Influence of Ammonium Thiocyanate on Scorching and Control of Bracken (Pteridium aquilinum) by Amitrole

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Abstract
The field performance of amitrole (3-amino-s-triazole) formulations containing a number of additives was tested on bracken [Pteridium aquilinum (L.) Kuhn var. aquilinum]. All formulations that did not contain thiocyanate produced severe foliar scorching within 72 h of spraying. The following season, control was relatively poor (63 to 77% reductions in frond density). The formulation of amitrole with sodium iodide produced particularly severe scorching followed by a nonsignificant reduction in frond density (2%) the following season. Formulations containing ammonium thiocyanate (molar ratios of 1:0.5, 1:0.75, 1:1, and 1:1.25 amitrole:ammonium thiocyanate) greatly reduced this foliar scorching, and control the following season was of the order of 94 to 97% in all cases. Translocation was proposed as being the limiting factor in control, ammonium thiocyanate increasing translocation by preventing scorching and hence decreasing decomposition in the foliage. To investigate scorching further, a bioassay, which consisted of floating bracken leaflets on amitrole solutions, was developed. The commercial 1:1 molar ratio of amitrole:ammonium thiocyanate may not be optimum for all conditions. Other additives were tested and cyanide and hexacyanoferrate (II) were found to be very active scorch inhibitors.