Treatment of Norway spruce and Scots pine stumps with urea against the root and butt rot fungus Heterobasidion annosum—possible modes of action

Abstract

The mode of action of urea in inhibiting infection of stumps of Norway spruce and Scots pine by Heterobasidion annosum was investigated. Stem discs from both species were treated with either urea, ammonium salt, ammonia or potassium hydroxide. Neither urea per se nor ammonium ions affected the growth of H. annosum, but the fungus was inactivated in wood at pH≥7. Urea treatment of pine stem discs or stumps caused a significant rise in both pH (to pH>8) and ammonia content within 24 h in sapwood but not in heartwood. In spruce, rises in pH started in the outer sapwood and were followed by sequential increases progressively towards the heartwood. After urea application to freshly-cut pine and spruce stumps, ammonia formation and pH rise started immediately, and initially there was a correlation between pH rise and ammonia content in the wood. High pH and high ammonia content were maintained for at least 3–4 months in warm humid weather. Ammonia was released to a greater extent in spruce and pine bark than in the adjacent sapwood but without a corresponding rise in pH. Studies on the urease activity suggested that the urea was initially decomposed by urease of host rather than microbial origin. The rate of ammonia decomposition and pH rise was found to be strongly dependent on temperature.