The toxicities of four candidate peptides, which have potential for engineering disease resistance into poplars and conifers, were tested and compared in vitro. Cecropin B, (Ala8,13,18)-magainin II amide, and the two synthetic membrane interactive molecules (Peptidyl MIMs(tm)) D2A21 and D4E1 inhibited germination of spores of the fungal pathogensCronartium ribicolaJ.C. Fisch.,Gremmeniella abietina(Lagerberg) Morelet,Melampsora medusaeThuem.,Nectria galligenaBres. in Strass.,Ophiostoma ulmi(Buisman) Nannf., andSeptoria musivaPeck. Minimal inhibitory concentrations (MICs) of peptides required to achieve >95% inhibition of germination of conidia and urediniospores ranged from 0.3 to >5 µM. Permutation analyses on MICs confirmed that peptides significantly reduced germination of fungus spores (P= 0.0038) and that MICs of the two synthetic peptides were, on average, lower than those of the two natural peptides (P= 0.0012). In contrast, peptides had much less of an effect on the pollen ofPicea,Pinus, andPopulusspecies and seeds of white pine; MICs from 10 µM to >50 µM of peptide were needed to arrest germination. Peptidyl MIM(tm) D2A21 combined high antifungal activity with low pollen toxicity. Similar to some antimicrobial peptides from plants, Peptidyl MIM(tm) D2A21 could not prevent germination of fungal spores in the presence of potato dextrose broth or chloride salts with divalent cations. This may explain why the peptide had only a limited effect on fungal mycelium grown in potato dextrose broth medium.Key words: antimicrobial peptides (AMP), white pine blister rust, scleroderris canker, poplar leaf rust, Dutch elm disease, septoria canker.