Interactive effect of ammonia and pH on viability and ¹⁴C-labeled exudation from fungal propagules

The effect of ammonia on exudation and viability of 14 C-labeled propagules of Cochliobolus victoriae, Cochliobolus sativus, and Macrophomina phaseolina was studied in a model fungistatic system with nutrient-independent or nutrient-dependent propagules or in soil amended with urea from which ammonia was generated. In soil, ammonia stimulated exudation of 14 C-labeled compounds, but their metabolism by the soil microbiota was suppressed. Short-term exposures of a few hours in a model fungistatic system with high free ammonia concentrations (up to 10 000 mg/L) stimulated a burst of exudation which was usually associated with death of the propagules. Lower concentrations increased exudation less, but exudation tended to be maintained at a level greater than that in nontreated controls. Viability was reduced when the propagules were continuously exposed to long-term doses which were nonlethal for shorter durations. High pH by itself increased exudation, but it did not account for the greater exudation effected by ammonia, nor for its lethal effects. In a fixed-volume system, which imposed low nutrient stress, ammonia (10–100 mg/L) had a sparing effect on the increased exudation brought on by high pH alone; as ammonia concentration was increased (100–1000 mg/L), the sparing effect was lost and exudation increased, accompanied by decreased viability.