The movement of sulfonylurea herbicide-resistant (R) kochia pollen was investigated in a spring barley field near Moscow, ID, using a Nelder plot design in 1991 and 1992. Each 61 m diameter plot had 16 rays spaced 22.5° apart and contained 211 kochia plants. There were 12 susceptible (S) plants and one R plant along each ray. The R and S plants were 1.5 m and 3.0 to 30.5 m from the center of the plot, respectively. Wind direction and speed in the 16 vectors, air and soil temperature, and rainfall were monitored continuously. Mature kochia seed was collected from individual plants, planted in the greenhouse, and sprayed with chlorsulfuron to test for resistant F1progeny. Results from the 2-yr study showed outcrossing of R pollen onto S plants at rates up to 13.1% per plant 1.5 m from the R plants and declining to 1.4% per plant or less 29 m from the R plants. At least 35% of the total R x S crosses occurred in the direction of prevailing southeastward winds. Predicted percentages of R x S crosses per plant ranged from 0.16 to 1.29 at 1.5 m, and 0.00 to 0.06% at 29 m. Thus, resistant kochia pollen can spread the sulfonylurea-resistant trait at least 30 m during each growing season.
Russian thistle plant movement and seed dispersal were studied in 1991 and 1992 by placing Russian thistle plants in the center of wheat fields in eastern Washington. Three adjacent site treatments, with 24 plants on each site, were used each year; wheat stubble, summerfallow planted to winter wheat, and a “stationary” site. Plants in the “stationary” site were anchored to the ground to prevent tumbling. Plants in the stubble and summerfallow sites were allowed to tumble naturally. Individual plant movement was monitored and recorded weekly by satellite global positioning systems technology. Average estimated seed number per plant at the beginning of the experiment was 57,400 in 1991 and 66,000 in 1992. The direction plants moved correlated highly with wind direction. Some plants moved a maximum distance of 4069 m in 6 wks, while other plants moved only 60 m because of variable winds and being compressed by snow or frozen into wheat stubble. Average percentage seed loss in 1991 and 1992 for stationary plants was 15 and 26%, and for tumbling plants was 48 and 66%, respectively.
The repeated use of sulfonylurea (SU) herbicides to control broadleaf weeds in wheat fields and right-of-ways has selected for herbicide-resistant Russian thistle populations. A survey was conducted in 1991 and 1992 to ascertain the relative occurrence of SU-resistant Russian thistle in eastern Washington state. The 55 574 km2survey area was divided into 149 equal sample areas. All sample areas were surveyed for Russian thistle and seed was collected from plants in 86 sample areas. No Russian thistle was found in the center of the remaining 63 sample areas. Seeds were collected, by plant, from 30 plants at each site. Site samples were tested in the greenhouse for resistance or susceptibility to chlorsulfuron. Populations that were either homogeneous or heterogeneous for chlorsulfuron-resistance were found in 70% of the sample areas and all of the plants were susceptible in 30% of the sample areas.
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