Experimental aspen waferboards, bonded with liquid or powdered phenol-formaldehyde resins and treated by various methods with a wide selection of preservatives, were tested for fungal resistance in accelerated laboratory trials. Mold growth on the sur face as well as weight and strength losses due to the actions ofdecay fungi were determined. Testing of board strength after decay in high and moderate hazard ex posure conditions required modification of decay tests used for solid wood. A range of protection was noted with no pre servative system exceedingthe efficacy of the inorganic salt formulations. Averaged over all treatments, strength loss and weight loss are well correlated. Field exposures of effective treatments are underway. Waferboard may become increasingly important as a structural panel product for residential and com mercial construction. Canada has several waferboard plants, and there presently are several in production in the United States. However, more waferboard/ flakeboard/OSB-typeplants may be built in the United States in the very near future (27). Projected demand for aspen waferboard includes many applications where durability against moisture and the deleterious effects of fungi and insects are necessary. Construction practices, paints, or sizings used to minimize moisture problems must be strictly maintained in service to effectively prevent damage by biological agents, and therefore, cannot be completely relied upon as permanent protection. In addition, decay in any portion of a structural sheet of waferboard would involve high replacement costs (16, 17). Although pres ervation of structural wood composites (nonveneered) has received scant attention in Australia and the Purchased by U. S. Department of Agriculture, Forest Service, for offical use. United States (12, 23), other nations such as the Federal Republic of Germany and New Zealand (9, 17) have developed guides for preservative treatment of such materials. It is therefore important to evaluate aspen (Populus tremuloides Michx.) waferboard, treated with various preservatives, by laboratory and field tests to provide data to assist in assessment of its potential service life in both high and moderate decay hazard usage. Waferboard and other nonveneered structural composites with decay and mold resistance would have potential application in numerous uses such as sheathing or subflooring in mobile homes and recreational vehicles and in the construction of ice-fishing shelters. Treated wa ferboard would also find use in certain watercraft com ponents and for some exterior uses, such as siding, within the United States. For example, in Puerto Rico 1/4-inchCanadian waferboard is being pressure treated with CCA and used for interior wall partitions. Previous reports have evaluated the weatherability and 1-year The authors are, respectively, Assistant Professor, Scien tist, and Professor, Dept. of Forest Prod., Univ. of Minnesota, St. Paul, MN 55108; and Technologist and Project Leader, USDA Forest Serv.,Forest Prod. Lab., P.O. Box 5130, Madison, WI 53705. T. Hubbard is gratefully acknowledged for as sistance. Mention of company or trade names is solely to iden tify the material used and should not be interpreted as an en dorsement by the Univ. of Minnesota or the U .S . Dept. of Ag riculture. This research was conducted jointly by the Univ. of Minnesota, St. Paul, Minn., and the USDA Forest Prod. Lab., Madison, Wis. under grant No. USDA FP-80-0252 and state agricultural research and McIntire-Stennis funds provided by the Univ. of Minnesota Agri. Expt. Sta.Published as Scientific JournalSeries No. 13,332 of theUniv. of MinnesotaAgri. Expt. Sta., Project Nos. 43-053 and 43-063. This article is based on information presented a t the Workshop on Durability of Struc tural Panels, Pensacola, Fla. (Oct. 1982) and the Fourteenth Annual Meeting of the International Research Group on Wood Preservation (IRG/WP/2195),Brisbane, Australia (May 1983). This paper was received for publication in February 1983. © Forest Products Research Society 1983. Forest Prod. J. 33(11/12):45-53. FOREST PRODUCTS J O U R N A L Vol 33, No 11/12 45 exposure durability of flakeboards made from ACA treated Ghanian hardwood flakes (14, 22). This study was made to determine the resistance of preservative-treated aspen waferboard to fungi in accelerated laboratory tests, and to determine the effects of fungi on board strength. Biodeterioration test ing followed the initial work which examined the effects of preservative, resin type, and treating methods on the mechanical and thickness swelling properties of aspen waferboard (15). Materials and methods