A method for the control of microorganisms, fungi or oomycetes in agriculture, said method comprising applying to plants or to the environment thereof, an anti-microbial, anti-fungal or anti-oomycetes effective amount of a combination of: (i) an agriculturally acceptable acidifying agent, and (ii) an agriculturally acceptable source of nitrite ions or a nitrate precursor thereof, wherein the combination of (i) and (ii) acts as a source of nitric oxide. Systems and compositions for use in this method are also described and claimed.