Abstract
Summary: The effect of the age of roots on their susceptibility to infection by a vesicular‐arbuscular mycorrhizal fungus was compared in leek (Allium porrum L.) and clover (Trifolium parviflorum Ehrh.). Plants were grown in soil lacking mycorrhizal fungi for up to 21 d (clover) or 32 d (leek) and then transplanted into soil infested with Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe. The pattern of infection which occurred was compared with that in plants grown continuously in infested soil.Leek roots up to 32 d old retained their susceptibility to infection whereas clover roots as young as 5 d old had become almost completely immune to penetration by G. mosseae and those which were 21 d old did not become infected at all. The most proximal part of the roots (9 mm in leek and 21 mm in the main root of clover) always remained uninvaded.A difference between leek and clover was also observed in the distance of the most distal entry point from the root tip (10 and 1 mm, respectively). Some examples of host resistance to infection are described. One clover plant was not colonized by G. mosseae despite the formation of many entry points.