82 Matsumoto et al.: Evaluation of Compressive Strength of Decayed Wood in Magnolia obovata A total of 195 decay test specimens were obtained, 15 of which were used as con- trols. Before incubation, the other 180 specimens were dried for one day at 60°C (140°F) and then weighed. All of the speci- mens were sterilized with propylene ox- ide for two days in a desiccator in vacuo. A total of 180 specimens were used to test decay by the brown rot fungus Fomitop- sis palustris FFPRI 0507, the white rot fun- gus Trametas versicolor FFPRI 1030, and the soft rot fungus Chaetomium globosum NBRC 6347. F. palustris FFPRI 0507 and T. versicolor FFPRI 1030 were provided by the Forestry and Forest Products Research Institute (Tsukuba, Ibaraki, Japan). C. globosum NBRC 6347 was obtained from the National Institute of Tech- nology and Evaluation (Kisarazu, Chiba, Ja- pan). Each fungus was precultured on a Potato Dextrose Agar (Difco Laboratories, U.S.) slant medium in a test tube (1.5 cm in diameter, and 15 cm in length). After mycelia had spread on the medium, the specimens, in which the mois- ture content had been adjusted to 50%–70% by dipping into sterilized distilled water, were put onto the medium in test tubes and then were in- cubated at 26°C (78.8°F) under dark conditions. In the decay test, 15 specimens per fungus were collected every 15 days up to 60 days immediately following the start of incuba- tion. After the specimens had been recovered from the culture, the mycelia were carefully removed from the surface of the decayed specimens. Then the wood samples were oven-dried for one day at 60°C. The dry weight of the decayed specimens was measured in order to calcu- late the percentage of weight loss. After the dry weight was mea- sured, the specimens were used for the subsequent experiments. A total of five sound and 60 decayed specimens were used to measure the compressive strength (CS) by Fractometer (Fractometer II). Five specimens of each fungal species were collected every 15 days up to 60 days after the start of incuba- tion. After the mycelia had been collected from the surface of the decayed specimens, the dry weight of the specimens was measured. The moisture content of the dried specimens was adjusted to 50%–70% with distilled water. To prepare the com- pressive test specimens (5 mm in diameter, 5 mm in length), decayed specimens were cut into small pieces about 5 mm in length. About five compression test specimens were prepared from one decayed specimen. Compressive strength parallel to grain was measured with a Fractometer. To calculate the mean values, about 25 compressive strength values were obtained from about five decayed specimens in each decay period per fungus. Wood meal was prepared from 15 specimens decayed for each period per fungus. Although residual agar medium on the surface of decayed wood specimen was not affected on the amounts of wood chemical components, mycelium, and agar medium, was carefully removed from surface of decayed wood specimen by tweezers. Wood chemical components (Klason lignin, holocellulose, and α-cellulose) were determined ac- Figure 1. Schematic diagram of the present study. cording to the TAPPI test methods (TAPPI 1991). Only one determination was done for chemical components, therefore the statistical analysis for chemical components was excluded. RESULTS AND DISCUSSION Time-Course Changes in Weight Loss, Compres- sive Strength, and Chemical Components of Wood Core Samples Figure 2 shows time-course changes in the weight loss ratios of core samples. On the 60th day after the start of decay, the weight loss ratios of wood core samples decayed by brown rot, white rot, and soft rot fungi were 27.4%, 45.7%, and 5.4%, respectively. The weight loss ratio by brown rot and white rot fungi increased as the decay period increased. Under the culture conditions in the present study, the weight loss ratio by the white rot fungus always showed higher values than those by the brown rot fungus through- out the decay period, indicating that white rot fungus decayed the specimens more severely than the brown rot fungus did. It is well known that the weight loss ratios of woods decayed by the brown rot fungus show higher values than those decayed by the white rot fungus (Fukuda and Haraguchi 1974). The results obtained in the present study did not correspond to those of the previous reports. However, Yoshimura (1966) reported that the weight loss of wood decayed by T. versicolor (white rot fungus) showed a much higher value than that decayed by F. palustris (brown rot fungus) when an agar medium was used for the wood decay ©2010 International Society of Arboriculture
March 2010
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