Abstract
Most species of plants are normally infected by mycorrhizal fungi, but some plant taxa do not usually form generally recognisable mycorrhizas. The identity and relationships of these "nonmycorrhizal" or weakly mycorrhizal taxa are considered in this review. Mechanisms that are used by the plants for avoiding infection are discussed. Exudation of fungitoxic compounds or simply low levels of exudation are not believed to inhibit infection. Similarly, the presence of fungitoxic compounds in root cortex cells is not believed to prevent mycorrhizal fungi from infecting weakly mycorrhizal plants. It is proposed that the control of mycorrhizal fungal penetration is exerted by interactions between the organisms at the level of the cell wall and (or) middle lamella.