Antimicrobial Silver in Maple Sap Collection

INTRODUCTION The ability of some metals to inhibit the growth of or to kill certain microorganism is well established. In particular, the use of silver as a method to control microorganisms to maintain water quality has been known for centuries (Landau 2006). Silver coins were added to pots or barrels of water to preserve freshness during extended land and sea voyages when potable water sources were uncertain. Ions of silver (and other oligodynamic metals) work by interfering with the metabolism and reproduction of microbes, resulting in their death. Contact between the microbe and silver-containing material is required to affect surface exchange of ions. Silver does not readily dissolve in water, so there is little or no residual action of silver in solution. The role of microorganisms in the premature cessation of maple sap flow (taphole drying) has been well recognized for some time (Naghski and Willits 1955). Bacteria, fungi, yeasts, molds and algae can colonize tapholes and sap collection systems, thereby reducing sap flow via a combination of vessel plugging by microbial biomass (Ching and Mericle 1960), or through the normal response of trees to close off the wound (taphole) to prevent microbial spread throughout the tree tissues (Shigo 1965, Walters and Shigo 1978). In both cases, the result is the same: sap flow is reduced and eventually stops. During the late-1950s and early-1960s, research was conducted to investigate ways to reduce or eliminate microbial contamination and attendant tap-hole drying in maple sap collection systems (Sheneman et al. 1958, Costilow et al. 1962). A number of different compounds were tried at that time, including paraformaldehyde (PFA), oligodynamic silver, antibiotics, sorbic acid, mer-curic iodine, and sodium hypochlorite (Sheneman et al. 1958). Of all the substances tried, ³.. . only paraformaldehyde appeared promising for commercial use.´ (Shenaman et al. 1958). Subsequently, PFA in tablet form was introduced as a taphole disinfectant, and registered by the E.P.A. as an approved pesticide for maple use. Using PFA in tapholes resulted in increased sap flow, especially in the latter part of the sap collection season, and could result in increases in sap yield up to 96% (Costilow et al. 1962). After a period of use in the maple industry, it was found that PFA interfered with the wound healing response of trees, resulting in greatly increased staining columns and decay, and higher levels of morbidity and mortality in trees in which PFA was used (Shigo …