Arbuscular mycorrhizal fungi (AMF) are ubiquitous soil microorganisms establishing mutualistic relationships with plants. AMF inoculation is used to increase plant productivity, including in horticulture, and AMF are introduced into commercial peat-based substrates. Little is known, however, about long-term persistence of AMF in these substrates, especially if presuming adverse storage conditions. The study tested long-term infectivity of AMF introduced to a peat-based substrate stored in five different temperature regimes (constant temperatures of -20, 5 or 20 degrees C) and two outdoor regimes simulating actual practice (shade and direct sunlight). Eight AMF inocula were tested, including monocultures of Claroideoglomus claroideum, Funneliformis caledonium, and F. mosseae plus two isolates of Rhizophagus irregularis, all applied as 4% (v) of the substrate, or their mixture in 1%, 4% and 8% (v) doses. Their infectivity was monitored for 56 weeks at 8-week intervals as measured by mycorrhizal root colonization of bioassay plants. In general, infectivity significantly decreased with time, but in three regimes including periods of cold (5 degrees C, shade and sunlight), cold stratification probably broke spore dormancy and led to temporal stimulation of AMF infectivity. Except for storage at 5 degrees C, where a wider AMF spectrum maintained sufficient long-term viability, infectivity became very low after 1 year. Temperature fluctuations in direct sunlight were less negative than expected, whereas infectivity decline was surprisingly strong also in mild storage conditions (20 degrees C). Higher inoculum dose lengthened AMF infectivity. R. irregularis isolates were found most resistant among the tested AMF. We ascribe the general decline of infectivity to unfavorable properties inherent to peat.