Screening of herbicides for selective weed control in African marigold (Tagetes erecta)
African marigold (Tagetes erecta L.) was screened for tolerance to preemergence and postemergence herbicides. Three preemergence herbicides, pendimethalin, bensulide, and trifluralin, had an acceptable margin of crop safety both in the greenhouse and in the field when applied at one, two, and four times their labeled rates. Two postemergence herbicides, topramezone and encapsulated acetochlor, caused minimal visual injury to the crop and encapsulated acetochlor had a minimal effect on plant dry weight. Dry weight response to topramezone varied among experiments. Additional studies were conducted to better define African marigold tolerance to topramezone. African marigolds tolerated rates up to 36.8 g topramezone ha-1. Visual injury five days after application (DAA) was 3.1 or less (one to nine scale) for plants treated with the label rate, 18.4 g ha-1, or less of topramezone. Fifteen DAA, injury was no longer evident. Applications of 73.7 g ha-1 and greater caused visual ratings exceeding 3.5 at the five day evaluation and eventually resulted in plant death. Topramezone and acetochlor were applied in a tank mix to African marigold in the greenhouse and in the field. In the greenhouse, visual injury was 4.6 or less seven DAA and plants made a full visual recovery by 15 DAA. In the field, visual injury was 3.3 or less one week after application (WAA). Plants treated with 18.4 g ha-1 topramezone recovered by three WAA, other visual injury was less than 2.7. Data from both studies indicated minimal injury and rapid visual recovery of treated plants. A study observing crop response to topramezone application when African marigold was grown in pendimethalin treated soils indicated that pendimethalin did not affect topramezone tolerance. A tank mix of topramezone with the insecticide malathion was evaluated to determine if tolerance of African marigold to topramezone is achieved through differential metabolism involving cytochrome P450. Five and 15 DAA, malathion caused no increase in visual injury (P = 0.3986 and P = 0.1315). This indicates African marigold tolerance to topramezone is probably due to a selectivity mechanism other than cytochrome P450 based metabolism.