry bean for several reasons: a) Selection of the annual life cycle from what originally primarily was a biennial species; b) High tolerance to many soil-applied herbicides, especially Sonalan, Prowl, Treflan, Dual and Lasso, and to postemergence herbicides, such as Pursuit and Raptor; c) Improper identification, since it frequently is confused with common ragweed; and d) Increased seed production during the current wet weather cycle, since biennial wormwood is most prolific in saturated soil areas.
A study was conducted at two sites in Minot, ND over the summer of 2000. Both sites were located at the North Central Research Extension Center farm and were necessary because of the erratic nature of emergence of biennial wormwood seedlings despite the fact that both sites had a history of infestation by this species. The general objective of this project was to identify the best control options for biennial wormwood in dry bean using registered and experimental herbicides.
The studies were initiated on May 30 with the application of preplant incorporated (PPI) herbicides. Maverick was seeded at Site 1 on June 2 and Othello at Site 2 on June 3. Preemergence (PRE) treatments were applied on June 6 and postemergence treatments were applied on July 15 (POST 1) and July 21 (POST 2). Control of biennial wormwood and dry bean injury were evaluated on July 19 and August 31 at Site 1, and on July 20 and August 30 at Site 2. The study included Spartan, Valor, Python, Broadstrike + Dual, Reflex and Raptor, which are not registered (Section 3) for use in dry bean.
Valor was the only treatment that caused significant crop injury at both sites but gave excellent control of biennial wormwood (Table 1 and Table 2). Broadstrike + Dual and Python did not control biennial wormwood. In a similar study conducted in soybean at Fargo, Broadstrike + Dual and Python provided excellent control of biennial wormwood. The reason for the contrasting results is probably linked to the soil pH. The soil pH was 5.5 and 7.5 for Minot and Fargo, respectively. Python and Broadstrike are recommended for use in soils with pH ranging from 5.9 to 7.8, therefore the soil pH at Minot was not adequate for biennial wormwood control with these treatments.
By the second evaluation date (Aug. 30) Spartan and Sencor effectively controlled biennial wormwood without causing significant crop injury (Table 1 and Table 2). A split application of Basagran was slightly more effective in controlling biennial wormwood then a single application. Tank mixes of Basagran + Raptor or Reflex + Raptor provided poor control of biennial wormwood.