**Project: Soil-borne fungi influence aspects of seed fate relevant to coexistence of desert winter annual plants** Soil-borne fungi influence coexistence of plant species in mesic environments, but little is known about their effects on demographic processes relevant to coexistence in arid and semi-arid systems. We isolated fungi that naturally colonize seeds of an invasive winter annual (Brassica tournefortii) in the Sonoran Desert, and evaluated their impact on seeds of B. tournefortii and a co-occurring native annual (Plantago ovata) under simulated summer and winter temperatures. Fungi isolated from seeds of the invasive plant impacted seed germination and mortality of both plant species in vitro. Rates of seed germination and mortality reflected a fungal strain × plant species interaction with differing outcomes under winter and summer temperature regimes. Some fungi affected germination under winter temperatures and thus were biologically active under those conditions, but fungi caused significant mortality of seeds only in the summer temperature regime. Under natural conditions, seasonal and species-specific effects of particular fungi on seed fate may be associated with promoting plant species coexistence by strengthening niche differentiation, but increased seed mortality and germination under unfavorable conditions may undermine coexistence by weakening seed banks.