Field plots infested withSynchytrium endobioticum pathotype 2 were amended with agricultural lime, NH4NO3 and/or urea and planted with cv. Arran Victory. A weekly survey of changes in pH from planting to harvest revealed constant pH in absence of the chemicals, fluctuating pH with lime, very large increase then falling off with urea, and depressed below normal pH with NH4NO3. Best disease control occurred in urea-treated plotsvs NH4NO3-treated plots. Addition of lime, although it elevated pH, did not suppress the disease with urea or NH4NO3 added. Mechanisms for suppression by urea are related to its role as an ammonia producer and microbial stimulator, and their implication for infection byS. endobioticum.