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Weekly UpdatesWood products are used extensively in applications where they can be biodegraded by various fungi, insects such as termites, and marine organisms. To prevent this biodegradation, most wood products for exterior use are treated with a biocidal preservative system. Wood preservation has undergone recent dramatic changes worldwide due to environmental and disposal concerns and governmental regulation (Preston 2003, Schultz et al. 2007). In North America the major residential preservatives are copper-rich systems, where copper (II) is combined with an organic cobiocide to control copper-tolerant fungi (Kamdem 2008, Freeman 2008). The two major commercial systems are copper quat and copper azole, with several formulations of each available.
The standardization or accreditation process for new or modified systems involves developing a comprehensive data packet with laboratory and field efficacy and depletion data, formulation, analytical procedures, environmental and health aspects, and proposed applications, that is submitted to the accrediting agency. This process is designed to ensure that the consumer receives a dependable and safe product. In the United States, most wood preservatives are standardized by the American Wood Protection Association (AWPA) or the International Code Council (ICC) (McIntyre 2008).
Some professionals are concerned about the adequacy of the process, with formal and informal discussions taking place. This includes the relevancy of laboratory efficacy tests to outdoor "real-world" field results, the field exposure duration necessary with small research stakes to ensure good performance during the long service life expected by consumers from commercial-size treated wood (Lebow et al. 2008), and if field trials for copper-organic ground-contact systems should be run at sites with copper-tolerant fungi. About 20 years ago, extensive laboratory studies were initiated on a number of copper-organic systems employing common laboratory efficacy tests (Nicholas and Sites 1995, Helmer 2008), and three promising candidates were identified. These three systems were subsequently employed to treat southern pine (SP) research size stakes which were installed at the Dorman, Mississippi, and Saucier, Mississippi, test sites, with copper-tolerant decay fungi known to be present at Donnan. Additionally, a set of larger SP 2 by 4 nominal stakes were treated and installed at Dorman. All of the stakes were treated with the same formulations for the three systems. This paper reports the average decay ratings for all three sets at 39 months and the long-term decay ratings for the 2 by 4s and the Saucier research sets after about 17 years of exposure.
The objectives were to:
1. determine field decay efficacy of the three systems that were selected based on promising laboratory tests,
2. compare the average 39 months' fungal ratings for the research size stakes at the two sites, one of which has copper-tolerant fungi, and
3. compare the ratings at about 3 years of exposure for the Dorman research size stakes to short- and long-term performance for commercial-sized material installed in an adjacent plot.
Methods
The three systems were initially selected following agar plate synergism (Schultz and Nicholas 1995) screening, soil block AWPA E 10 tests which employed the supposedly copper-tolerant fungus Postia spp. and other decay fungi, and fungus cellar AWPA E14 tests for soft-rot efficacy (Nicholas and Sites 1995) which included depletion samples. The latter two laboratory screening tests are commonly employed in the initial laboratory screening of potential commercial systems (Helmer 2008). The three soluble copper(II) systems that showed promise were copper plus benzoic acid, copper plus naphthenic acid, and copper plus relatively small amounts of the azole (or triazole) propiconazole. The latter system is similar to current commercial copper azole (CA) systems except that ammoniacal copper sulfate was used in the experimental formulation and the azole employed commercially is either tebuconazole or a propiconazole/tebuconazole mixture. The copper plus naphthenic acid formulation was intended to result in the in-situ formation of copper naphthenate (not confirmed by chemical analysis), but generally employed much higher copper levels than the commercial waterborne copper naphthenate system. Even though there are similarities in the actives between the latter two research formulations and current commercial systems, results from these systems should not be directly compared to the commercial systems due to formulation differences. For example, all of the systems were formulated using copper sulfate and later studies showed increased copper depletion with copper sulfate than copper carbonate.
Field testing consisted of two sets of research size stakes 19 by 19 by 457 mm (0.75 by 0.75 by 18 in.) and one set of nominal 2 by 4 by 18 inch/457 mm stakes, all cut from defect-free SP sapwood. All of the samples were treated with the same formulations by a full-cell process, with the retentions based on weight gain and the copper retention based on copper as metal. The water-based formulation for the copper/benzoic acid and copper/azole treatments consisted of copper sulfate plus benzoic acid or propiconazole with ammonia and sufficient acetone to dissolve the organic biocide. The copper sulfate plus naphthenic acid waterborne formulation contained ammonia plus small amounts of propylene glycol.
Two sets of research size stakes were installed at the Dorman, Mississippi, and Saucier, Mississippi, test sites, with 10 replicate stakes per treatment at each location. One set of nominal 2 by 4 stakes were installed adjacent to the research size stakes at Dorman. The Dorman plot is located in northeast Mississippi, has an acidic heavy clay soil that is poorly drained, and is in a high (AWPA 4) deterioration zone. Copper-tolerant decay fungi are sporadically present (Schultz et al. 2000). The Saucier plot is located in the Harrison National Forest near the Gulf Coast, has an acidic sandy loam soil that is well drained, and is classified as a severe (AWPA Zone 5) deterioration zone. Copper-tolerant fungi are known to exist near this area but this specific plot has much lower prevalence than at Dorman (Schultz et al. 2000, Lebow et al. 2003). The stakes were annually inspected up to 3 years, based on an earlier AWPA E7 10, 9, 8, 7, 6, 4, and 0 rating system where 10 is no attack, 9 is a trace to 3 percent, etc.; the "6" rating was employed a few years prior to it being adapted by the AWPA. The 2 by 4 Dorman and Saucier research stakes were reinspected after about 17 years of exposure. Unfortunately, the Dorman research stakes were pulled at about 15 years to make room for another study but only about 200 of the initial approximately 700 stakes were saved. The area was scanned several times with a metal detector and an additional approximately 100 stakes that failed were identified. Due to the large number of missing stakes, however, the long-term data from this set were not used. Also, intermediate ratings were not conducted on the three sets. Only the decay ratings are given in this paper as minimal termite attack generally occurred.
Results and discussion
Table 1 shows the treatments, average retentions for the nominal 2 by 4 stakes, average decay ratings, and the number of stakes that had an efficacy rating of "7" or less at 39 months for all stakes (an AWPA rating or "7" or less is generally considered as cause for concern), and the decay ratings at 212 months for the Saucier research size stakes and at 210 months for the Dorman 2 by 4 stakes. Since all of the stakes were treated with the same formulation, only the retentions for the 2 by 4 stakes are shown.
Laboratory vs. outdoor ground-contact efficacy
As previously mentioned, all three systems were selected from a large list of candidates following extensive laboratory efficacy studies employing agar plate, soil block, and fungus cellar tests. Despite the extensive initial screening employing standard tests and fungi to select promising systems, the copper/benzoic acid system performed very poorly at both field test locations (Table 1). Specifically, a number of failures were noted after only 39 months at Dorman and a few failures also occurred at Saucier. The other two systems, copper/ naphthenatic acid and copper/propiconazole which have similarities to current commercial systems, performed better.
The poor prediction of the laboratory efficacies to field trials for the one system, especially in the plot with copper-tolerant fungi, may be partially caused by many strains of the copper-tolerant Postia placenta fungus typically employed in laboratory tests having relatively poor copper tolerance. P. placenta (Fries) M. Larsen et Lombard is listed as one of two standard brown-rot fungi in laboratory soil block (AWPA E10 and E22) tests. Alternatively, the standard laboratory soil-block test employs sterilized soil and, therefore, does not have the microorganisms, such as bacteria and mold fungi, which are in a field site and can slowly detoxify an organic cobiocide.
Decay rates at Dorman Lake vs. Saucier in research stakes
More rapid or equivalent decay and a greater number of stakes with efficacy ratings of "7" or lower generally occurred at Dorman than Saucier. This is likely due to the presence of copper-tolerant fungi at Dornaan. By contrast, for treatments which only employed the organics naphthenic acid or propiconazole, more decay occurred at Saucier, which is in a higher AWPA deterioration zone than Dorman. Prior work has shown that when matched stakes treated with one of several different systems were installed at five different sites, the relative efficacy of the systems varied. For example, a sodium pentachlorophenate formulation was the most effective preservative in the harsh climate of Panama but was least effective in the relatively mild climate at Madison, Wisconsin (Lebow and Highley 2008). Thus, it is apparent that ground-contact results obtained from one site are not necessarily comparable to results from another site.
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