Immune function of a plant can be modulated by administering a composition comprising beta glucan, where the beta glucan includes unbranched beta-(l,3)-glucan. Such unbranched beta-(l,3) -glucan can be obtained from Euglena, including heterotrophically grown Euglena. The unbranched beta-(l,3)-glucan can be in the form of paramylon and/or can be part of algae meal. The composition can further include a fertilizer, a pesticide, a fungicide, a bactericide, combinations thereof, as well as one or more various plant immune system modulators other than beta glucan. The wellbeing of the plant seed, seedling, mature plant, or harvested plant product can accordingly be improved.
