The effects of simulated rain acidified with H2SO4 were studied on 5 host-parasite systems. Plants were exposed in greenhouse or field to simulated rain of pH 3.2 or pH 6.0 in amounts and intervals common to weather patterns of North Carolina [USA]. Simulated acidic rain resulted in an 86% inhibition of the number of telia produced by Cronartium fusiforme on willow oak (Quercus phellos); a 66% inhibition in the reproduction of root-knot nematode (Meloidogyne hapla) on field-grown kidney beans (Phaseolus vulgaris ''Red Kidney''); a 29% decrease in the percentage of leaf area of field-grown kidney beans affected by Uromyces phaseoli; and either stimulated or inhibited development of halo blight on kidney bean (caused by Pseudomonas phaseolicola), depending on the stage of the disease cycle in which the treatment were applied. The effect varied as follows: simulated acidic rain applied to plants before inoculation increased disease severity by 42%; suspension of bacteria in acidic rain resulted in no infection; and acidic rain applied to plants after infection inhibited disease development by 22%.