Division of Forest Protection, Taiwan Forestry Research Institute, 53 Nan-hai Road, Taipei, Taiwan

Background and objectives
Phellinus noxius (Corner) Cunningham is widely distributed in tropical regions in Southeast Asia, Oceania and Africa, where it causes a brown root rot that is responsible for decline of numerous orchard and forest tree species. In recent years, the disease has become one of the most serious problems of fruit and forest trees in central and southern Taiwan at elevations less than 800 m [1]. Because basidiocarps of P. noxius are rarely observed on diseased trees in the field, long-distance dispersal by basidiospores is apparently uncommon. Several slowly expanding, circular disease patches extending from infection centers were observed in the field, indicating that the disease caused by P. noxius is mainly spread from diseased to healthy trees by root contact. The fungus can survive in soil and in roots of dead host plants for a long time, and infected root debris is the primary source of inoculum [2]. Hence, removal of infected root debris or killing the fungus within debris has been considered as the most effective control method for the disease. However, for even a small tree, complete removal of the infected root debris is very difficult. Therefore, integrated methods of removing the coarse infected roots and killing the fungus within small debris will greatly enhance the effectiveness of control practices. In the preliminary study, urea amendment to soil was found to be fungicidal to P. noxius infested wood in soil. It has been reported that urea is able to reduce populations of certain soilbome fungi through release of free toxic NH3 upon hydrolysis. However, the effects and mechanisms of urea on survival of woody debris in soils harboring root rot fungi have not been documented. The purpose of this study was to investigate the mechanism of urea fungicidal to P. noxius in infested wood.

Results and conclusions
Phellinus noxius was not recovered from pieces of artificially infested wood placed in soil and on soil amended with urea, (NH4)2CO3 or ammonia water, while high concentrations of volatile NH3 were detected from these treatments, indicating that volatile NH3 generated from these chemicals was fungicidal to P. noxius . A high concentration of volatile NH3 was detected from the treatment of urea amended to nonautoclaved soil, but was not detected from autoclaved soil, indicating that soil microorganisms are involved in NH3 generation from urea in soil. A concentration of 3000 ppm of urea was required to kill the fungus completely. Urea generation of volatile NH3 which was fungicidal to P. noxius was associated with alkaline soil, but the effect was not observed in acidic soil. Urea was able to generate similar quantities of volatile NH3 from 12 to 32C that was fungicidal to P. noxius . Urea generated a high quantity of NH3 in lower soil matric potentials ranging from -0.75 to -0.15 MPa which was fungicidal to P. noxius , while high soil matric potentials (>0.025 MPa) or flooded soil hindered the production of volatile NH3 from urea. Ammonia was more effective in killing P. noxius growing in smaller sized pieces of wood, the fungus was not recovered from wood sizes smaller than 3 cm in diameter. In addition to P. noxius , volatile NH3 was also fungicidal to the six root rot fungi, Ganoderma australe, G. lucidum, G. tropicum, Rigidoporus vinctus, Heterobasidion annosum, and Rosellinia necatrix, indicating that it has a wide fungicidal spectrum to root rot fungi parasitic on woody plants.

1. Chang TT, 1995. Plant Disease 79, 962-965.
2. Chang TT, 1996. Phytopathology 86, 272-276.