Efficacy of copper:propiconazole and copper:citrate systems in ground contact exposure at a site with copper tolerant fungi
D D Nicholas, T Schultz
Southern yellow pine (SYP) sapwood field stakes were treated with copper alone (ammoniacal copper carbonate, ACC) at four levels, or three levels of copper (1.6, 3.2, or 6.4 kgm-3, as CuO), air-dried, then re-treated with propiconazole in a light organic solvent at 0.07, 0.3, or 0.7 kgm-3 retentions. In a separate study, SYP field stakes were treated with three levels of ACC to give 6.7, 13.4 or 29 kgm-3 retentions, or the same ACC levels plus citric acid at 38% of the CuO level. These ground-contact stakes were installed at a test plot which has copper tolerant fungi, and inspected regularly for fungal and termite degradation. At the most recent inspection, the copper azole stakes had been exposed for 118 months and the copper citrate stakes for 100 months. For the copper azole stakes, copper alone was only effective at the highest copper retention (10 kgm-3, CuO basis). In contrast, the copper azole-treated stakes were adequately protected with 1.6 kgm-3 CuO and the highest (0.7 kgm-3) level of propiconazole, or 3.2 kgm-3 CuO and 0.3 or 0.7 kgm-3 propiconazole, or 6.4 kgm-3 CuO and 0.07 kgm-3 (or greater) propiconazole. For the copper citrate stakes, stakes treated with copper alone performed slightly better at all three retentions than the copper:citrate-treated stakes. We conclude that the co-addition of propiconazole provides increased protection against copper tolerant fungi and other wood-destroying organisms, with increased azole levels necessary as the copper retention is lowered. In contrast, the co-addition of citric acid did not increase the efficacy of copper.