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Three tri-substituted spermidines, di-p-coumaroyl-caffeoylspermidine, tri-caffeoylspermidine and tri-p-coumaroylspermidine, isolated from pollen of Quercus alba, were examined for antifungal activity. Both di-p-coumaroyl-caffeoylspermidine and tri-p-coumaroylspermidine reduced mycelial growth of the oat leaf stripe pathogen, Pyrenophora avenae and reduced powdery mildew (Blumeria graminis f. sp. hordei) infection of barley seedlings when applied as a post-inoculation treatment. When used as a pre-inoculation treatment, only di-p-coumaroyl-caffeoylspermidine reduced powdery mildew infection significantly. Growth of P. avenae in the presence of 100 microM di-p-coumaroyl-caffeoylspermidine reduced activity of S-adenosylmethionine decarboxylase (AdoMetDC), and led to a reduction in the incorporation of labelled ornithine into spermidine. The other two spermidine conjugates increased AdoMetDC activity and the flux label from ornithine into spermine in P. avenae significantly.