Graduate Thesis Or Dissertation
 

Selective control of Cirsium arvense (L.) Scop. in Mentha piperita L. with 3,6-dichloropicolinic acid

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  • A series of field experiments was conducted during 1975 and 1976 to examine peppermint tolerance and Canada, thistle response to Dowco 290 (3, 6- dichloropicolinic acid). Peppermint oil yield and Canada thistle density were considered most important in evaluating experimental results. Peppermint tolerance to Dowco 290 was studied in weed-free peppermint (cv. Mitcham).. Rates of 0.25 lb/A or more reduced peppermint oil yield. Time of application appeared to be less critical than rate of herbicide as oil production decreased with increasing rates. Mint injury was sufficiently severe at high rates to cause yield reductions the year of treatment, but recovery was good and no reduction in oil yield was found 1 year later. Spring application of Dowco 290 to peppermint (cv. Mitcham and Todd's Mitcham) infested with Canada thistles resulted in erratic oil yield but good thistle control. Oil yield was difficult to evaluate because of the variability of the peppermint stand. All rates of Dowco 290 tested gave good short-term thistle control and rates of 0.5 lb/A or more gave excellent seasonal control, There was no advantage to split applications of Dowco 290 (fall plus spring) over a single application in the spring. An excellent combination of good thistle control and high oil yield was obtained from sequential treatments in the spring when 0.125 lb/A was applied 10 weeks prior to harvest and 0.063 lb/A was applied 2 weeks later. Trans location of Dowco 290 through underground thistle parts was demonstrated by treating parent plants and observing the response in a connected daughter plant. The herbicide was not lethal at rates that translocated to connected plants, but it did cause abnormal floral development. Dowco 290 was sufficiently active when applied at 0.25 lb/A to control all underground plant parts of test thistles even when treated plant parts were removed as soon as 1 hour after treatment. Root mortality was measured by a modified tetrazolium test. Seedling Canada thistle plants were slightly more sensitive to Dowco 290 than plants which developed from mature rootstock. After seedling plants had developed a more complex root system, regrowth from rootstock of plants grown from seed and from mature rootstock was similar.
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Déclaration de droits
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  • File scanned at 300 ppi (Monochrome, 24-bit Color) using Capture Perfect 3.0.82 on a Canon DR-9080C in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
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