Penicillium fungi from Picea glehnii seeds protect the seedlings from damping-off

•  In a coinoculation test of seed-epiphytic Penicillium species and pathogenic Pythium vexans to Picea glehnii seedlings, the number of surviving seedlings which were inoculated with P. vexans 5 d after preinoculation with Penicillium damascenum PGS-O7 increased compared with those inoculated with P. vexans alone. •  The phytotoxicity of antifungal compounds to P. glehnii seeds and seedlings was assessed in growth experiments. Production of these compounds was quantitatively analysed in the rhizosphere of the host seedlings using chromatography techniques. •  Dense mycelial growth of P. damascenum was observed microscopically around the roots of the inoculated seedlings. Furthermore, the antifungal and phytotoxic compound citrinin produced by P. damascenum was detected as a component released around the roots of the normally growing fungus-inoculated seedlings. •  We propose two possible mechanisms for the protection of P. glehnii seedlings by P. damascenum from P. vexans; first, chemical protection by an antibiotic agent, citrinin produced by P. damascenum, and second, the occupation of the space around the P. glehnii roots by the P. damascenum mycelia.