EFFECT OF PRESERVATIVE TREATMENT ON SURVIVAL OF FUNGI IN WESTERN RED CEDAR UTILITY POLES.
The ability of fungi to survive in western red cedar utility poles through the thermal treatment process was explored by removing increment cores from 200 poles before and after treatment. The treatment processes were varied to produce maximum temperatures ranging from 17[degrees] to 80[degrees]C at the pith center of the largest pole in a given charge. Although approximately 20 percent of the poles contained some evidence of visible decay prior to treatment, only one decay fungus was isolated from these poles. It is generally difficult to culture decay fungi from western red cedar heartwood and our results confirm this problem. As an alternative, we used the incidence of non-decay fungi before and after treatment as an indicator of heat efficacy. The incidence of non-decay fungi declined by 93 to 100 percent, depending on the treatment conditions, suggesting that the treatment processes were capable of eliminating most fungi from the poles. The paradox between non-sterilizing temperatures and low fungal surv ival was believed to reflect the fact that most of the fungi were present in the sapwood, where they were affected more directly by heat and preservative treatment.
Living trees are remarkably resistant to colonization by most fungi. Once cut, however, trees lose this resistance and can be colonized by a variety of fungi and insects. The rate of colonization depends on the wood species, as well as moisture content (MC) and temperature. For example, the heartwood of species with natural durability, such as redwood (Sequoia sempervirens) or western red cedar (Thuja plicata), will be colonized more slowly than species with thicker, less durable sapwood, such as southern pine (Pinus spp.). Colonization is less of a problem with lumber, since these materials are generally either kiln-dried shortly after sawing or dipped in fungicides to provide surface protection until the wood can be dried below the fiber-saturation point. In poles and other large timbers, however, portions of the wood may remain at or above the fiber-saturation point for many months after felling, providing an opportunity for colonization by an array of decay fungi .
The potential for fungal damage during seasoning and before either drying or
treatment has encouraged the development of both storage limitations and sterilization requirements . The most common sterilization requirement is 66[degrees]C for 60 minutes at the pith center . The amount of time required to achieve sterilization can vary widely depending on MC, pole size, and temperature conditions . Although sterilization is an excellent approach for eliminating decay fungi, questions have arisen concerning whether it is necessary for species with naturally durable heartwood, such as western red cedar.
The specifications and methods of treatment for western red cedar differ markedly from those used for other species because the thin sapwood shell of western red cedar generally does not require substantial pressure treatment to achieve complete penetration . As a result, western red cedar is often treated by using either relatively short thermal processes or very low pressures that may not result in internal sterilization. In addition, the specification for western red cedar typically allows for the presence of some internal decay in the butt . This defect is allowed because most heartrot fungi present in the standing tree do not continue to grow once the tree is cut and processed . One other important feature of western red cedar is that the naturally durable heartwood should be less susceptible to colonization by decay fungi during air-seasoning . As a result, there may be less need to eliminate fungi that might become established during the seasoning process. In fact, the risk of fungal invasion in the heartwood should be sharply lower than for species with little or no heartwood durability. For example, Douglas-fir, which contains a high proportion of moderately durable heartwood, is far less susceptible to degradation during air-seasoning than is southern pine, which consists primarily of more decay-susceptible sapwood [9,10]. Western red cedar heartwood should be even less prone to colonization during the season ing period.
Over the past decade, the processes used to treat western red cedar utility poles have evolved so that the treatment cycle is shorter and enclosed cylinders are used instead of the open tanks more typical of the older thermal process. Concerns were raised by utilities that the shorter treatment times might allow fungi to survive the treatment process, and become a problem for in-service poles. There is little information on the effects of these shorter cycles on survival of fungi in cedar poles. The following study was undertaken in order to better understand the potential for fungal survival.
MATERIALS AND METHODS
Two hundred western red cedar poles ranging in size from 15.0 to 22.5 m long (Class 2 to Class 3, according to the American National Standard Institute Standard ANSI 05.1) were selected for study. Fifty poles were evaluated at each of four sites: Olympia, Washington; Sandpoint, Idaho; Galloway, British Columbia; and New Brighton, Minnesota. The poles were inspected prior to treatment by measuring the amount of visible sapwood on the butt, and measuring MC at the butt and tip with an electrical resistance-type moisture meter 25 mm from the surface. The presence of decay at the butt, as well as in knots along the length of each pole, was noted. Increment cores were taken to the pith at the intended groundline (10% of the length plus 0.6 m) as well as at the midpoint and near the tip. The increment borer holes were plugged with tight-fitting wood dowels. The poles were numbered so that they could be identified after treatment. The cores were placed into plastic drinking straws, which were labeled and stapled shu t prior to being shipped to Oregon State University, Corvallis, Oregon. The cores were removed from the straws and small chips (approximately 1 mm square) were cut from five locations along each core. These chips were briefly flamed to eliminate possible contaminating surface fungi, then placed on the surface of 1.5 percent malt extract agar in plastic petri dishes. The plates were observed for 4 weeks, and any fungi growing from the wood were examined for characteristics typical of basidiomycetes, a class of fungi containing many important wood decay fungi. Fungi were classified as decay (basidiomycetes) or non-decay (all other fungi) for reporting purposes.
The poles were then commercially treated using various cycles (Table 1). Then additional increment cores were taken within 300 mm of the original sampling sites and these cores were cultured as described previously. The incidence of both non-decay and decay fungi in cores from poles at the four sites before and after treatment was used as a measure of treatment effectiveness.
After treatment, the relationship between treatment cycle and fungal colonization was examined. The maximum temperature at the pith was extrapolated by using heating curves generated by MacLean  from the diameter of the largest pole in a given charge. For simplicity, the total heating times were combined to produce an average temperature over the treatment cycle and the initial wood temperature was considered to be 15.6[degrees]C. This was probably higher than the actual pole temperature since several of the treatments occurred during the winter, but it provided a basis for comparison among charges.
RESULTS AND DISCUSSION INTERNAL POLE TEMPERATURES
The four treatment facilities used very different treatment cycles (Table 1). One plant used a more traditional thermal process with a total treatment time of 27.0 to 27.5 hours at temperatures between 90[degrees] and 107.8[degrees]C. The remaining plants used lower treatment temperatures and shorter treatment times.
Estimated pole temperatures (based upon the reported treatment cycles) varied widely. The target time and temperature required by the American Wood Preservers' Association (AWPA) for sterilization of Douglas-fir utility poles is 66[degrees]C at the pith center . The average internal temperature for the test poles ranged from as low as 16.7[degrees]C for the two charges that used relatively short treatment times (4.3 to 4.4 hr.) and lower treatment temperatures (71.7[degrees] to 80.0[degrees]C) to 80.0[degrees]C for the two charges that used more traditional thermal treatment processes. The target temperature (65.6[degrees]C) for fungal elimination was only achieved in four of the nine charges .
PRE-TREATMENT POLE CONDITION
Poles from the four sites varied slightly in MC at the butt (Table 1). The Washington poles were the wettest at 21 percent, while those from Minnesota were the driest at 13 percent. The presence of visible decay in the poles also varied by site. Six of fifty poles from Washington contained visible decay at either the butt or in knots along the length, while 14 of 50 poles from the Minnesota site contained decay. At the British Columbia site, 9 of the 50 poles contained visible decay, whereas at the Idaho site, 12 poles had visible decay. A total of 20.5 percent of the poles in the test contained visible decay. It is unknown whether this level of decay is typical of current pole production, but it suggests that decay fungi were already present in many poles.
PRE-TREATMENT FUNGAL COLONIZATION
Relatively few decay fungi were isolated from poles prior to treatment (Table 2). The low levels of isolation are typical of western red cedar. Previous studies have shown that decay fungi are not broadly distributed in the wood of this species and even isolations adjacent to visible decay pockets often fail to isolate the causal organism [5,6,8,11]. No decay fungi were isolated from poles prior to treatment in Minnesota or British Columbia, while 2.7 percent of cores from the Washington poles and 2.0 percent of cores from Idaho contained decay fungi. Only one decay fungus was isolated from a pole where decay was evident. No decay fungi were isolated from the other 40 poles showing visible evidence of decay. Inconsistent isolation of decay fungi is typical of this species. These low levels of isolation make it difficult to accurately assess the potential impacts of the treatment cycle on survival of decay fungi.
Colonization by so-called non-decay fungi tended to be higher at all sites, although there was considerable variation among the four sites (Table 2). Fungal colonization was highest at the Washington and Idaho sites. It is interesting to note that the pre-treatment MC at those two sites was slightly higher than the MC found at either the British Columbia or Minnesota sites (Table 1). Fungal colonization was somewhat lower in British Columbia and was only 12 percent of that found in Washington. Once again, MC of the British Columbia poles was only 13 percent, compared with 21 percent at the Washington site. Wetter poles should be more receptive to fungal colonization. Although the moisture levels in the poles were at or below the levels typically considered acceptable for microbial colonization , the differences imply that moisture conditions might have been suitable for fungal growth for a longer time in the Washington poles.
In addition to the isolation of fungi, a number of poles contained various bacteria prior to treatment. The role of bacteria in wood is poorly understood, but as with the non-decay fungi, they can serve as indicators of treatment efficacy. Bacteria were most abundant in the British Columbia poles, but were present at nearly similar levels in poles from Idaho and Minnesota. No bacteria were isolated from poles from Washington.
A single decay fungus was isolated from one core from a pole treated in British Columbia. The significance of this isolation is difficult to determine, particularly since the pole did not contain visible decay nor were decay fungi isolated from any poles at this site prior to treatment. Treatment conditions at the British Columbia site also entailed the longest heating periods of any site. As a result, the single isolation should be viewed cautiously.
Given the higher isolation frequencies of non-decay fungi in the poles prior to treatment, we elected to use these fungi as potential indicators of heating effectiveness. Although isolation levels of non-decay fungi were high before treatment, isolations declined precipitously after treatment. Isolations declined 99, 99, 93, and 100 percent in poles from Washington, Idaho, British Columbia, and Minnesota, respectively. The higher isolation level from British Columbia is again puzzling, particularly given the long heating cycles.
The low levels of fungi in most poles suggest that the combination of the naturally durable heartwood, which initially limits fungal colonization, and the heating cycles are adequate for eliminating most fungi initially colonizing the wood. A similar trend was found with bacteria. No bacteria were isolated from poles treated in British Columbia, Idaho, or Minnesota. Bacteria had not been isolated from poles in Washington prior to treatment, but were present at low levels after treatment. Bacteria are often overgrown by fungi and may have been missed in the pre-treatment samples. Although the species of the isolates were not identified, they may be heattolerant Bacillus. The role of these organisms in red cedar performance is difficult to assess, but it is clear that, like their fungal counterparts, they were mostly eliminated by treatment.
The relatively low isolation levels following treatment temperatures that would not normally be classified as lethal to fungi may be because the fungi were established near the wood surface. Although maximum temperatures at the pith might remain relatively unchanged over the shorter treatment cycles, the temperature near the surface will be higher. In many cases, more fungi will be isolated from the sapwood of the poles than from the heartwood, reflecting the availability of nutrients in this zone (data not shown). Temperatures in the relatively thin sapwood of western red cedar are likely to approach and exceed the lethal temperature, even with the shortest treatment cycle employed (7). In addition, preservative penetration in the zone should kill any established fungi. Thus, a major component of the reduced incidence of fungi and bacteria might be attributed to the combined action of the near-surface heat and penetrating preservative.
Although many poles contained visible evidence of decay, it was difficult to culture active decay fungi from the wood. Exposure to various treatment processes produced marked declines in the incidence of non-decay fungi, suggesting that the relatively low temperatures achieved during these processes were adequate for eliminating these fungi. Further studies to characterize the distribution of fungi in western red cedar logs prior to treatment might help to better understand the relationship between heating at various depths and fungal survival.
The authors are, respectively, Professor and Senior Research Assistant, Dept. of Forest Prod., Oregon State Univ. (OSU), Corvallis, OR 97331; and Treating Manager, Western Div. Timber Prod. Inspection, 105 SE 124th Ave., Vancouver, WA 98684. This is paper 3413 of the Forest Res. Lab., OSU. This paper was received for publication in July 2000. Reprint No. 9155.
(*.) Forest Products Society Member.
[c] Forest Products Society 2001.
Forest Prod. J. 51(9):69-72.
(1.) American National Standards Institute. 1992. American National Standard for Wood Poles: Specifications and Dimensions. ANSI 05.1-1992. ANSI, New York. 26 pp.
(2.) American Wood Preservers' Association. 1999. Western redcedar, Alaska yellow cedar, northern white cedar, and western larch poles - preservative treatment by thermal processes. Standard C35-97. AWPA Book of Standards. AWPA, Granbury, TX. pp. 12 1-123.
(3.)-----. 1999. Standard for purchase of treated wood products. Standard M1-96. AWPA Book of Standards. AWPA, Granbury, TX. pp. 295 - 297.
(4.) Chidester, M.S. 1939. Further studies on the temperatures necessary to kill fungi in wood. In: Proc. American Wood Preservers' Assoc. 35:319-324. AWPA, Granbury, TX.
(5.) Duncan, C.G. and F.F. Lombard. 1965. Fungi associated with principal decays in wood products in the United States. Res. Pap. WO-4. USDA Forest Serv., Washington, DC.
(6.) Eslyn,W.E. 1970. Utiuity pole decay. Part II. Basidomycetes associated with decay in poles. Wood Sci. and Technology 4:97-103.
(7.) MacLean, J.D. 1952. Preservative treatment of wood by pressure methods. Agri. Handb. 40. USDA Forest Serv. Washington, DC. l6O pp.
(8.) Scheffer, T.C., B.S. Goodell, and F.F. Lombard. 1984. Fungi and decay in western redcedar utility poles. Wood and Fiber Sci. 16:543-548.
(9.) Sexton, C.M., S.M. Smith, J.J. Morrell, B.R. Kropp, M.E. Corden, and R.D. Graham. 1992. Identity and distribution of Basidiomycotina colonizing Douglas-fir poles during three years of air-seasoning. Mycological Res. 96:321-330.
(10.) Smith, S.M., R.D. Graham, and JJ. Morrell. 1987. Influence of air-seasoning on fungal colonization and strength of Douglas-fir pole sections. Forest Prod. J. 37(9):45-48.
(11.) Southam, C.M. and J. Ehrlich. 1950. Etiology of some sap rots of western redcedar poles. Phytopathology 40:439-444.
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(13.) Zabel, R.A. and JJ. Morrell. 1992. Wood Microbiology: Decay and Its Prevention. Academic Press, San Diego, CA. 474 pp.
TABLE 1. Characteristic of western red cedar poles and the conditions used to heat them during preservative treatment at four treatment plants. Largest pole Charge Initial Poles with no. Site MC Class Diameter visible decay (%) (cm) 93-187D WA 21 2-75 43.5 6/50 93-188D 2-75 43.5 B14 BC 16 2-65 41.0 9/50 B16 2-65 41.0 18F13 ID 19 2-70 42.3 12150 18F14 2-75 43.5 62 MN 13 2-75 43.5 14/50 64 2-75 43.5 67 2-65 43.5 Time and temperature Charge ([degrees]C) no. Conditioning Pressure/soak 93-187D 2.5 hr. @ 82.2[degrees] 2.0 hr. @ 82.2[degrees] 93-188D 2.5 hr. @ 82.2[degrees] 2.0 hr. @ 82.2[degrees] B14 6.0 hr. @ 107.8[degrees] 20 hr. @ 93.9 [degrees] B16 6.5 hr. @ 107.8[degrees] 20 hr. @ 90.0[degrees] 18F13 9.25 hr. @ 110[degrees] 4.0 hr. @ 87.8[degrees] 18F14 9.0 hr. @ 110[degrees] 35 hr. @ 87.8[degrees] 62 2.5 hr. @ 80.0[degrees] 0.3 hr. @ 78.3[degrees] 64 1.75 hr. @ 80.6[degrees] 0.25 hr. @ 76.1[degrees] 67 1.83 hr. @ 78.3[degrees] 0.25 hr. @ 74.4[degrees] Charge no. Expansion Vacumm 93-187D 2.0 hr. @ 87.8[degrees] -- 93-188D 2.0 hr. @ 87.8[degrees] -- B14 1.0 hr. @ 103.3[degrees] -- B16 1.0 hr. @ 103.9[degrees] -- 18F13 1.75 hr. @ 101.7[degrees] -- 18F14 2.0 hr. @ 101.7[degrees] -- 62 0.5 hr. @ 77.2[degrees] 2 hr. @ 73.9[degrees] 64 0.3 hr. @ 75.0[degrees] 2 hr. @ 71.7[degrees] 67 0.3 hr. @ 74.4[degrees] 2 hr. @ 74.4[degrees] Maximum Charge temperature no. at pith [a] ([degrees]C) 93-187D 21.1 93-188D 21.2 B14 80.0 B16 80.0 18F13 65.0 18F14 59.4 62 18.3 64 16.7 67 16.7 (a)Values estimated from a starting temperature of 15.6[degrees]C at the pith center using the data of MacLean (7). TABLE 2. Microbial colonization of western red cedar poles seasoned at four sites as measured before and after preservative treatment with pentachlorophenol in P9 Type A oil. Isolation frequency [a] Pre-treatment Post-treatment Test Decay Non-decay Decay site fungi fungi Bacteria fungi (%) WA 2.7 76.7 0 0 ID 2.0 68.7 6.0 0 BC 0 45.3 10.7 0.7 MN 0 9.3 8.7 0 Test Non-decay site fungi Bacteria WA 0.8 2.0 ID 0.7 0 BC 4.0 0 MN 0 0 (a)Values represent cultures of 150 cores removed from 50 pols per test site before and after preservative treatment.
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|Author:||MORRELL, J.J.; FREITAG, C.M.; EDDINGTON, C.|
|Publication:||Forest Products Journal|
|Date:||Sep 1, 2001|
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