Sequence of introduction determines the success of contrasting root symbionts and their host

Many crop species play host to a diverse range of soil-borne symbionts ranging from parasitic, such as potato cyst nematodes (PCN), to mutualistic, including arbuscular mycorrhizal fungi (AMF). Each of these organisms may establish symbiosis with the host prior to the arrival of another which may impact the fitness of all parties involved. We simulated a range of arrival time scenarios for both AMF and PCN and determined their consequences on potato host plants and subsequent symbionts to reflect the likely complexity of symbioses that occur in the field. Simulations were focussed on the first few weeks of plant growth to identify the importance of symbiont interactions during early plant development. Our data indicate that the order in which symbionts are introduced to crop roots is not only important for their own success, but also for that of the host and its additional symbionts. The presence of AMF increased the PCN population on the host, with earlier introduction of AMF increasing the magnitude of the effect. However, presence of AMF also increased the potato's tolerance to PCN, ameliorating the negative effects of the increased PCN burden. This tolerance was stronger the earlier the AMF were introduced and was sustained even when AMF were introduced after PCN. Overall, we show that the initial few weeks of crop emergence and growth may reflect a window of opportunity where the prosperity of the crop and its tolerance of parasites can potentially be influenced by coordinating application of AMF propagules. Additionally, these timings impact the success of below-ground plant parasites that can persist and impact crops for several years.