Sensitivity of fungi to urea, ammonium nitrate and their quimolar solution UAN

The sensitivity of oomycota, saprophytic and pathogenic fungi to urea, ammonium nitrate and UAN (urea plus ammonium nitrate in equimolar solution) was studied in laboratory trials. The compounds were applied in agar in concentrations of 0.06, 0.19 and 0.6M. The most toxic was urea. Ammonium nitrate inhibited the growth of fungi only in higher concentrations. In contrast, the growth of Gaeumannomyces graminis was stimulated by even the highest concentration of 0.6M ammonium nitrate. The fungi most sensitive to urea and UAN were Alternaria tenuissima, Botrytis cinerea, Cladosporium cladosporioides and Pseudocercosporella herpotrichoides. No synergistic effect between the two compounds in UAN was found. Urea was toxic also to Colletotrichum acutatum which does not produce urease. Likewise, the urease inhibitor NBPT did not decrease the toxicity of urea to fungi; the urea degradation product ammonia should, therefore, not be assumed to be the only toxic agent. Application of urea in agricultural practice can decrease the population of a pathogen not only by the stimulation of antagonists, but also by the direct toxic effect. The tested concentrations of 0.06-0.6M correspond to 0.36-3.6% (w/w) solution of urea and to 0.64-6.4% UAN used in agricultural practice as a 75% water solution. If the dilution and metabolisation under natural conditions is taken into account, the concentration of urea 0.06M (0.36%) was too low to have an effect of practical importance on fungi. While after application of urea on plants or on plant debris its concentration is increasing due to water evaporation, the concentration of the extremely hygroscopic UAN is decreasing. Therefore, the control effect will depend more on the applied rate than on the concentration.