Competition for ammonium and nitrate forms of nitrogen between some phytopathogenic and antagonistic soil fungi

The role of competition for nitrogen in interactions of pathogenic fungi and antagonists was investigated. Five phytopathogenic fungi were used in the experiments, three of Fusarium spp. (Fusarium solani, Fusarium sambucinum, and Fusarium moniliforme) together with Rhizoctonia solani and Sclerotinia selerotiorum. Of the five agonistic fungi tested, four belonged to the Trichoderma spp. (Trichoderma longibrachiatum, Trichoderma harzianum, Trichoderma viride, and Trichoderma koningii), additionally Gliocladium roseum was included in the study. Compared to the phytopathogenic fungi, in most cases the antagonistic fungi used nitrogen as ammonium at statistically significant higher rates during the first six days of cultivation. After first using nitrogen as ammonium antagonistic fungi switched to using the nitrate form of nitrogen as ammonium was nearly used up. In contrast the phytopathogenic Fusarium spp. used nitrate significantly faster than the antagonistic fungi. R. solani and S. sclerotiorum first used ammonium and when it became limited, they switched to nitrate. The Fusarium spp. used the two sources of nitrogen simultaneously. Even at the beginning, when there was ample ammonium they used nitrite at the same time. If the use of nitrogen is regarded as only one variable, Fusarium spp. make up for the relative advantage of the antagonistic fungi by the fact that they are able to use both forms of nitrogen simultaneously. (C) 2003 Elsevier Science (USA). All rights reserved.